A N E XPLORATION OF THE V IABILITY OF SPARK PLASMA SINTERING FOR THE COMMERCIAL FABRICATION OF NUCLEAR FUEL PELLETS By TIMOTHY IRONMAN A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFI LLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2017
2017 Timothy Ironman
To my friends and family, who expect more out of me then I expect out of myself
4 AC KNOWLEDGMENTS I would like to thank my friends and family for their unwavering support. I would like to thank Professor Tulenko for giving me the opportunity and for entrusting me with all the duties and responsibilities of a laboratory manager. The experi ence has led to immeasurable personal growth. I would like to thank Professor Subhash for all of his assistance and his honest critiques of myself and my work. I would like to thank Professor Lear and Professor Yang for sitting on my committee and having p atience with me. I would like to thank all of the members of the Laboratory for the Development of Advanced Nuclear Fuels and Materials for their assistance and support over the past four years. I would like to give a special thanks to Patrick Moo and Jite sh Kuntawala.
5 TABLE OF CONTENTS page ACKNOWLEDGM ENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 7 LIST OF FIGURES ................................ ................................ ................................ .......... 8 ABSTRACT ................................ ................................ ................................ ................... 10 CHAPTER 1 MOTIVATION AND SIGNIFICANCE ................................ ................................ .......... 11 2 LITERATURE REVIEW ................................ ................................ .............................. 14 2.1 Commercial U O2 P ellet P roduction ................................ ................................ .. 14 2.2 Spark Plasma Sintering ................................ ................................ .................... 15 2.3 SPS of UO2 ................................ ................................ ................................ ...... 19 2.4 Graphite SPS Tooling ................................ ................................ ....................... 21 2.5 Commercial SPS C apabilities ................................ ................................ ........... 22 3 METHODOLOGY REVIEW ................................ ................................ ........................ 25 3.1 Spark Plasma Sintering ................................ ................................ .................... 25 3.2 Commercial Pellet Qualification ................................ ................................ ........ 26 3.3 Pellet Loadability Tester ................................ ................................ .................... 28 3.4 Laser Profilometer ................................ ................................ ............................. 29 4 FABRICATION OF COMMERCIAL UO2 PELLETS ................................ ................... 34 4 .1 Introduction ................................ ................................ ................................ ....... 34 4.2 Experimental Procedure ................................ ................................ ................... 34 4.3 Results and Discussion ................................ ................................ ..................... 36 4.4 Conclusions ................................ ................................ ................................ ...... 39 5 COMPOSITE PELLETS ................................ ................................ ............................. 44 5.1 Introduction ................................ ................................ ................................ ....... 44 5.2 Experimental Procedures ................................ ................................ .................. 44 5.3 Results and Discussion ................................ ................................ ..................... 47 5.4 Conclusions ................................ ................................ ................................ ...... 50 6 SPS TOOLING ................................ ................................ ................................ ........... 55 6.1 Introduction ................................ ................................ ................................ ....... 55 6.2 Tooling Considerations ................................ ................................ ..................... 56
6 6.3 Conclusions ................................ ................................ ................................ ...... 67 7 SCALING UP PRODUCTION ................................ ................................ ..................... 77 7.1 Introduction ................................ ................................ ................................ ....... 77 7.2 Experimental Procedures ................................ ................................ .................. 78 7.3 Results and Discussion ................................ ................................ ..................... 82 7.4 Conclusions ................................ ................................ ................................ ...... 88 8 ATR PELLET PRODUCTION ................................ ................................ ................... 100 8.1 Introduction ................................ ................................ ................................ ..... 100 8.2 Experimental Procedures ................................ ................................ ................ 101 8.3 Results and Discussion ................................ ................................ ................... 103 8.4 Conclusions ................................ ................................ ................................ .... 105 9 ECONOMIC CONSIDERATIONS ................................ ................................ ............ 111 9.1 Introduction ................................ ................................ ................................ ..... 111 9.2 Capital Costs ................................ ................................ ................................ .. 112 9.3 Production Costs ................................ ................................ ............................. 114 9.4 Conclusions ................................ ................................ ................................ .... 117 10 CONCLUSIONS AND FUTURE WORK ................................ ................................ 120 REFERENCES ................................ ................................ ................................ ............ 122 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 125
7 LIST OF TABLES Table page 4 1 Dimensions and density of si ngle pellets produced via SPS .............................. 43 4 2 Pellet grain size versus position as measured ................................ .................... 43 4 3 Dimensions of UO 2 pellets fabricated in two runs ................................ ............... 43 4 4 Pellet end geometry measurement statistics ................................ ...................... 43 5 1 Diamond physical dimensions ................................ ................................ ............ 52 5 2 SiC p hysical d imensions ................................ ................................ ..................... 52 5 3 Comparison of dish depth in composite and pure UO2 pellets ........................... 54 6 1 Pellet density with varying die outer diameter ................................ ..................... 69 6 2 Comparison of dish depth in a five pellet run with graphite punches .................. 69 6 3 Comparison of dish depth in a four pellet run with YZX punches ....................... 69 6 4 Comparison of dish depth in a three pellet run with B4C punches ..................... 69 7 1 Statistics of pellets produced as stacked doubles ................................ .............. 90 7 2 Density and dimensions of parallel produced pellets ................................ .......... 90 7 3 Pellets produced by combining series and parallel setup ................................ ... 91 7 4 Pellets produced by 19 pellet die ................................ ................................ ........ 91 8 1 Planned ATR pellet sintering parameters. ................................ ........................ 106 8 2 ATR Blanket pellet results ................................ ................................ ................ 107 8 3 ATR Base pellet results ................................ ................................ .................... 108 8 4 ATR Diamond pellet results ................................ ................................ .............. 109 8 5 ATR SiC pellet results ................................ ................................ ...................... 110
8 LIST OF FIGURES Figure page 2 1 Example SPS die setup ................................ ................................ ...................... 23 2 2 Diagram showing heating at particle contacts during SPS. ................................ 23 2 3 Three stage commercial SPS machine ................................ ............................. 24 3 1 An unloaded die with punches. ................................ ................................ ........... 29 3 2 A loaded die w ith graphite feltr. ................................ ................................ .......... 29 3 3 A representative temperature and pressure curve for the SPS process. ............ 30 3 4 A die loaded in the SPS m achine. ................................ ................................ ...... 31 3 5 The pellet load tester. ................................ ................................ ......................... 32 3 6 The Bruker optical profilometer ................................ ................................ ........... 33 4 1 Sintered UO 2 pellets. ................................ ................................ .......................... 40 4 2 Grain size measurements ................................ ................................ ................... 41 4 3 Central ring artifact. ................................ ................................ ............................ 42 4 4 Profilometer measurement of the dish geometry ................................ ................ 42 5 1 Diamond c omposite p ellet l ot ................................ ................................ .............. 51 5 2 SiC c omposite pellet lot. ................................ ................................ ..................... 52 5 3 Graphite on surface of SiC pellet ................................ ................................ ........ 53 5 4 Diamond pellet showing surface chip ................................ ................................ 53 5 5 Fracture s urface of d iamond/UO2 p ellet ................................ ............................. 54 5 6 Dish depth for SiC, diamond and UO2 pellet ................................ ...................... 54 6 1 Fracture surface of pellet fabricated with Al 2 O 3 spray ................................ ......... 70 6 2 SEM image showing UO 2 powder inclusions ................................ ...................... 70 6 3 Images of the punch wear with usage ................................ ................................ 71 6 4 Comparison of dish depth for graphite tooling ................................ ................... 71
9 6 5 Pellets produced with YZX punch tip ................................ ................................ .. 72 6 6 YSZ p unch tips after use ................................ ................................ .................... 73 6 7 Comparison of dish depth using YSZ punches ................................ ................... 74 6 8 Comparison of dish depth using B4C punches ................................ ................... 74 6 9 Profiles of SPS run of YSZ punch tips vs graphite punch tips ............................ 75 6 10 Profiles of SPS run of YSZ punch tips at 1050 C vs 900 C ................................ 76 7 1 Schematic showing stacking powder samples in two pellet die .......................... 92 7 2 Schematic of the die geometry used to fabricate multiple pellets ....................... 92 7 3 Die used to fabricate 19 pellets ................................ ................................ .......... 93 7 4 Sintering of 19 pellet die ................................ ................................ ..................... 94 7 5 Stacked pellets as fabricated ................................ ................................ .............. 95 7 6 Temperature profiles for dies fabricati ng multiple pellets I ................................ .. 95 7 7 Voltage profiles for dies fabricating multiple pellets I ................................ .......... 96 7 8 Electrical power profiles for di es fabricating multiple pellets I ............................. 96 7 9 Z axis displacement profiles for di es fabricating multiple pellets I ...................... 97 7 10 Temperature profiles for dies fabricating multiple pellets II ................................ 97 7 11 Electrical power profiles for dies fabricating multiple pellets II ............................ 98 7 12 Voltage profiles for dies fabricating multiple pellets II ................................ ......... 98 7 13 Z axis displacement profiles for dies fabricating multiple pellets II ..................... 99 9 1 Dr. Fuji Dr. Sinter largest standard furnace ................................ ..................... 118 9 2 Dr. Fuji Dr. Sinter example tunnel furnace ................................ ....................... 119
10 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy AN EXPLORATION OF THE VIABILITY OF SPARK PLASMA SINTERING FOR THE CO MMERCIAL FABRICATION OF NUCLEAR FUEL PELLETS By Timothy Ironman December 2017 Chair: James Tulenko Major: Nuclear Engineering Sciences The viability of spark plasma sintering (SPS) for fabrication of industrial grade nuclear fuel pellets is explored b y utilizing die designs for single and multiple pellet manufacturing. Traditional UO2 pellets were also manufactured by systematically varying processing temperature and pressure as needed for single and multi pellet fabrication. The pellets were then qua lified against commercial fuel specifications for density, shape, microstructure and surface flaws. Pellets produced one at a time met all commercial specifications except for grain size. Pellets produced in batches of 2, 4 and 8 pellets showed sub optimal density indicating that further changes to sintering conditions are warranted. Additionally, commonly used graphite tooling for pellet fabrication was shown to be ineffective in producing large numbers of fuel pellets, as the die and punches were shown to undergo severe wear in each run thus decreasing the reliability of the tooling for production of pellets as per the specification. Finally, additional discussion is provided for identifying the avenues for scale up of SPS to meet the current commercial de mand of 400 million pellets per year. These studies are viewed as a first step towards assessing the ability of SPS technology to meet the quality specifications and quantity demands of the production of nuclear fuel pellets.
11 CHAPTER 1 MOTIVATION AND SIGNIFICANCE The primary material for nuclear fuel, worldwide, is uranium oxide (UO2). Compared to its metal counterpart (Uranium), UO2 h as higher melting point (2865 C vs 1132 C for U), which allows nuclear reactors to run much hotter. UO2 also is chemi cally and physically more stable in reactor environments . It has been used and extensively researched for over the 70 year history of commercial nuclear power. However, a key drawback of UO2 is its low thermal conductivity (3.5 W/mK at 1000 C for UO2 v s 38.7 W/mK at 1000 C for U). The negative effects of the low thermal conductivity of UO2 include a large radial temperature gradient (in excess of 500 C over a radial distance of 4 mm) in the fuel pellets and reduced operating power (reactivity) of the r eactor. These temperature gradients create hi gh thermal stresses which cause cracks to develop in the pellets which further reduce thermal conductivity and increases gaseous fission product diffusion out of the pellet. Despite six decades of research and practice, there have been no major changes in pellet shape or its processing method  Advances in enhancing either UO2 thermal conductivity or microstructure and properties using the conventional sintering process are also limited. In recent years, dopi ng UO2 with chromia (Cr2O3) has been shown to increase the grain size of fuel pellets up to 45 microns [ 3 ].The increased grain size is expected to provide improved fission gas retention and decreased cracking in the pellet. Efforts to further increase ther mal conductivity of UO2 with 2nd phase particles and dopants have been met with limited success due to the extreme difficulty in sintering these powders in the conventional processing method.
12 While the conventional sintering method has remained unaltered i n the past few decades, in recent years spark plasma sintering (SPS) has emerged as a promising alternate method for processing nuclear fuel pellets into desired shape and dimensions in significantly lower processing times (30 45 min) and at lowe r sinterin g temperatures (900 C) [4 ]. The SPS method also requires a significantly fewer number of steps, time, energy and provides net shaped pellets in a single step. The SPS process has also been shown to be capable of successfully sintering UO2 powders with 2nd phase particles such as diamond, SiC and even carbon nanotubes (CNTs) [ 5 ]. SPS has many other benefits over conventional sintering. It can sinter novel materials and hard to sinter materials  to a desired density in a significantly reduced processing ti me. The SPS heating rates allow for reduced sintering times and minimize formation of undesirable chemical by products. There have been numerous instances of SPS being used to produce precise ceramics that may be cost prohibitive to machine to desired shap e [ 3 ]. SPS has been successfully used by the University of Florida to consolidate commercial UO2 powder into pellet form. Ge et al [ 4 ] achieved over 96% theoretical density at a peak temperature of 1050 C for just 0.5 minutes of sintering time. Novel compo site materials, with high thermal conductivity, such as UO2 SiC [ 5 ], UO2 diamond [ 6 ] and UO2 CNT [ 7 ] have been successfully fabricated by UF via SPS. The above studies have unequivocally demonstrated the ability of SPS to fabricate UO2 fuel pellets rapidly and at lower sintering temperatures than conventional methods. However, the method has previously been shown to be effective only for fabricating a single pellet each time, thus questioning the utility of SPS for a scale up to
13 multi pellet batch manufactu ring and, eventually, for industrialization of the technique for fabrication of hundreds of millions of pellets per year. Towards this end, this current study intends to explore the following features to determine the viability of commercial SPS manufactur ing UO2 pellets. 1. Can the SPS process be used to fabricate commercial grade nuclear pellets with strict dimensional control, pellet density, microstructural homogeneity, and surface quality? 2. Can the SPS process be scaled up to make multiple pellets in each production run? 3. Are there manufacturing defects in SSP produced pellets and what is the potential rejection rate? 4. What are the tooling failure modes and wear rate? How does tooling affect the pellet surface quality? 5. Can the pellets produced by the SPS process pass a screening test for loading into a cladding for evaluation of their performance in the Advanced Test Reactor? 6. Finally, can SPS be used to fabricate commercial scale pellet quantities at equal or cheaper cost than the conventiona l sintering process? It is vital to seek answers to these questions before one can make sound judgments about the feasibility of SPS for scale up fabrication and its potential for industrialization.
14 CHAPTER 2 LITERATURE REVIEW 2.1 Commercial UO2 P e llet P roduction Commercial UO2 pellet production can be broken up into three parts: the production of UO2 powder from the UF6 feedstock, the fabrication of dense pellets from UO2 powder and the production of the metal assembly framework along with loading the pellets into the assembly framework  The only aspect that concerns this process is step two, the fabrication of dense pellets from UO2 powder. After the raw UO2 powder is produced, the powder is conditioned. Conditioning involves blending powders t ogether to ensure the required particle size distribution and specific surface area. U3O8 may be added to reduce the required sintering temperature and improve the sinterab ility of the powder overall Additionally pore formers, lubricants and burnable pois ons may be added in the powder conditioning stage. The powder is then placed in a metallic die and pressed with hundreds of MPa into 50 60% dense green pellets. The pressing of pellets is done individually in high speed pressing machines. The axial edges o f the green pellets are produced with a desired end geometry through the use of a machined punch tip, which is made of a highly wear resistant material. The green pellet is then removed from the die and placed in large trays and sent to the sintering furna ces. The furnaces heat the green pellets to temperatures in excess of 1750 C to densify the pellet f rom 50 60% dense to above 95% dense. The heating process requires in excess of 15 hours  The fully dense pellet is not yet in the engineered shape. The sintering process deforms the pellets, so they are not perfectly cylindrical. The pellet are centerless grounded into the desired shape This process also produces a very smooth outer pellet
15 surface along with a carefully controlled outer diameter. The UO2 ground off is then recycled back to the beginning of the process. Typical throughpu t for a commercial facility is 4 5000 pellets per hour. A representative sample size is removed from the line and tested for quality every hour. 2.2 Spark Plasma Sintering S park Plasma Sintering Spark Plasma Sintering (SPS) is a field assisted sintering technique. SPS uses a low voltage, pulsed DC current and an applied pressure to activate sintering mechanisms for material synthesis. SPS is capable of synthesizing new materi als, difficult to sinter materials and conventional materials all at a faster rate [ 10 ]. SPS also allows for sintering near net shape components. SPS is similar to hot pressing, in that heat is applied and a pressure assists sintering. The major difference s are the method of heating, Joule Heating, and the current activates other sintering mechanisms in conducting and semi conducting samples. The most common SPS setup uses a graphite die, a pair of graphite punches, graphite foil separating the powder sampl e for the die and graphite felt exterior to the die as a thermal ins ulator. An example is shown in F igure 2 1 below. Due to using the pulsed current, extremely high heating rates (>1000C/min) can be used. This offers numerous advantages including both mate rial advantages and economic advantages. The process b egins by loading the die compact The compact is placed in a furnace and set between two conducting steel rams. The rams apply the force to compress the powder and pressurize the powder, along with ensu ring electrical contact. A pulsed DC current is run through the compact leading to Joule heating which quickly heats the sample and die. Depending on the conductivity of the powder sample and the specific tooling used, the current can be driven through the sample or the die
16 allowing for process control. When using purely insulating materials, such as Al2O3, the technique is comparable to hot pressing with high heating rates. The SPS technique dates all the way to 1906 with a patent for a current a ssisted si ntering arrangement [ 11 ]. The process matured slowly until the 1960s when Inoue designed the modern form of SPS [ 11 ]. The maturation of the technology stalled while Inoue s patents were intact, but after they expired the technology was spread worldwide. Cu rrently there a re three major producers of SPS machines for research and commercial needs. These are Fuji Electronic Industrial Co., Ltd in Japan, FCT Systeme GmbH in Germany and Thermal Technology LLC, INC in the United States. The commercialization of SP S machines has led to a worldwide increase in patents and academic literature. The technology has exploded with over 400 patents between 2000 and 2008 compared to under 80 before 1990. The exact mechanisms involved in SPS are still under discussion  Many of the recent work s ha ve treated SPS as a black box. Researchers tend to just report mechanical properties and microstructure of various materials sintered without concrete mechanistic explanations [13 22] SPS does feature three classes of effects: m echanical, thermal and electrical effects  These effects work in tandem and are not truly separable. The mechanical effects originate from the compressive force applied by the furnace rams. This effect increases the contact between powder particles an d modifies the morphology of particle contact areas. The applied pressure enhances conventional sintering mechanisms while activating other mechanisms. Plastic deformation allows for increased densification at high temperatures and densities. Typical press ures are limited to 80 150 MPa due to the strength of the graphite tooling;
17 however, alternative tooling including Tungsten Carbide and Silicon Carbide have allowed for pressure up to 1 GPa. The thermal effects are primarily due to the heating rate. The h igh heating rates allow for minimizing unfavorable densification mechanisms such as coarsening. The high heating rates make high activation energy mechanisms, such as grain boundary diffusion, dominant. Th ese effects increases densification rates, decreasi ng processing times. The small processing times further discourage grain growth. The high heating rate therefor, allows for high densification rates without significant grain growth. There are detrimental thermal effects due to the high heating rates, such as both axial and radial thermal gradients within the sample and the tooling. This may lead to non uniform microstructure or even localized melting. Th ese can be avoided with tooling design  The electrical effects involve both field effects and curre nt effects. This is further complicated by the variable conductance of the sample. Conducting and non conducting samples obviously behave differently, but individual samples have a variable effective electrical conductivity due to densification and tempera ture. In conducting samples, a significant portion of the current flows directly through the sample. In non conducting samples, the current runs essentially exclusively through the tooling. In nonconductors, such as Al2O3, sintering behavior is similar to hot pressing. In conducting samples there are numerous effects including the Peltier effect between the powder and the tooling, percolation effects, electrochemical reactions and electromigration. When the current flows through the sample, joule heating le ads to local temperature gradients at particle contacts as shown in F igure 2 2 This creates very large differences from local to global temperature. These junctions sinter first
18 further increasing conductivity. These current pathways can be traced all the way through the process. Tooling design can control the current path  By using conducting and non conducting tooling or coatings, current can be driven into low conductance samples or driven away from conducting samples. This makes SPS complicated b ut quite malleable. The Peltier effect which is directional heating due to electrons travelling across a junction, leads to anisotropic temperature gradients at the tooling powder interfaces  This is particularly detrimental when making complicated sh apes. This can be minimized by further tooling design. Electrochemical reactions can be manipulated for fabrication benefits or can lead to detrimental material properties but must be considered when designing a particular component process. Electric field s effects are typically discarded due to the low voltage in SPS, but may be important in certain systems. SPS has been used to fabricate numerous materials that are either difficult or impossible to sinter through other means. Materials that have been inve stigated include refractories, high temperature ceramics, nanostructured materials, functionally graded materials, transparent ceramics, and metastable materials [27 28 ] High temperature ceramics such as ZrB2 and B4C are materials which SPS provides nume rous advantages. Very high melting temperatures lead to high sintering temperatures which makes fabrication through other means difficult without extended grain growth. SPS allows for the production of these materials without that grain growth. Refractori es have very high sintering temperatures and tend to oxidize. They require applied pressure to reach high density. SPS allows for atmospheric control, along with applied pressure to reduce
19 oxidation and short sintering times to minimize grain growth. A com parative study sintering ruthenium via SPS and hot pressing showed SPS produced sintered mate rial with lower residual stress  Non equilibrium materials cannot be fabricated through conventional sintering or other alternatives due to chemical reaction s or phase transitions that would occur. SPS allows for the fabrication of these materials due to the low time at temperature and the applied pressure. These materials include composite materials with various enhance thermal, electrical and mechanical prop erties. SPS provides unique advantages in fabricating functionally graded materials. The temperature gradients inherent in SPS setups can be manipulated to create variable environments in different area of a sample. Simply modifying the OD of a die enable s controlling the amount of joule heat generated in various parts of the die. This enables variable environments for different sections of the sample. Also the temperature gradients are continuous reducing failu re origins and delamination 2.3 SPS of UO2 The University of Florida has pioneered processing UO2 via SPS. Ge et al [ 4 ] successfully sintered UO2 with an SPS sintering temperature of only 1050 C for 30 seconds. He produced 96% TD pellet s consistent with con ventionally produced UO2. The total run time was only 10 minutes. This is a large improvement over the many hours required to sinter UO 2 conventionally. Chen et al [ 30 ] continued this work and produced the master sintering curve for UO2 via SPS. Chen deter mined the apparent activation energy for SP was lower than the conventional sintering process, which he attributed to the coupled current field and
20 pressure field in SPS. SPS leads to an increased diffusional transport rate and the pressure allows for more grain boundary contact. Chen does state that more study would be required to quantify the basis for the reduce activation energy. The master sintering curve allows for the designed path to achieve high density. Chen determined which temperatures and hold times are r equired for densification. Yeo Chen and Cartas sintered UO2 with high thermal conductivity additives to produce ceramic matrix composites with improve thermal performance. Yeo produced UO2/SiC composites with both SiC whiskers and particles. Th e SiC/UO2 composites improved thermal conductivity consistently without undesired reaction products. The above study showed that UO2/ SiC composites could not reach high density and unfavorable chemical reactions occurred in conventional sintering but in SPS high density and pure materials were produced. Chen produce d UO2/diamond composites with improved thermal and mechanical performance. UO2/diamond composites did exhibit diamond graphitization above 1500 C. Cartas produced UO2/ CNT composites with high thermal conductivity. Cartas determined the material could be densified well as long as the CNTs were well distributed in the UO2 matrix. Other more exotic nuclear fuels have also been explored via SPS fabrication. These include simulated UO2 fuel containi ng CsI , Tungsten UO2 Cerments , and Uranium Nitride . These materials have a wide variety of applications. The studies performed on these materials tend to focus on making laboratory quality samples and not with the geometry of the various app lications.
21 2.4 Graphite SPS Tooling There are two elements to the graphite tooling used in SPS that may negatively impact the sintering of UO2 fuel pellets. These two issues are the reaction of UO2 with graphite and the localized overheating problem. UO2 is known to chemically react with Carbon. This reaction is useful for the reduction of the UO2 powder to stoichiometric UO 2.00 without the need for extra steps in processing. Various studies have claimed this reaction starts between 1000 C and 1300[34 3 7]. The reaction products are UC, UC 2 or UC 1 x O x depending on the amount of the residual oxygen in the UC lattice [35 37 ]. Gosse  found for hyper stoichiometric UO 2 powder, the reaction starting temperature could be as low as ~700 and in this reaction, UO 2+ x is reduced into UO 2.00 without the formation of carbides. The reaction between graphite and UO 2 is more likely to happen on the surface contact between the UO2 and graphite punch because of the higher temperature on this surfa ce. The following sequence of reactions are possible in the surface layer . UO 2+ x + x C UO 2 + x CO UO 2 + 4C UC 2 + 2CO 3UC 2 +UO 2 4UC +2CO The localized overheating problem occurs in many SPS experiments. This phenomena involves undesired extreme temperat ure gradients forming in the SPS tooling. This typically present in the punches. Giuntini  showed the difference in temperature between the punch and the powder may be as high as 1000 C. The origin of this phenomena may be related to the resistance as sociated with contact points in the SPS setup. Wei  showed electrical contact resistance dominates the
22 temperature evolution in SPS tooling compacts. The combination of these two issues may present problems at higher temperatures. 2. 5 Commercial SPS C apabilities Currently the commercial capabilities of SPS are limited. There is no published material describing commercial use but there are vendors producing large scale SPS machines. Any knowledge on the commercial use of SPS is protected tightly by for profit companies. There are three main companies producing SPS machines they are FCT in Germany, Thermal Technologies in the USA, and Fuji electronic company in Japan. The commercial type machines ( Figure 2 3 ) have three compartments. The first compartme nt is pre sintering and accomplishes all tasks required before the high heating rate is applied. These tasks include loading powder into dies and preheating dies to an elevated tem p e r ature before sintering occurs. The second stage is the actual SPS sinteri ng. In this stage the powder is heated with the electrical current and consolidates. In the third stage, the tooling and sintered pellet is cooled naturally to room temperature and the pellets are removed.
23 Figure 2 1. Example SPS die setup Figur e 2 2. Diagram showing heating at particle contacts during SPS [ 10].
24 Figure 2 3. Three stage commercial SPS machine from Fuji Electric, Japan Reproduced with p ermission from FUJI ELECTRONIC INDUSTRIAL CO., LTD. (Kawasaka, Japan 2010).
25 CHAPTER 3 METHODOLOGY REVIEW 3.1 Spark Plasma Sintering SPS is a field assisted sintering technique utilizing a large pulsed current to quickly heat graphite tooling, leading to high sintering rates in the powder loaded in that tooling. Heating rates can be as larg e as 1000 C/min. In addition to the current, SPS features an applied pressure which compresses the powder body, increasing particle contact and enhancing densification. This method allows for minimal powder preparation and sintering is accomplished in a si ngle step. Applied pressure is limited by either the capacity of the SPS machine or by the tooling. In this study, a pressure limit of 80 MPA is set because this value is the fracture strength of the graphite tooling used. All samples are loaded into a gra phite die by hand by pouring the powder into th e die. The tooling is shown in F igure 3 1. The powder is slightly compressed in between two punches with the pressure simply applied by hand. Typically the powder is physically separated from the die via a th in layer of graphite. This layer of graphite comes in two forms: a thin yet rigid graphite foil or a graphite spray. The graphite foil is the standard used in SPS research but is not viable for large scale industrial production. The graphite foil may act a s a lubricant and/or current pathway. After the powder is loaded, a thick graphite felt is then wrapped around the die for insulation purposes. A loaded die, re ady for sintering, is shown in F igure 3 2. A small hole is cut into the felt to allow for measu ring the temperature of the die. The die is loaded into the SPS machine in between two graphite spacers which are in physical contact with steel rams at the top and bottom of the machine. This completes the circuit for the current to flow. The complete di e compact, ready for
26 sintering, is shown in F igure 3 3. A pyrometer, which measures the temperature of the die, focuses on the axial center of the die. The pyrometer is synced with the SPS controller. This controller modifies the current to achieve a prepr ogrammed temperature profile. A typical t emperature profile is shown in F igure 3 4. The pyrometer has a minimum temperature it can read, which is 600 C. Typically there is a dwell time of 5 7 minutes at which the die set temperature is held at 600 C to all ow the pyrometer to take a consistent reading. The temperature profile includes setting the heating rate to the maximum temperature. At the maximum temperature, the compact is held at temperature for a set time. During this time, an external pressure is ap plied via the pressure controller which requires manual input. The die may migrate slightly (on the order of 5 mm) vertically during sintering. This means the spot the pyrometer is measuring may slightly change during the process. After sintering, the cons olidated pellet is removed by hand pressing the punch through the die. If the pellet cannot be removed by hand, a carver press is used to force the pellet out of the die. Then the die is cleaned and reused. 3.2 Commercial Pellet Qualification Typical com mercial pellet qualification requires a sample of 20 pellets, out of millions, to be randomly tested against the requirements stated below. Pellet diameter is measured via a digital micrometer three times and the average of these three measurements is use d to calculated density. The three measurements are taken at 120 degree azimuths to provide further insight into pellet shape. The diameter of the pellets shall be 8.1915 0.0254 mm, with a goal of meeting a tolerance that is reduced by 1/2, i.e. 0.0127 mm.
27 The length of each pellet is taken as a single measurement. The length of the pellets shall be 10.16 1.27 mm, with a goal of meeting a tolerance that is reduced by approximately 1/3, i.e. 0.423 mm Mass is a single measurement. Density is the n ca lculated using the calculated pellet volume and measured mass. A correction factor of 0.9909 used to correct the volume due to the missing volume of the dish and chamfer. For each pellet type, the average pellet density shall be greater than or equal to 96 .5% of theoretical density and individual pellet densities shall be 2.0 % of that average. The pellets shall have a spherical dish and chamfer on each end. The width of each dish shall be 4.52 mm and the depth shall be 0.0254 mm. The target width (radial dimension) and depth (axial dimension) of each chamfer shall be 0.381 mm and .127 mm, respectively. Pellet end geometry is measured via the profilometer described below. Surface quality is determined by visual inspection. Any chip or crack that may be lar ger than the allowable size are further examined with a magnifying lens with a built in scale. Each pellet shall be inspected at 1 magnification for cracks and chips. The length of any individual axial crack shall not exceed 1/3 of the pellet length. The length of any individual circumferential crack shall not exceed 1/3 of the pellet circumference. The total area of all chips on the cylindrical surface of a pellet shall not exceed 5% of the area of the cylindrical surface. Chips on the cylindrical surface that exceed a circumferential width of 1.5 mm or larger shall be recorded. The total area of all chips on any end surface of a pellet shall not exceed 1/3 of the area of the end surface.
28 Each pellet type shall be subjected to a loadability test. A minimu m of two ten sintered pellets. Each test stack shall be loaded axially in compression to a maximum of 100 lb. The load at which a 1.5 mm (maximum dimension) chip or la rger is produced shall be recorded. If the sample withstands a 100 lb load without chipping, the chip loading for the sample shall be recorded as 100 lb. The expectation is that pellets can withstand a 40 lb load without producing a 1.5 mm (maximum dimensi on) chip. Grain size is measured via thermal etching following by the linear intercept method  The pellet grain size measured at three representative points (edge, midway and center) shall be greater than or equal to 10 m, with a goal of 20 m or lar ger. The pellet porosity greater than 20 m shall be less than or equal to 0.5% by volume. 3.3 Pellet Loadability Tester Pellets must be able to survive an axial force of 100 pounds without chipping or cracking, to ensure they would survive insertion int o the clad. The pellet loadability tester, provided by Areva, was used to accomplish this. The standard loadability test was performed on all the pellets [ 42 ]. In this test, a pre inspected stack of ten pellets were laid horizontally in between the space p rovided by the fo ur rods in the tester shown in F igure 3 5 and the tester slowly exerted 100 pounds of force on the stack. T he force was then removed and the pellet inspected for chips and cracks.
29 3.4 Laser Profilometer The Contour GT I Bruker Optical P rofilometer is a benchtop instrument used to measure the profile and surface roughness of a sample. The device uses a laser to measure the vertical location of the focused spot to a resolution of 0.1 nm. The device can automatically make a map of the verti cal locations of the laser as it moves across the sample surface. Post processing can remove any intrinsic tilt in the sample The profilometer is shown in F igure 3 6 Figure 3 1. An unloaded die with punches Photo courtesy of the author. Figure 3 2. A loaded die with graphite felt. Photo courtesy of the author.
30 Figure 3 3. A representative temperature and pressure curve for the SPS process.
31 Figure 3 4. A die loaded in the SPS machine Photo courtesy of the author.
32 Figure 3 5. The pellet load tester which is used to measure pellet strength Photo courtesy of the author.
33 Figure 3 6. The Bruker optical profilometer being used to measure pellet end geometry Photo courtesy of the author.
34 CHA PTER 4 FABRICATION OF COMMERCIAL UO2 PELLETS 4.1 Introduction Commercial grade nuclear fuel pellets are specifically engineered parts, with tight tolerances and precise dimensional requirements. Previous SPS work at UF focused on producing dense UO 2 sampl es from feedstock powder without concern for the size and shape of the sample. These previous samples were flat disks with a length of 4 8 mm and a diameter matching Mark B fuel pellets (12.5 mm). Thin disks are both easier to fabricate and easier to char acterize. The commercialization of this process requires pellets to be fabricated according to the i ndustry standards described in C hapter 3. The major changes are the modification of the diameter from 12.5 mm to 8.19 mm, the change in L/D ratio of the pel lets and the shape change from a flat disk to a modified cylinder with a d ish and chamfer. As stated in C hapter 2, the current standard for achieving the desired pellet shape is a combination of prepressing the UO2 powder and using centerless grinding. SPS has been used previously to make various shapes while not requiring these other processing steps. 4.2 Experimental Procedure Initially, 20 single pellets were produced to meet the requirements set in the pellet q ualification plan described in C hapter 3. The powder used was feedstock powder supplied by AREVA Federal Services. This powder was raw UO2 which was not modified with any additives or prepared in any special way. The O/M ratio of the powder is approximate 2.11. The average grain size of the raw po wder is 0.1 m to 0.4 m. The pellets were sintered at 1050 C for 5 minutes with a heating rate of 100C/ min and an
35 applied pressure of 40 MPa. The initial dwell time was 6 minutes, leading to a total run time of 22 minutes. All pellet dimensions were mea sured to determine dimensional consistency and process reliability. These 20 pellets used a variety of freshly machined punches and dies. The tooling was replaced as often as deemed necessary after visual examinations of the tooling, to maximize the qualit y of the pellets. Most punches were used 2 3 times, with 5 dies making the 20 pellet batch. Pellet surfaces were cleaned with alcohol and visually inspected with both a 1X magnified lens and a 10X magnified lens for defects, chips, cracks and inclusions Typical industrial specification is that the total chip area is required to be less than 5% of the total surface area. Any cracks must be less than a third of the pellet circumference. The pellet ends were inspected for the characteristic dish and chamfe r shape. To ensure pellets were fabricated with the required end geometry, a laser profilometer (Bruker Contour GTI) was used. The instrument had a vertical resolution of 3 nm and a horizontal resolution of 0.15 microns. The laser was focused on the pel let land area and the measurement was taken toward the center of the pellet creating a profile showing the dish dimension and then on the other side measuring the chamfer dimensions. The pellet density was calculated from mass and dimensional measurements The length of each pellet was measured once, while the pellet diameter was measured at three orientations separated by 120 degree. The pellet volume was calculated from the average diameter and length, while accounting for missing material from the dish and chamfer. A pellet was cut at three axial locations and the cut surfaces were
3 6 polished. These segments were then thermally etched for ten minutes at 1100 C in a CO2 atmosphere to reveal grain structure. The grain size was measured via the linear interc ept method. A standard loadability test was performed on all the pellets [ 42 ]. In this test, a pre inspected stack of ten pellets were laid horizontally and a force of 100 pounds was slowly applied to the stack. Then, the force was removed and the pellet inspected for chips and cracks. This test checks if pellets would survive loading into cladding without chipping or cracking. Additionally, a second round of ten additional pellets were produced to determine the relationship between tooling reliability an d pellet reliability. In each of two runs, 5 pellets were fabricated with the die and punches being reused for all five pellets, consecutively. The tooling and pellets were then examined for any sign of tooling wear or degradation. 4.3 Results and Discu ssion Table 4 1 shows the compiled results for the 20 pellets initially produced. It reveals that the pellets were successfully fabricated to the specified (goal) dimensions. In SPS, the die inner diameter confines the pellets, so the diameter of all pelle ts was consistent. The pellet length was shorter than required, as this may be due to incorrect estimation of powder weight required for correct length of pellet. However, the resulting length of the pellet was consistent. Also, measured pellet density was consistently acceptable.
37 Fig ure 4 1 shows 19 of the 20 produced pellets. The excluded pellet was removed and processed for the grain size measurements. The surface of all the pellets appeared smooth and clean. However, there was an obvious gray ring in the center of all pellets, ( Fig ure 4 1 a ). The origin of this artifact was unknown but was smooth to the touch. All pellets were produced with a clear dish as shown in Fig ure 4 1 (b). Some pellets had light chipping along the pellet edges, as indicated. The chipping was within the 5% of surface area allowed in the specification. To observe the microstructural homogeneity throughout the pellet, one of the pellets from the above batch was cut at , , and length, as shown in Fig ure 4 2 Using the linear int ercept method, the grain size on each cross section was measured at locations near the circumference and at the center. The pellet average grain size at each position are shown in Table 4 2 It is clear that the smallest grains are in the center of the pe llet and the largest grains are near the edges of the pellet. This is consistent with previous SPS studies [ 4 ]. The average grain size at the center of the pellet was 5.52 0.6 microns. Average grain size at all points was 6.37 0. 9 microns The second b atch of pellet production, featuring two runs of 5 pellets each, is described in Table 4 3 Pellets 1 5 were fabricated one at a time using a single die and set of punches as run 1, while pellets 6 10 were fabricated in another die and utilizing another se t of punches as run 2. From Table 4 3 it is seen that the diameter and standard deviation of the pellets increased from 1st pellet to 5th pellet in each run. Pellet #1 was very symmetric and had the smallest OD. Pellet #5 showed a noteworthy increase in O D and was the least symmetric. This increase is attributed to wear of the
38 inner diameter of the die in each run, which will be discussed separately later in C hapter 6. Despite this wear, all pellets achieved a density above 96.5 % theoretical density and all ten pellets survived axial loading up to 100 pounds. Fig ure 4 3 shows the central circumferential ring that appears on all pellets fabricated (Fig 4 1 (a)). A laser profilometer scan of the surface near this ring, shown in Fig ure 4 3 (b), revealed a loc alized increase in surface roughness. This artifact consistently appeared at halfway of each pellet length. At this point, we are unable to identify its origin. This artifact is not an indention but only surface roughness S ome pellets showed slight chipp ing around their edges ( Fig ure 4 1(b)) This may be due to a mismatch in the punch diameter and die inner diameter. If the gap between the punch and the die is too large, upon sintering, some UO2 may form a sharp edge which can fracture during pellet remov al. The chips may have occurred during the sintering process due to the axial load being applied or after sintering during removal. The end surface of the first pellet produced in each run (with a new graphite punch) was measured with a profilometer t o obtain the dish and chamfer profiles. This measurement length was taken from the land area (at the top of the chamfer) to the dish center on one side (as shown in the inset of Fig ure 4 4 ) and then from the land to the end of the chamfer on the other side A summary of the results of these measurements are listed in Table 4 4 These parameters were within 10% of the goal confirming the acceptability of dimensions of the dish. The only parameter that deviated more than 10% was the chamfer depth. This is mos t likely because as the laser reaches the edge of the chamfer (close to the cylindrical surface) it goes out of focus and the rest of the
39 measurement becomes unreliable. Therefore, the measurement given here does not imply that the pellet chamfer was not to the specifications, but also does not confirm that it was to the specifications. 4.4 Conclusions SPS has been shown to be capable of producing single fuel pellets meeting the specifications of the nuclear industry, with the exception of grain size. Hi gh density pellets, conforming to the nuclear industry qualification requirements, were fabricated and evaluated. These pellets met all required criteria except for grain size. The process is reproducible, over a 95% success rate, when pellets are produced under ideal conditions: one at a time and with fresh tooling. An unknown artifact which may be surface roughness, is present in the form of a ring along the pellet axial center. This ring does not appear to have an impact on pellet density or the pellets ability to stand up to 100 lbs of loading force The quality of the pellets decreases as the tooling is reused. As tooling is reused, the surfaces of the pellets have a greater likelihood of a defect.
40 (b ) Figure 4 1. Sintered UO 2 pellets A ) P ellet artifact (central ring on surface) B ) D ish and chamfer along with slight chatter on chamfer edge Photo courtesy of the author.
41 (a) ) (b) (c) (d) Figure 4 2. Grain size measurements A ) Schematic of the grain size m eas urement cross section locations B ) G rain size measurement surface locations C ) M icrograph revealing grain structure of pellet edge D ) M icrograph of pellet center. Photo courtesy of the author.
42 Figure 4 3. Central ring artifact A ) Image of a pellet with ce ntral circumferential ring B ) L aser scan of the ring revealing high surface roughness Photo courtesy of the author. Figure 4 4. Profilometer measurement of the dish geometry. The inset reveals the measurement location Photo courtesy of the author.
43 Table 4 1 Dimensions and density of single pellets produced via SPS Goal Measured Sample Size 20 20 Sample mean diameter (mm) 8.1915 0.0254 8.194 0.013 Sample mean length (mm) 10.16 9.83 0.26 Density (% TD) 96.5 96.8 Table 4 2 Pellet grain size versus position as measured Position Average Grain Size (m) 1 6.3 1.0 2 5.52 0.6 3 7.28 0.9 A 6.79 1.3 B 6.15 0.9 C 6.88 1.1 D 6.33 1.4 E 5.7 0.9 Table 4 3 Dimensions of UO 2 pel lets fabricated in two runs Pellet Density (%)TD Length(mm) Diameter (mm) Goal 96.5 2.0 10.16 1.27 8.192 0.0254 1 96.9 10.26 8.184 0.0031 2 96.9 10.26 8.192 0.0076 3 97.0 10.26 8.189 0.0046 4 96.9 10.26 8.189 0.0033 5 96.7 10.26 8.199 0.0160 6 97.2 10.21 8.176 0.0025 7 97.7 10.19 8.176 0.0076 8 97.2 10.21 8.181 0.0081 9 96.9 10.29 8.192 0.0026 10 96.6 10.29 8.199 0.0104 Avg 97.0 0.3 10.24 0.04 8.188 0.0081 Table 4 4 Pellet end geometry measurement statistics Parameter Goal (mm) Measured (mm) Dish Depth 0.254 0.247 Dish Radius 2.260 0.50 2.350 Chamfer Length 0.381 0.17 0.400 Chamfer Depth 0.127 0.05 0.075
44 CHAPTER 5 COMPOSITE PELLETS 5.1 Introduction Previous research into composite pellets at UF had focused on the many facets of SPS including sintering parameters, optimal volume fraction of additives, optimal additive particle size, and dispersion techniques. None of these facets were extensively examined in this study. The focus of this study was producing commercial grade fuel pellets. This required the fabrication of composite pellets to net shape, while still meeting the density, cleanliness and other requirements of the pellet qualification plan described in Chapter 3. All previous studies wer e performed with thin flat disks with an L/D ratio of 0.32 0.75. The increase in L/D ratio may make achieving a high density more difficult. The addition of the pellet end geometry requires additional tooling modifications and adds stresses to the pellet. The composite pellets require greater temperatures than the pure UO2; fabrication may be more difficult than for pure UO2 and the behavior may be different than for the pure UO2 studies previously discussed. All previous studies have shown the additives prevent grain growth during SPS so it is expected that large grain sizes will not be produced in this experiment. Sintering parameters were used as suggested by previous studies, which is a temperature around 1400 C. 5.2 Experimental Procedures The UO 2 powder was the same powder used in the pure UO 2 experiments above and was obtained through AREVA. The powder had a bulk density of 2.3 g/cm 3 a mean diameter of 2.4 m, and a starting O/M ratio of 2.11. The powder was depleted UO 2 with
45 an enrichment of les s than 0.71%. The raw powder was stored at room temperature and pressure for period of time of over one year after being obtained from AREVA. The SiC whiskers used in these experiments was obtained from Advanced Composite Materials, Inc. The whiskers had an average length of 11 m, an average diameter of 0.65 m, an aspect ratio of 15:1, and were reported to be greater than 98% pure. The whiskers were found to have trace amounts of Carbon (0.1 wt %) and SiO 2 (0.43 wt %). The diamond particles used were pur chased from Advanced Abrasives Corporation. The particles were single crystals with a reported average diameter of 3 m and a standard deviation of 0.475 m. The constituent powders were mixed through the use of a vibrational ball mill acting as a mixer. Powder batches totaling 20 grams were produced by loading raw UO 2 powder and the requisite additive powder into a zirconia vial; a mixing aid, Decafluoropentane (DECA) was also added to enhance facilitate mixing. Deca is a stable organic compound that do es not react with UO 2 SiC (w) or diamond. Additionally, Deca evaporates quickly at room temperature. Powders were mixed to attain a 10 % by volume fraction of the additive. The SiC (w) /UO 2 mixture contained 19.37 grams of UO 2 and .69 grams of SiC (w) Th e diamond/UO 2 mixture contained 19.31 grams of UO 2 and .63 grams of diamond particles. The zirconia vial was then sealed via a rubber O ring and a generous amount of electrical tape. The SiC (w ) and UO 2 were mixed in the mill for 60 minutes. Three steel ba lls were added to the UO 2 /SiC (w) mix to break up SiC (w) agglomerates The diamond and UO 2 powders were mixed for 60 minutes. Steel balls were not used to mix the diamond because the hard diamond particles led to iron contamination. After mixing, the mixe d powders were removed and
46 placed in paper cups and dried in a dry keeper from 24 hours. Twenty four hours was enough time for the Deca to evaporate away. The dried powders were loaded, by hand, into the graphite tooling. To prevent a reaction with the di e wall, a thin graphite foil was placed between the powder and the die wall. This foil acted both as a barrier and as a lubricant. Additionally, the graphite foil facilitates the flow of current from the punches to the die. The punch tips were sprayed with a thin layer of graphite spray, to prevent the powder from reacting with the punches. The mass of powder was calculated by assuming the desired pellet volume and the desired 96.5 % theoretical density would be attained. The mass of a diamond/UO 2 powder bo dy was 5.3 grams and the mass of the SiC/UO 2 powder body was 5.25 grams. Ten pellets of both types were sintered according to the sintering conditions recommended by previous researchers. The sintering parameters used for both SiC/UO 2 and diamond/UO 2 comp osites were a peak temperature of 1400 degrees centigrade held for five minutes. The pressure at the hold for diamond/UO 2 composites was 40 MPa, while the SiC/UO 2 composites used 60 MPa. The difference was mainly due to a reluctance to increasing the tempe rature of the UO2/ SiC (w) composites out of concern that the punches would react with the powder. The extra pressure should lead to a small increase in attained density. The heating rate was held at 100 degrees centigrade per minute during heating and coo ling. After the sintering cycle was completed, the die compact was allowed to cool naturally for one hour. Then the die was removed and taken to the laboratory for pellet extraction.
47 Pellets were extracted through the use of a carver press. After removal from the die, pellets required light polishing to remove the graphite foil attached to the outer diameter of the pellets. After the foil was removed, pellets were cleaned with alcohol and stored. At a later time, pellets were weighed and measured to determ ine the actual diameter, length and density. The pellet end was visually inspected to determine if the dish and chamfer were present. Pellet 1 from both the diamond and SiC pellet lots were further inspected via the laser profilometer to determine if the d ish was formed as designed. 5.3 Results and Discussion The diamond pellet lot is shown in Figure 5 1. The pellet measurements are described in Table 5 1. The average density of 95.2% TD was below the 96.5% goal. These pellets had a larger standard deviat ion, than previous pure UO2 pellets, but it is within the requirements. Three pellets were below the 94.5% minimum. These pellets were longer than the specified goal. This implies these pellets did not shrink as desired. It is not clear why this occurred f or these three pellets. Pellet number five was egregiously long and of poor density. It was 10% longer than desired and only 91.5% theoretical density. This pellet may simply be an outlier, perhaps due to human error. Pellet 3 was very short (8.8 mm). The pellet was dense (95%), so the pellet mass was too small. This is either due to human error when measuring out the powder or human error in loading the powder causing a significant portion of the powder to be lost. These pellets did not have as clean an O D as UO 2 pellets. This result is primarily due to the use of graphite foil on the exterior. Light polishing was required to remove the graphite foil. On pure UO 2 pellets, graphite could be removed via sandblasting, but with
48 the composite pellets sandblasti ng was insufficient. Additionally, there was some chipping on the pellet edge. This is from the chamfering. The chipping is most likely caused by a size mismatch between the designed chamfer and the actual powder dimensions after pressing. The foil recoil s a bit during pressing leaving the powder to expand further than the designed OD. This is a very small effect but enough to lead to some chipping ( Figure 5 1 ) With the pure UO 2 the rigid die wall does not allow for the powder OD to expand. The pellets s howed a visible dish and chamfer. A visual inspection indicated graphite from the punch adhered to the dish. Most of this graphite could be removed via sandblasting, but not entirely. Certainly these pellets required post processing after removal, which is not ideal. On the diamond pellets there was slight chipping. The largest chip, located on a diamond composite pellet, is shown below in Figure 5 4. The chipping may be due to tooling size mismatch or a localized flaw in microstructure. A mismatch in the punch tip creating the chamfer could cause the flaw shown. Figure 5 5 shows a micrograph of the fracture surface shown in Figure 5 4. The image shows a large amount of diamond particles on the fracture surface. The localization of diamond particles could h ave led to the chipping. This implies the mixing of diamond particles was not adequate. The SiC (w) pellet batch appears respectable ( Figure 5 2 ) The physical characteristics are described in Table 5 2. The pellet batch had an average density of 95.3 % T D. These pellets had a tighter standard deviation than the diamond pellets. Two pellets had densities under 94.5%, meaning 80% of the pellets met the required density. The pellets were shorter than desired. This mismatch is most likely due to powder lost during loading. A small amount of powder is lost in the loading procedure
49 since it is done by hand. The SiC (w) pellets did attain the desired diameter, this is of course constrained by the die inner diameter. The SiC (w) shing to remove the graphite foil from their exterior. The polishing required was significantly more arduous than for the diamond pellets. This is most likely due to the increased pressure applied during sintering. The larger pressure leads to a larger pre ssure on the pellet exterior exerted by the die wall. The foil is believed to include deeper into the pellet exterior which requires polishing more material to achieve a smooth surface. Most pellets had very clean surfaces. Some pellets had graphite foil adhesions as shown in Figure 5 3. This is due to graphite foil crunching up upon powder loading. This problem is exacerbated when decreasing the die ID. This produces a non uniform pellet OD filled with residual graphite. There was some light chipping and significant chatter on the pellet ends. The chipping present was less severe than on the diamond pellets. There was a dish present as indicated by the visual inspection. This dish was present, and was comparable to the dish in the diamond pellets. Fabrica ting both the diamond and the SiC (w) pellets led to punch failure in a single fabrication run. The dish surface on the punches is visibly reduced after a single run. It is unclear if this is due to the increased temperature or the composite material itsel f. Figure 5 6 shows the dish depth of pellet 1 of the SiC (w) and diamond pellet lots as measured by the laser profilometer. A representative dish measurement for a UO2 pellet is also shown on F igure 5 6 for comparison. Table 5 3 compares the maximum dept h achieved for the SiC (w) diamond and UO2 pellet. The composite pellets
50 achieve the desired depth, even attaining a dee per dish than the UO2 pellet. I t should be remembered that the diamond and SiC (w) pellets were post processed; whereas, the UO2 pelle t did not required this post processing. The post processing included a sandblaster removing graphite located in the dish. The effect the post processing has on the dish geometry is unknown. It is obvious that the shape of the dish differed between the thr ee pellets. The SiC dish was the smoothest and most spherical, but the SIC also required the greatest amount of post processing. The diamond pellet had a flatter and more linear dish. This showed the composites did form a dish and at the depth desired, but the behavior is slightly different than for the pure UO2 pellets. Also the punches were essentially single use. No comparison of fabricated multiple composites pellets was performed. More work is required to guarantee composite pellets would pass qualifi cation. 5.4 Conclusions Fabricating 17x17 composite pellets proved to be more difficult than pure UO 2 pellets. It appears the 1400 C sintering temperature is nearing the capacity of the graphite tooling. There is not much leeway when working at thes e temperatures. Even a small increase to 1450 C may cause a reaction between the UO 2 and the graphite tooling. As it stands now, the tooling can only survive a single run before needing replacement. Even at 1400 C and the available applied pressures, the c omposite pellets were of significantly lower quality than the pure UO 2 pellets described previously. Only 70% of the diamond pellets passed the density requirements and even less passed the entire pellet qualification. The SiC pellets had superior density but graphite foil inclusions were seen on all pellets.
51 Currently a significant amount of post processing is required to ensure pellets meet requirements. The graphite foil produces flaws that must be removed in the future. The best case scenario require s polishing off on graphite that bonds with the pellet. In the worst case scenario, the foil bunches up and then includes into the outer surface of the pellets. These inclusions may not be removable while still maintaining the required outer diameter. Diamond pellets showed signs of particle agglomeration, hinting that the mixing was inadequate. These may exacerbate chipping near the pellet edge. Using steel or ceramic ball to break up agglomerates will introduce contaminates and still may not break up the hard diamond agglomerates. A longer mixing time than 60 minutes is suggested. Pellets do approach 96.5 % TD, indicating studies could move forward fabricating multiple composite pellets in a single run. Pellets acquired a dish as deep as required but the shape of the dish was not as good as for pure UO2 pellets. More precise measurements of the pellet end geometry of composite pellets, including the effect of post processing, should be performed. Using the information learned, optimized pellets were fabricated for insertion into the Advanced Test Reactor. Figure 5 1. Diamond c omposite p ellet l ot Photo courtesy of the author.
52 T able 5 1. Diamond physical dimensions Pellet Density Length (mm) Diameter (mm) Goal 96.5 2.0 10.16 1.27 8.1915 0.0254 1 10.08 8.1940 0.0051 2 10.9 8.1915 0.0127 3 8.81 8.1839 0.0203 4 10.92 8.2093 0.0102 5 11.13 8.1813 0.0076 6 10.52 8.2067 0.0127 7 10.77 8.2093 0.0076 8 10.49 8.2093 0.0102 9 10.77 8.2144 0.0254 10 10.03 8.1712 0.0229 Avg 10.44 0.69 8.1966 0.0203 Table 5 2. SiC Physical Dimensions Pellet Density % TD Length (mm) Diameter (mm) Goal 96.5 2.0 10.16 1.2 7 8.1915 0.0254 1 10.24 8.1737 0.0127 2 9.53 8.1483 0.0203 3 9.63 8.1788 0.0203 4 9.96 8.1610 0.0076 5 9.73 8.1636 0.0025 6 10.11 8.1661 0.0254 7 9.75 8.1594 0.0102 8 9.93 8.1407 0.0152 9 9.68 8.1788 0. 0203 10 10.03 8.2931 0.0051 Avg 9.86 0.254 8.1661 0.0508 Figure 5 2. SiC Composite p ellet l ot Photo courtesy of the author.
53 Figure 5 3. Graphite on surface of SiC pellet Photo courtesy of the author. Figure 5 4. Diamond p ellet s howing s urface c hip Photo courtesy of the author.
54 Figure 5 5. Fracture s urface of d iamond/UO2 Pellet Photo courtesy of the author. Figure 5 6. Dish depth for SiC, diamond and UO2 pellet Table 5 3. Comparison of dish depth in composite a nd pure UO2 pellets Parameter Goal Depth (mm) UO2 (mm) Diamond (mm) SiC (mm) Dish Depth 0.254 0.237 0.255 0.263
55 CHAPTER 6 SPS TOOLING 6.1 Introduction The SPS process is firmly tied to the tooling used in the process. The tooling considerations o f SPS include the geometry of the tooling apparatus, the barrier between the powder and the die wall, and the actual tooling material. The geometry of the tooling setup has large effects on the SPS process. This is in large part because the tooling act ually creates the heat that is used to sinter the powder. The outer diameter of the die determines the diameter of the resultant pellets. The tips of the punches mirror the surface profile of the pellets. The size and shape of the punches and dies have a l arge effect on the power consumption of the process and on the temperature gradients within the tooling. During the SPS process, there is a small gap between the punches and the die. In the majority of SPS experimental work, a thin graphite foil fills this gap and prevents the powder body from coming in contact with the die wall. Furthermore, a thin graphite spray prevents the powder body from coming in direct contact with the punch tip. The graphite in the tooling reacts with oxygen freed from the powder f orming carbon dioxide. The punch is also applying the pressure on the powder body. So the tooling provides the heat, the pressure and contributes to the gaseous environment the powder experiences. The tooling material must have a high electrical and therm al conductivity so the current can pass through the tooling compact and thermal gradients are minimized. The primary method of heating comes from the resistive heating of the tooling material. The typical tooling material used is graphite. Graphite is unm atched in electrical
56 main drawback is that it is soft and has limited strength and poor wear resistance. Additionally, UO 2 does react with both graphite and Carbon M onoxide at temperatures over 1000 C, which is experienced during SPS. 6.2 Tooling Considerations Typically there is a graphite foil acting as a barrier between the powder and the die wall. This barrier prevents the powder from reacting with the die wa ll. It also prevent the powder from eroding the die wall. The barrier also directs current from the punch around the powder and into the rest of the tooling. Most S PS studies use a graphite foil as the barrier. For the contact point where the punch comes i n contact with the powder, this is impossible. The punch end is a precisely machined surface, designed to imprint a shape onto the powder body. A rigid foil would prevent this. For the punch ends, a graphite spray has been u s ed. The graphite spray was atta ined from ZYP coatings. A thin layer is sprayed onto the punch ends. This seems to do the trick; however, at high temperatures, the punch ends still react with the UO2 powder. To determine i f the graphite foil and graphite spray could be removed, prelimin ary studies were conducted with alternative coatings. A boron nitride coating, an alumina based coating and a Yttria based coating were explored. All of these were acquired from ZYP coatings. The Alumina based coating consistently produced pellet failure The pellets would continuously fracture immediately upon removal. After SEM examination, Figure 6 1, it was concluded that Al 2 O 3 was coating the fracture surface. It is believed that the Al 2 O 3 being an insulator, created a large temperature spike at the punch/powder
57 interface. This melted the Al 2 O 3 and allowed it to find its way into the powder body interior leading to failure. The Al 2 O 3 spray was scraped. The Yttria based spray proved to be nearly indistinguishable from the graphite spray. Sometimes the Y 2 O 3 spray outperformed the graphite spray but no conclusively. Additionally, the Y 2 O 3 spray was significantly more difficult to remove form pellets during post processing. The Boron Nitride spray prove to be unsafe to use. Other studies have used BN spr ay but on the single attempt made during this study, the die blew apart during sintering. For safety reasons, no more study were performed. There was no barrier found to be adequate. For most studies, the powder was allowed to interact with the die direct ly. This had negative consequences described below. There are six major factors to consider in the SPS tooling geometry: The outer diameter of the die, the inner diameter of the die, the length of the die, the diameter of the punches, the length of the p unches, and shape of the punch tip. All of these can play a large difference in the results of pellet fabrication. Additionally, the gap between the punch and the die wall can actually prevent the process from even starting. The most obvious factor to co nsider is die inner diameter. One of the key benefits of SPS is the ability to fabricate cylindrical pellets at a specific diameter; thus, removing the need for centerless grinding or other post processing. Is determined primarily by the inner diameter of the die. This surface acts as a physical barrier to the powder as it is being compressed upon startup. The ID continues to restrain any radial expansion of the pellet as pressure is applied during sintering. When fabricating pellets with a
58 diameter of 8.19 mm, the die ID was set at 8.20 mm as long as there was no graphite felt being used as a barrier. The graphite felt typically used in these experiments was .025 mm thick. So, an additional 0.05 mm would be required in the die ID making it 8.25 mm. In addit ion to acting as a physical barrier for the powder expansion, the die ID acts as a conduit, transferring current from the punch into the rest of the die. For this reason, if the die ID is too big or if there are inclusions on the die ID, then there will no t be uniform current flow. Non uniform current flow leads to temperature gradients and non ideal sintering. For this reason, the die ID must be cleaned routinely with alcohol and/or a brush. The die outer diameter plays more subtle roles in the SPS proces s. On first with the punch, and there should not be any wear on that surface. All the die OD does is provide a spot for the pyrometer to read the die temperature. On fur ther inspection, the die OD has large effects on the process. The die OD determines the distance from the pyrometer measuring spot to the sample, it determines the mass of graphite that needs to be heated, it determines the resistance of the apparatus and thus the current required, and it is a contributing factor in the maximum force that can be applied to the pellet by the rams. This is due to the hooped stress placed on the die via the powder pressing into the die ID. Recall that all temperature measure ments are conducted via the optical pyrometer focused on the die OD. The larger the OD, the farther from the sample the pyrometer is reading. This also allows for the possibility of increased uncertainty in the measurement. Table 6 1 shows a simple study i n which four dies of OD 2 in, 1.5 in, 1 in,
59 and 0.5 in were used to fabricate depleted UO 2 samples at a sintering temperature of 1100 C for 5 minutes, with a pressure of 40 MPa. The ID was kept constant. Three samples were fabricated in each die. As shown in Table 6 1, as the die OD decreased so did the average density of the pellets. It is notable that density decreased almost a full 2% purely by altering the die OD. Graphite has a high thermal conductivity, so it was surprising that the effect was so la rge. It is believed that the electrical consequences are at least as important as the thermal consequences of altering the die size. The die OD also determines the total volume of the graphite and thus the total mass of the graphite tooling. The larger t he die OD, the more energy is required to heat the whole apparatus. This is simply a matter of efficiency but, the total volume is highly dependent of the OD. The ID is set so as the OD begins to grow, the volume of the tooling grows quickly. The surface a rea to volume ratio increases with a decreased die OD, which would lead to higher energy loss due to radiative heat loss. This is neglected because the use of the graphite felt should minimize radiative heat loss. Also the larger the die, the more current is used to heat the die. This increases the current density in the punches and leads to localized overheating. For these reasons the die OD should be minimized. This may not be true if new materials were developed for the punch or dies tooling. While mini mizing the die OD is a good idea, there is a limiting factor in the strength of the die. While the powder compresses, the powder exerts a hoop stress on the die wall. Additionally, graphite is a brittle material. A larger die OD allows for a larger pressu re to be applied to the sample while not leading to die fracture. For safety reasons, it was determined that the ID to wall thickness ratio should not drop below 2.0.
60 So the thickness of the die walls was set to be the same thickness of the sample and the OD of the die is 3 times the diameter of the sample to ensure the dies maintained structure integrity By setting the die dimensions, the density of the pellets could be controlled by changing sintering parameters. The final consideration for the die OD is the effect on the current and the curr ent path. The resistance of a ho llow cylinder to current flowing along its axis is proportional to the reciprocal of the difference of the squares of the radii. So decreasing the die OD, increases the resistance of the die. This requires a larger voltage to push the current through the die. The power of joule heating is proportional to resistance. Also, the current path is related to the relative resistance of the sample and die. This means the die OD may affect the amount of current that flows through the sample itself. In an insulator, this effect would be negligible. In a conductor, this effect could be large. Since, UO 2 is a semi conductor, it is unclear the effect die OD had on current flow through the sample. The specifics of the thermo electrical effects of die size are beyond the scope of this study, but it is obvious that the die OD affects the thermo electrical environment of the tooling and most likely the sample as well. The final dimension of consequence on the die is its length. The die length has similar efficiency consequences as the die thickness but without many advantages of a long die. A longer die requires more energy to heat, may have larger temperature gradients, and is costlier to make. There a re two factors creating a minimum die length. The limiting factor was found to be ability of this researcher loading powder into the die. The funnel density of the powder is about 10% TD and even with some pressure
61 applied by hand, the powder is about 20% TD before being placed in the SPS machine. With a powder mass of 5.5 g, the powder body is about 50 mm in length before consolidated in the SPS machine. Any die smaller than 65 mm proved to be either difficult or impossible to load. The other consideration is, the shorter the die, the smaller the interface between the punches at the die wall are. A larger die/punch interface optimizes heat and current transfer. The dimensions of the punch affect the SPS process. The punch diameter is simply as large as pos sible while still freely moving in the die. For most processes, if the die ID was 8.20 mm, the punch diameter was set at 8.15 mm. A larger punch diameter was desired but it was found than a larger punch diameter prevented the punch from moving freely withi n the die. This was usually due to either powder being stuck between the punch and the die or due to flaws in machining either the punch or die. The punch length is a major contributor to the condition of the final pellets, specifically the pellet end g eometry. All of the current travels through the punches; this means the current density of the punches is much higher than the current density in the die. This leads to higher temperatures in the punches. The higher temperature in the punches may be mitiga ted by heat transfer to the die. Heat transfer to the die is a function of the contact area between the punches and die. Joule heating is also proportional to the length of the resistor. A longer punch attains a higher temperature during the sintering pro cess. A longer punch is also more susceptible to buckling during the process. SPS studies at UF started with 40 mm long punches. When the first studies shifted to 17x17 pellets, the pellet ends were constantly being fabricated with poor quality. Reducing the punches to 30 mm prevented most of those problems. Any
62 sample. During the course of this project, four materials have been used as the material for the die. Overwhel mingly, graphite has been the material of choice from the die. This is the industry standard for SPS studies. Tungsten Carbide, Molybdenum, and Silicon Carbide were also used to machine typical single pellet dies. These three materials were chosen in attem pt to prevent negative consequences of graphite tooling. In all materials, the punches were made from the same material as the die. Graphite has great electrical properties, adequate thermal properties and high strength at elevated temperatures. Graphi te (Hardness 15 MPa)) is much softer than UO 2 (Hardness 8.5 GPa) and hence it erodes easily. While not having a barrier between the powder and the die led to excellent surface finish on the pellets, it also led to surface erosion on the die interior wall. T he UO 2 (white) wears away the graphite (grey) and then adheres to the die wall ( Fig ure 6 2) This reduces the dimensional reliability of the pellets. As the die wall is progressively worn away, the pellets get slightly wider at these locations. This proce ss may lead to pellet chipping, surface deformities or even fractures upon removal. As the UO 2 powder sticks to the graphite die, the dimensions of the pellet also get disturbed. The process is less severe compared to punch tip erosion discussed earlier, but still undesirable. The stuck UO 2 on the die surface may also cause scratches on the pellet while it is being removed from the die and contribute to poor surface quality of the pellet. Of the 3 additional materials investigated, none made it through s intering a single UO 2 pellet. The SiC die fracture upon initiating startup. This die fractured radially. The
63 Molybdenum die failed to complete a circuit. The Tungsten Carbide die worked as planned but at a temperature around 900 C, the punches buckled and failed. These studies made it obvious that at this time, graphite would still be required. Other materials may be possible, but they must be capable of reaching very high temperatures while maintaining strength. To test the repeatability of and wear pr operties of various punch materials, punch tips were machined out of three different materials, graphite, Yttria Stabilized Zirconia, and Boron Carbide. These punch tips were then used to consecutively fabricate five UO2 pellets each. After pellets runs began, this number was altered to 5 for graphite, 4 for YSZ and 3 for B 4 C. The dimensions of the fabricated pellets were then measured and the profile of the pellet ends were measured via the profilometer. These results were compared to better understand the effects of using the various materials. The graphite punches were machined with the same vendor used for the rest of the study. These are the standards used in every SPS study conducted at UF. The YSZ was pure tetragonal phase, fine grain material, ma chined into the desired punch tip shape. The Boron Carbide punch tips were fabricated here at UF via SPS. Punch tips with the mirror image of the desired profile were used to fabricate B4C parts with the desired end profile. The dies used in all pellets we re new graphite dies with no barrier between the die wall and the powder. The initial design was for all pellets to be sintered at a peak SPS temperature of 1050 C for 5minutes with a pressure of 40 MPa applied. This was altered for the YSZ pellets. These alterations are discussed below.
64 Many graphite pellets were fabricated during this project, but five were fabricated specifically for the purpose of evaluating tooling wear. After each pellet, the physical changes in the die and punches were observed. Fi g ure 6 3 shows the erosion of the dish forming part of the punch. The punch to the far left was used 5 times. The one to the far right is a virgin punch, fresh from being machined. The dish forming region of the punch erodes as it is used multiple times. T his is due to graphite being soft while UO 2 is very hard. The resulting erosion of the dish is shown in the profilometer measureme nt of the pellets, shown in Figure 6 4 and described in Table 6 2. The virgin punch created a dish 0.247 mm deep, which met d esign specifications. As the punch was reused, the dish became gradually shallower until the fifth pellet in the run, which at 0.097 mm deep was only forty percent of the original dish depth. In an attempt to overcome this occurrence, an intermediate pun ch tip has been introduced between the punch and powder. The punch is then flat on both sides, with the punch tip being flat on one end and machined on the other. The material for this was chosen to be harder than graphite. Two choices were made; one with a higher electrical resistance than graphite and one with a low electrical resistance. A high resistance leads to the current diverging at the interface with the punch tip. This situation would effectively put less stress on the interface of the dish of t he pellet. This material was the YSZ. For this process to be a beneficial addition to our sintering method the punch tips have to have a reusability of more than 6 while still producing satisfactory dishes.
65 Using YSZ punch tips, the first pellet was sinte red at 1050C for 5 mins. This pellet came out so good that the temperature was decreased to 1000C for the 2nd test. A similar result was observed with 3rd pellet. The 4th pellet was sintered at 900C for 5mins. None of these sintering processes used added p ressure (usually 40MPa). Pellets 1 and 4 are shown in Figure 6 5. High density was not the original intent of this experiment but this sintering result has offered a means of achieving high theoretical density at a lower sintering temperatures and with les s time needed for processing. It is believed that even higher density can be achieved at lower sintering temperatures with the application of pressure (40MPa) at the hold temperature. This result does not imply the actual temperature inside the die is in f act 100 degrees lower, but the temperature distribution changed quite significantly with the addition of the ceramic punch tips, allowing for a lower tooling temperature. It was noticed that the even after cleaning the punch tips they still keep residual graphite spray, the amount increases with each successive test. After the 3rd pellet a graphite foil inclusion was noticed on both the punch tip and the pellet itself. The dish surface appears to be just as defined after the 4th test as it was after the 1s t. The punch tips are shown in Figure 6 6. Profilometer measurements confirm that observation and are shown in Figure 6 7. The dish remains close to the ideal depth throughout the four pellet run. These measurements are described in Table 6 3. It is clear that the YZS punches did not wear like the graphite punches. The pellets are not quite as cosmetic as some of the pellet made with graphite punches but they are sufficient. The central
66 ring, present to previous pellet, is not only present but may be more prominent with the YSZ punch tips. The B 4 C punch tips could not be purchased externally. These were fabricated in house at UF. To fabricate the B4C punch tips, graphite punches with a dished end were fabricated. Then B4C powder was obtained and sinte red with those graphite punches. This produced the goal punch tip shape in the B4C; however, there was not any quality control of this process so the actual dimension of the B4C punch tip are not known. The pellets made with the B4C punch tip were all hig h density (above 95% TD). The profilometer measurements for those pellets is shown in Table 6 4. The Profilom eter measurements are shown in F igure 6 8. It is obvious the dish is a different shape than when produced with the graphite or YSZ punch tips, but the shape remains stable over comes to reliability, is its softness and tendency to wear away. To further understand the effect the punch tips had on the SPS process, th e temperature, voltage, displacement and power profiles of the YZS punch tips used to fabricate a pellet at 1050 C were compared with a typical graphite punch used for the same purpose. These profiles are shown in Figure 6 9. T he temperature profiles mimi c each other exactly. The Z axis displacement and power profiles are also very similar and nearly identical. The only difference is the voltage profile. The ceramic YSZ requires more voltage to drive the current through the tooling. This is because the cer amic has a higher resistance. This difference is obvious when watching the process as the punches get very hot with the ceramic punch tip. This is because the higher contact resistance between the graphite punch and the ceramic punch tip as opposed to
67 just having a pure graphite punch and tip. The interesting bit is that the power output was still the same, indicating the current was lower for the ceramic punch tips. The same product was produced, but the environment was slightly different. As stated ab ove, the ceramic punch tips enabled full density pellets to be formed with a die temperature of only 900 C. This was interesting because with graphite punch tips, full density required 1050 C. The temperature, voltage, displacement and power profiles of t he YZS punch tips during the 900 C run are compare to the 1050 C run in Figure 6 10. As shown, the temperature profiles follow each other, with the exception of the peak temperature of course. The voltage profiles follow similarly. With the lower temperatu re, the power profile shows the 900 C run required half as much power as the 1050 C run. The displacement profiles followed each other and the densification plateau occurs between 850 and 900 C. In the graphite run, the plateau occurs at 1050 C. The cause of this phenomena is unknown. It would seem either the temperature distribution in the tooling is different, or the current path is different. Whatever, the cause of this phenomenon, the pellet achieved full density at half the power requirement and withou t any tooling wear. A similar study with the B4C punch tips was not performed. B4C is of interest because it has a much lower resistivity than YSZ. 6.3 Conclusions This study shows the large impact the tooling has. Every aspect of the tooling has a sig nificant effect on the process and must be designed. The die dimensions allow for modifications if the rest of the tooling must meet certain requirements. The most important aspect of the tooling is the tooling material itself. Even the small punch tips, while achieving their initial goal of forming the pellet end, radically changed the current
68 and temperature gradients in the compact. These punch tips were far superior to graphite punch tips, at least in regards to wear resistance and reactivity with the UO 2 This complicates the process but allows for vast possibilities previous unexplored. Advanced tooling may considerably improve the process, even to the point of being more efficient than conventional sintering. The most interesting change with the cer amic punch tips was the effect on the required sintering temperature. The YSZ punch tips led to a reduction in the required sintering temperature from 1050 C to 900 C. It was implied that this may even be as low as 800 C. The sintering temperature is of co urse the temperature at the die exterior. The power requirements were not significantly different for YSZ as compared to graphite, when the same temperature was achieved. This indicated the 900 C YSZ run required half the energy to get the same product. Further investigation is require to determine the nature of this result. The most obvious requirement for commercialization of SPS is the removal of the graphite foil. A high conductivity barrier seems to be required to get a high density product; however this is both clumsy and negatively impacts the pellet exterior. The work with other materials as a die material proved fruitless. The studies with the YSZ and B4C punch tips show other material can be used to interact with UO2 directly. Additional work is needed to determine a barrier between the powder and the die wall. Graphite tooling may still be required in some capacity. Every die fabricated with a non graphite material failed immediately. The graphite seems to be required for current flow and hea t generation. It is suggested that the outermost portions of the die and punch remain graphite, while the inner portion, in contact with the powder, be
69 modified to a different material. This also makes sense economically, since graphite is cheap and easy to machine into the required shapes. Table 6 1 Pellet density with varying die outer diameter Die OD Average Density (%TD) 98.1% 97.7% 96.6% 95.3% Table 6 2 Comparison of dish depth in a five pellet run with graphite punc hes Parameter Goal Depth (mm) 1st Pellet (mm) 2ndPellet (mm) 3rd pellet (mm) 4th Pellet (mm) 5th Pellet (mm) Dish Depth 0.254 0.247 0.221 0.198 0.172 0.097 Table 6 3. Comparison of dish depth in a four pellet run with YZX punches Parameter Goal Depth (m m) 1st Pellet (mm) 2ndPellet (mm) 3rd pellet (mm) 4th Pellet (mm) 5th Pellet (mm) Dish Depth 0.254 0.245 0.244 0.246 0.244 N/A Table 6 4. Comparison of dish depth in a three pellet run with B4C punches Parameter Goal Depth (mm) 1st Pellet (mm) 2ndPelle t (mm) 3rd pellet (mm) 4th Pellet (mm) 5th Pellet (mm) Dish Depth 0.254 0.262 0.266 0.257 N/A N/A
70 Figure 6 1 Fracture surface of pellet fabricated with Al 2 O 3 spray showing Al 2 O 3 c oa ting fracture surface Photo courtesy of the author. Fig ure 6 2 SEM image showing UO 2 powder inclusions on die interior surface. Photo courtesy of the author.
71 Fig ure 6 3 Images of the punch wear with usage. The number of times each punch was used is indicated on the top. Photo courtesy of the author. Figure 6 4 Comparison of dish depth from 1st pellet produced to 5th pellet showing tooling wear
72 Figure 6 5. Pellets p roduced with YZX punch tip A ) First pellet produced. B) Fourth pellet produced. Photo courtesy of the author.
73 Figure 6 6 Punch t ips a fter use A) Pellet after f irst r un. B) Pellet after s econd r un C) Pellet after t hir d r un D) Pellet after f ourth r un Photo courtesy of the author.
74 Figure 6 7. Comparison of dish depth from 1st pellet produced to 5th pellet showing tooling wear with YSZ punches Figure 6 8. Comparison of dish depth from 1st pellet produced to 3rd pellet sh owing tooling wear with B4C punches
75 Figure 6 9. Profiles of SPS of the YSZ punch tips vs the graphite punch tips A) P ower profile B) T emperature profile C) D isplacement profile D) V oltage profile
76 Figure 6 10. Profiles of SPS run of YSZ punch tips at 1050 C vs 900 C A) P owe r profile B) T emperature profile C) D isplacement profile
77 CHAPTER 7 SCALING UP PRODUCTION 7.1 Introduction The commercial production of nuclear fuel is a large scale, twenty four hour a day, mature process. A single fabrication facility produces 400 million pe llets per year. While the results discussed above are encouraging, producing pellets one at a time, once per hour, would require over 45,000 SPS machines working 24 hours a day. Obviously, multiple pellets must be produced in a single production run. In th e conventional process, pellets are pressed in batches of about 60 then loaded onto large containers and sent off to the furnaces. While the process to sinter any individual pellet is long (up to 17 hours), hundreds or thousands of pellets are being fabr icated at any given time. Conversely, the SPS process sinters a single batch very fast (as little as five minutes), but the batch sizes have yet to be determined and will undoubtedly be smaller than the batches in the conventional process. To this resear che s knowledge, there are no published studies on fabricating multiple samples via SPS. The most straightforward and easiest batch SPS method to implement is to continue to use a single die, but increase the number of pellets that can be loaded and sinte red in that die. Alternative methods would instead use multiple dies within a single SPS machine. The use of multiple dies would make the application of pressure to the powder bodies much more complicated. This study focusses on increasing the number of pe llets in single die and determining how many pellets can be produced in a single run of the Dr. Sinter SPS 1030. There are two obvious ways of increasing the number of pellets in a die: stack powder bodies in a single cavity (series production) and machine more cavities in the die (parallel production).
78 In this study these two methods were explored and then paired to determine if multiple commercial grade pellets could be successfully fabricated in a single SPS run. The pellets underwent the same qualific ation procedures used on the single UO 2 samples. The methods were compared and discuss. Then, a POC experiment was performed to test the maximum number of pellets the Dr. Sinter SPS 1030 could handle. 7.2 Experimental Procedures All attempts at multiple pellet fabrication were performed using calcined depleted UO 2 powder with a nominal enrichment of 0.50 %, as supplied by Areva. The powder had a reported bulked density of 2.5 g/cm 3 a BET surface area of 3.11 m 2 /g, and an average particle size of 2.4 micr ons. The grain size of the UO 2 was 100 400 nm. The O/M ratio of this powder was 2.11. This powder comes directly off the AREVA fuel line after U 235 was removed during the enrichment process. The powder contained no additional additives. The process to fa bricate two pellets in series was similar to fabricating a single pellet. The die length, punch length, and the powder loading procedure in the die were different. The die was made longer (105 mm) to accommodate two pellets and the punch lengths were exten ded to 40 mm, as shown schematically in Figure 7 1. A punch was first inserted on one end of the die and then the powder for one pellet was poured. A graphite spacer punch with mirrored pellet end profiles was then introduced. All surfaces of the graphite spacer are covered with graphite spray. This is done to prevent reaction between the powder and the spacer, and to decrease friction between the spacer and the die wall. Then the powder for the second pellet was poured and the upper punch was inserted. Th e die was then placed in the SPS machine.
79 The temperature profile used included a seven minute initial dwell time at 600 C to allow the pyrometer to calibrate. The die was then heating 100 C/min for 4.5 minutes to achieve a peak temperature of 1050 C. Thi s temperature was held for 5 minutes while applying 40 MPa of pressure. The die was then cooled at 100 C/min for 4.5 minutes, followed by natural cooling for one hour. This process is the same as a single pellet except for the longer initial dwell time. T o achieve this temperature profile, a larger current (1200 A) was needed than for the single pellet production. Both sintered pellets were then removed and characterized. While removing from the die, it was noticed than no additional pressure was required to remove the pellets. They basically just fell right out. In this manner, twelve pellets (6 batches of 2 each) were produced and characterized. Dies were reused for 3 batches each, making two dies required for the whole fabrication. The die design was t hen modified to fabricate four pellets in each run. Four pellets were fabricated in parallel in a single run by using the larger die with four cavities shown in Figure 7 2(b). This die was 65 mm long with an outer diameter of 42 mm. The 6.35 mm from the die exterior and 6.35 mm from the die central axis. 5.50 grams of UO 2 powder were placed in each of the four cavities. Eight punches (four on top and four at the bottom) were then used to fill the cavities and apply axial pressure to the powder. Graphite spray was applied to the tips of these punches. The typical graphite felt was placed around the die exterior. The die was then placed in the SPS machine. The sintering parameters for this large die were slightly different than for the sma ller die. First the dwell time, the hold at 600C to allow the pyrometer to calibrate, was 10 minutes. This is double the dwell time
80 used for a single die. The heating rate was kept at a constant 100 C/min. The die was heated for 4 .5 minutes to 1050 C and h eld for 5 minutes with 40 MPa of applied pressure. To attain this pressure, an effective diameter and thus effective area of the punches had to be programmed into the SPS controller. This simply required telling the SPS controller the diameter of pressure application was twice as large as for a single cavity. The temperature was measured, as before, on the die exterior, not in the powder body. Twelve pellets in total were produced with the die shown in Figure 7 2(a). To fabricate eight pellets in a single run, the series and parallel methods discussed above were combined as shown in Figure 7 2(b). This die has the same dimensions as the four pellet die except it is 110 mm long as opposed to the 65 mm length of the four pellet die. This die uses eight machi ned graphite punches, four graphite inserts, and a large thick graphite felt, which insulates the die. The standard pellet mass (5.5 g) was used. The loading process was to place a single punch in a single cavity, then pour 5.5 g of UO 2 powder into that ca vity. This powder was compressed by hand via a metal punch to make room for the second powder body. After compressing the first powder body, the insert was placed into the die cavity. The second powder body followed the insert and a second punch was placed on top of the second powder body. This whole system was then compressed by hand and fastened with electrical tape. Then the other three cavities were filled with the same methods. The process of loading this die took around 30 minutes as opposed to the on e minute it takes to fill a die for a single pellet. This was due to the awkward handling of the large die and the need to constantly secure the die before moving. An automated system would greatly reduce the loading time. After the four cavities were fill ed, a thick graphite
81 felt was wrapped around the die with a 1cm by 1cm hole in the felt. The hole is so the pyrometer can measure the die temperature. The sintering parameters were identical to the 4 pellet die as described above with the one exception tha t the peak temperature was 1150 C instead of the 1050 C. The higher temperature (1150 vs 1050) was chosen because preliminary work [ 43 ] suggested that this temperature provides the desired density in each pellet. Finally, a large proof of concept batch w as fabricated. The POC used the largest die that could fit in the Dr. Sinter SPS 1030 and 19 cavities were drill through the die. These cavities were filled with 5.5 g of UO 2 powder for a parallel production run of 19 pellets. The spacing between cavities was kept constant and identical to the four pellet die described above. A complete schematic is shown in Figure 7 3(b). This die (Figure 7 3a) required 38 machined graphite punches. The loading time for this die was over one hour and took two researchers. The di e was placed in the SPS furnace and the temperature was brought up to 600 C. The dwell time at 600 C was 10 minutes to ensure the temperature was measured correctly. The temperature was then increased to 1150 C in 5.5 minutes at 100 C/min. The die was then held at 1150 C for 5 minutes then allowed to cool naturally for 1 hour. No pressure was applied during the hold leaving the pressure on the pellets at around 1 MPa. This was done for safety reasons. The large die would require a much high er force to produce to 40 MPa typical of sintering UO 2 If there was some small contact deviation so that a single punch felt this whole force, then the entire die could fail. This was just a first step to prove so many pellets would be sintered. The density was ex pected to be less than previous pellets made. After sintering, the 19 pellets were
82 removed, clean ed and characterized. The die during sintering is shown in Figure 7 4. As shown, there is plenty of energy being lost due to radiation. 7.3 Results and Discuss ion The twelve pellets produced as stacked doubles are described in Table 7 1. Consecutive pellets were made in a die with the odd numbered pellet on top in the die and the even number below it in the die. Three pellets were at a lower density than the specifications, with two pellets under 92% TD. The reason for this results may be an increased friction with the die wall. There is twice as much powder in contact with the die wall in this fabrication scheme than in a single pellet fabrication, thus twice as much friction. Additionally, the graphite insert between the two powder compacts also appeared not to slide smoothly during powder packing due to stray UO 2 powder being lodged in the small space between the insert and the die wall. However, nine of th e twelve pellets have met the specification. These twelve pellets all had the grey ring at their center. There was a visible difference in the quality of the dish on two surfaces of a pellet made in this stacked configuration. The surfaces of the pellets t hat were in contact with the central insert were superior as compared to the other two surfaces that were in contact with the punches, ( Figure 7 5 ) The surfaces in contact with the punches had small amounts of graphite in the dish. In the SPS process, t he current flows through one punch into the die and down through the other punch. The heat produced is mostly through resistive heating throughout the punches and die. Neither the heating nor the radiative loss are uniform throughout the entire tooling ass embly. These factors, along with the geometry variations along the tooling assembly, lead to temperature gradients in the tooling. These gradients are even more severe in the two pellet stacked
83 arrangement. Our research group is currently conducting a coup led electro thermo mechanical modeling of the SPS tooling process [ 43 ]and our preliminary modeling efforts have shown that the temperature gradient indeed exists across the specimen length and especially in the two pellet configuration. A t emperature grad ient in the tooling is undesirable and can produce deleterious effects on the sintered samples. The results of the four pellets fabricated in parallel in a single die are presented in Table 7 2. Once again, these pellets did not achieve adequate density, but they did display the desired dish and chamfer shape. The low density may be due to two reasons: (i) the maximum current available in the Dr. Sinter1030 machine is 3000A and it was noticed during the fabrication that this value reached its limit and h ence the maximum desired temperature could not be reached. (ii) It is also quite likely that, severe temperature gradients existed among the four pellets, resulting in uneven temperature distributions and possibly lower temperatures in the powder when comp ared to the single pellet production. Finally, the applied pressure on the four pellets was not sufficient to cause full densification. Furthermore, the pressure may not have been equally distributed to compress the four powder compacts equally. This would have led to reduced green pellet density prior to sintering and hence a lower final pellet density. The surfaces of these pellets had no visible defects. The dish and chamfer geometry were visible on all pellets; however, the pellets had lower densities than the desired values. The parallel production leads to reduced density with the available amperage. These four pellets had the grey ring at their center.
84 The details of the eight pellets produced in series and parallel in a single die are described in Table 7 3. All pellets had a smooth surface finish and a clearly visible dish and chamfer. All pellets had the grey ring at their center. The pellets were produced with the desired length and diameter. The average density was low but within the acceptabl e range according to the specifications (Table 7 3). There is a loose correlation between pellet diameter and pellet density. Pellets 1 and 2 are both less dense and smaller than the other 6 pellets. Additionally, pellets 1 and 2 were produced in the same cavity. This cavity had a slightly smaller diameter than the other three cavities, due to a machining error. This led to the smaller diameters. It is unclear if this led to the reduced density in pellets 1 and 2. Other possibilities are temperature gradie nts within the larger die. The temperature distribution is expected to be more uneven than in the smaller dies. If cavity 1 was off center, then it would have a lower temperature than the other three cavities. All pellets have a higher density than the fou r pellets produced in parallel but a lower density than pellets produced only in series. These pellets were sintered with a higher external die temperature than the pellets produced as singles (Table 7 1) or in series (Table 7 2), but still were less dense The 19 pellets fabricated in the 19 pellet die are briefly summarized in Table 7 4. It is clear the pellet are not dense. All pellets average 63.7 % TD. The die has three rings of pellets starting at the center. The interior ring is the single pellet i n the center. The middle ring is the 6 pellets outside the interior ring. The exterior ring is the 12 pellets on the die exterior. There is not a large difference in the density of the pellets in any ring. This study took place with no added pressure. Ther e was very little force compressing the powder and it shows. Without the pressure application, the SPS
85 process is not the same, obviously. 19 pellets could still be heated in parallel and there were sintered. They were not just green bodies. Future experi ments should include the application of pressure. The various profiles extracted from the SPS machine are edifying. The temperature profiles for the fabrication of one (single), two (double), 4 (quad), and 8 (Octo) pellets are compared in Figure 7 6. The eight pellet die was heated to 1150 C as opposed to the 1050 C of the other dies. Also the four and eight pellet dies had a longer startup time. These conditions are easily read from the figure. Everything else looks identical. This is of course because th at is what the SPS controller does; it simply ensures the desired temperature profile by adding current until the desired temreapture is reached The voltage profiles found in Figure 7 7 begins to show the variation that occurs during the SPS process for the various die sizes. The initial voltage spike implies the relative resistance of each die. This is because the SPS controller does not have any input for die geometry etc. It simply uses a set punch diameter to calculate pressure. The voltage spikes fo r the stacked double die more than the other dies ( Figure 7 7 ) This implies the stacked double die had a higher resistance than the other dies and required the higher voltage for current flow. The voltage profile is similar for each die but with variatio n based simply on magnitude and the startup time. The voltage during the process is required for heating up the compact and follows the expected trend of the larger the die the more required voltage to achieve the maximum temperature. Figure 7 8 shows the electrical power profiles of the sintering of various dies. The plot is normalized on a per pellet basis. This means that the power output for the eight
86 pellet die was divided by eight so the power requirement on a per pellet basis is shown The double di e appears to have the lowest power requirement per pellet This was not expected but may be somewhat skewed. The surface area to volume ratio of the four and eight pellet dies is smaller than for the single and double dies. This normally implies that the h eating should be more efficient. The SPS process is different due to the heating mechanism and the graphite felt insulation. First the insulation is only on the die exterior, not on the top and bottom of the die and not on the punches. The surface area of the punches for the four and eight pellet dies are four times as large as for the single and double dies. This implies a larger loss of radiative heat. Additionally, they have a much larger cross sectional area that radiates heat from the top and bottom o f the die. This may be a root cause of those dies seemingly being less efficient that the double die. Finally the heating mechanism must be considered. The different cross sectional area and length of the dies will have an effect on current and temperatur e gradients in the tooling. This is visually proven due to the single and double die clearly showing the localized over heating problem in their punches. Those punches, two in total, get heat and glow a bright red, maybe even a full on white glow. This doe s not occur with the four and eight pellet die. Those pellets stay relatively dark for the entire run. This is due to the current density in the punches. Figure 7 8 shows the displacement profiles for the various dies. The most obvious aspect of this figu re is the relative displacement. The four and eight pellet dies experience less pressure during the startup. This is due to their punches having a larger cross sectional area to absorb the holding force of the machine. This leads to the enormous displaceme nt of the eight pellet die. The four and eight pellet dies are
87 basically the same but with the eight pellet die having twice the magnitude. The single and double die behave differently. The double die has a very low displacement. This is due to two reasons First the powder is loaded into this die and compacted so it fits. It is compacted tighter than for the single die so it presumably has a higher green density. Additionally, the double die follows the trend of the single die but plateaus quicker. This m ay be due to the central insert not gliding. If the insert gets jammed, the powder consolidation changes. Figure 7 9 shows the temperature profiles for the single, four and large (19) dies. These were chosen to compare with the large die because they have all pellets in a single plane without stacking, so trends may be more visible without the complication of the pellets stacking. The large die barely reached its desired temperature of 1150 C. This was because the machine was at its capacity in terms of cu rrent. The die could not be heated any faster. From the plot it appears that a heating rate of 100 C/min was attainable until about 1000 C, at which time the radiative heat loss must have become too great. At this point the large die drifted down to a heat ing rate of about 50 C/min. Figure 7 10 shows the electrical power profiles for the single, quad and large die, gained in the power requirements for the larger dies. T his continues the trend from Figure 7 8. Expanding the die radius and adding cavities did not make the dies more efficient. This may be due to the same reasons stated above, namely a lack of insulation causing a large amount of radiative heat loss (Figure 7 4) The punches radiat ed a large amount of heat. Insulation could be added to the top and bottom surface of the die, but adding insulation to the punches may not be possible.
88 Figure 7 11 shows the voltage profile for the single, four pellet and large d ie. The voltage profile of the large dies differs from the other two dies. That voltage profile rises slower but stays high. This is because energy was constantly needed to keep the die hot. It was harder to achieve the desired heating rate so there was n ever an overheating that needed to be corrected by the machine. Figure 7 12 shows the displacement profiles for the single quad and large dies. As shown, the large die never truly levels off. In fact it is still densifying even when the powder is cut. Thi s is primarily due to the lack of pressure. The lack of pressure both required a greater amount of densification due to lower green density and would require greater densification rates to achieve full density or conventional sintering times (12 hours) Si nce the densification did level off, it is implied that temperature was not the driving force behind the low density pellets. This experiment should be repeated with added pressure. 7.4 Conclusions The batch processing of pellets was partially successfu l, but did provide valuable information for further investigation. When producing eight pellets via parallel and series production, the success rate was 50%. This is most likely due to increased die wall friction, along with uneven temperature and pressure gradients in the die compacts, in addition to a limited power supply currently available in the SPS facility. Producing pellets in parallel will require a higher temperature and/or higher pressure to achieve full density. Application of pressure through the sintering process is suggested. This may require a novel approach to pressure application. Ensuring pressure is evenly applied to all powder compacts is not straight forward. Expanding dies radially did not lead to
89 more efficient use of power. This m ay be due to exaggerated radiative heat loss. The pellets made through the parallel production were lower in density but this is primarily due to pressure application. Stacking pellets in series led to pellets of lower density. The pellets also were produ ced with a wide range of densities (standard deviation of over 2.0%). These results show that there are non uniform temperatures and/or pressures within the die. The displacement curve for the double die showed a leveling off of densification before the si ngle die. This may be due to the insert getting stuck. The double die did provide the greatest power benefit. This was counterproductive due to having a large surface a rea to volume ratio; however, the double die also has the greatest percentage of surface area covered by insulation. This implies the insulation and maybe not the double die is the cause of this efficiency. Further investigation should be conducted before the stacking production of pellets would be successful. The large scale proof of conce pt showed many of the difficulties with scaling up the SPS process. The addition of so many punches (38 vs 2 for the single die) reduced any efficiency gains in power usage. The number of pellets made applying pressure complicated. Even if a single punch w as out of alignment, massive pressures would destroy that punch. For this reason, no pressure was applied and the pellets could not densify. This experiment did show that the DR. Sinter SPS 1030 could heat this large die to 1 1 00 C at 100C/min. It is theori zed the full temperature of 1050 C or 1150C could be attained with the desired heating rate with added thermal insulation. The ability to fabricate full density pellets with this setup have yet to be proven.
90 Table 7 1. Statistics of pellets produced as stacked doubles Pellet Density % TD Length (mm) Diameter (mm) Goal 96.5 2.0 10.16 1.27 8.1915 0.0254 1 90.4 10.77 8.1940 0.0038 2 97.0 9.93 8.1991 0.0030 3 96.7 9.98 8.1966 0.0064 4 95.4 10.11 8.2042 0.0053 5 97.4 9.75 8.2017 0.00 43 6 96.4 9.50 8.2093 0.0053 7 96.7 9.83 8.2017 0.0038 8 91.8 10.34 8.1915 0.0053 9 96.4 9.83 8.2042 0.0038 10 96.7 9.53 8.1686 0.0064 11 95.2 9.91 8.1940 0.0124 12 93.0 10.16 8.1966 0.0038 Avg 95.3 2.3 9.98 0.34 8.1964 0.0069 Table 7 2. Density and dimensions of parallel produced pellets Pellet Density (% TD) Length (mm) Diameter (mm) Goal 96.5 2.0 10.16 1.27 8.1915 0.0254 1 93.8 10.35 8.1737 0.0067 2 92.0 10.50 8.1670 0.0039 3 94.0 10.21 8.1763 0.0025 4 94. 4 10.12 8.1763 0.0044 Average 93.5 1.1 10.30 0.17 8.1733 0.0044
91 Table 7 3. Pellets produced by combining series and parallel setup Pellet Density % TD Length (mm) Diameter (mm) Goal 96.5 2.0 10.16 1.27 8.1915 0.0254 1 92.6 10.35 8.1686 0.0067 2 93.3 10.27 8.1678 0.0059 3 95.3 10.01 8.1890 0.0025 4 95.6 9.96 8.1873 0.0015 5 95.6 9.95 8.1830 0.0053 6 94.5 10.03 8.1932 0.0039 7 94.2 10.08 8.1890 0.0051 8 94.7 10.07 8.1898 0.0082 Average 94.5 1.1 10.09 0.14 8.1835 0.0049 Table 7 4. Pellets produced by 19 pellet die PELLET AREA AVERAGE DENSITY INTERIOR 63.1 MIDDLE 63.5 EXTERIOR 63.8 WHOLE 63.7
92 Figure 7 1. Schematic showing stacking powder samples in two pellet die Figure 7 2. Schematic of the di e geometry used to fabricate multiple pellets A ) F our pellet die B ) E ight pellet die.
93 Figure 7 3. Die used to fabricate 19 pellets A) P hotograph B) S chematic Photo courtesy of the author.
94 Figure 7 4. Sintering of 19 pellet die showing localized overheating of punches Photo courtesy of the author.
95 Figure 7 5. Stacked pellets as fabricated A) Pellet edge in contact with punch showing some deformation B) P ellet edge in contact with insert showing clear dish. Photo courtesy of the auth or. Figure 7 6. Temperature profiles for dies fabricating one (single), two (double), 4 (quad), and 8 (Octo) pellets.
96 Figure 7 7 Voltage profiles for dies fabricating one (single), two (double), 4 (quad), and 8 (Octo) pellets. Figure 7 8. Elect rical power profiles for dies fabricating one (single), two (double), 4 (quad), and 8 (Octo) pellets.
97 Figure 7 9. Z axis displacement profiles for dies fabricating one (single), two (double), 4 (quad), and 8 (Octo) pellets. Figure 7 10. Temperature pro files for dies fabricating one (single), 4 (quad), and 19 (Large) pellets.
98 Figure 7 11. Electrical power profiles for dies fabricating one (single), 4 (quad), and 19 (Large) pellets. Figure 7 12. Voltage profiles for dies fabricating one (single), 4 (q uad), and 19 (Large) pellets.
99 Figure 7 13. Z axis displacement profiles for dies fabricating one (single), 4 (quad), and 19 ( l arge) pellets.
100 CHAPTER 8 ATR PELLET PRODUCTION 8.1 Introduction All lessons learned from the above stud ies were incorporated into another proof of concept pellet fabrication. In this production cycle, pellets would be produced for insertion into the Advanced Test Reactor at Idaho National Lab. This study was a partnership with Areva to place 4 pellet types into the ATR and irradiate them. In the future, post irradiation examinations will be performed on the fuel to determine the reliability of the fuel, but for this particular work, the fabrication of the pellets is the only concern. The four pellet types pr oduced were pure depleted UO 2 pellets labeled Blankets, pure enriched UO2 pellets labeled Base, 10% by volume SiC (w) /UO 2 pellets labeled SiC (w) and 10% by volume diamond/ UO 2 pellets and the Base pellets used 4.95% e nriched powder. These pellets must strictly pass all requirements previously stated. In addition, to prove fabrication of multiple pellets could be accomplished and to save time, all pellets were initially fabricated in the stacked arrangements explained e arlier This arrangement fabricates two samples per run. All other procedures are discussed below. Throughout the production cycle, all procedures were continuously reevaluated. dlets being Base pellets, two of Dia, and two of SiC (w) The rodlets were a stack of ten pellets. Each rodlet had a Blanket pellet on the top and bottom of the rodlet leaving the other eight pellets as Base, Dia, and SiC. This requires 16 Base pellets, 1 6 Dia pellets, 16 SiC pellets and 12 Blanket pellets. Areva wanted to keep a few extra pellets for
101 further testing and archive. It was decided that 20 pellets of each type would be fabricated. A total of 80 pellets should take 40 sintering runs during opti mal conditions. 8.2 Experimental Procedures The depleted UO 2 powder was the same powder used in the experiments earlier and was obtained through AREVA. The powder had a bulk density of 2.3 g/cm 3 a mean diameter of 2.4 m, and a starting U/M ratio of 2.11. The powder was depleted UO 2 with an enrichment of less than 0.71%. The raw powder was stored at room temperature and pressure for period of time of over one year after being obtained from AREVA. The 4.95% enriched UO2 powder was supplied by Areva Fuels. It contained trace amount of U 234. The U/M ratio was 2.038. The powder is straight of f the Areva prepped dry conversion calcined UO 2 powder. The bulk density was 1.65 g/cc. No surface area or othe r metrological information was provided by Areva. The SiC whiskers used in these experiments was obtained from Advanced Composite Materials, Inc. The whiskers had an average length of 11 m, an average diameter of 0.65 m, an aspect ratio of 15:1, and were reported to be greater than 98% pure. The whiskers were found to have trace amounts of Carbon (0.1 wt %) and SiO 2 (0.43 wt %). The diamond particles used were purchased from Advanced Abrasives Corporation. The particles were single crystals with a reporte d average diameter of 3 m and a standard deviation of 0.475 m. The constituent powders were mixed through the use of a vibrational ball mill acting as a mixer. Powder batches totaling 20 grams were produced by loading raw UO 2 powder and the requisite a dditive powder into a zirconia vial; a mixing aid,
102 decafluoropentane, was also added to enhance facilitate mixing. Deca is a stable organic compound that does not react with UO 2 SiC or diamond. Additionally, Deca evaporates quickly at room temperature. Po wders were mixed to attain a 10 % by volume fraction of the additive. The SiC/UO 2 mixture contained 19.37 grams of UO 2 and 0.69 grams of SiC. The diamond/UO 2 mixture contained 19.31 grams of UO 2 and 0.63 grams of diamond particles. The zirconia vial was th en sealed via a rubber O ring and a generous amount of electrical tape. The SiC and UO 2 were mixed in the mill for 60 minutes. Three steel balls were added to the UO 2 /SiC mix to break up SiC agglomerates. The diamond and UO 2 powders were mixed for 180 minu tes. Steel balls were not used to mix the diamond. After mixing, the mixed powders were removed and placed in paper cups and dried in a dry keeper from 24 hours. The loading process was more strictly controlled to ensure no enriched UO 2 was lost. T he mas s of powder required was measured T he powder was loaded into the die using the glass funnel, as needed. The loading was performed over a tray to catch any spills. Next the powder was compressed with the other punch, using moderate hand pressure. After loading the first powder mass, a clean ste e l punch was run through the die to remove any residual powder from the die wall. A cold press with the steel punch was to compact the first powder mass. Then the insert was sprayed with graphite spray and add ed t o the die. T he mass of powder required to produce a second pellet was weighed and load ed into the die using the glass funnel, as needed. The cold press and metal punch was used to compact the second powder mass. The second punch was then inserted and the punch/ die compact was secured with electrical tape. The loaded dies were then transported for sintering.
103 Pell ets were sintered according to the parameters listed in T able 8 1 Initial Base and Blanket pellets were sintered at 1050 C for 5 minutes with an applied pressure of 40 MPa. Dia and SiC (w) pellets were sintered at 1400 C for 5 minutes. Dia pellets had an applied pressure of 40 MPa while SiC had an applied pressure of 60 MPa. After sintering was completed, all pellets went through the rigorous pelle t qualification plan as previous ly outlined These parameters changed after preliminary pellets were fabricated. The goal of the study was to produce at least 16 acceptable pellets of each pellet type. While each pellet was characterized, pellets were con tinuously fabricated until 20 acceptable pellets were produced or the required powder ran out. The study was continuously altered and these alterations are discussed below. 8.3 Results and Discussion The Blanket pellets were the first pellets fabricated. The first pellet run was a disaster. The pellets were not loaded correctly and led to both pellets failing all tests. Additionally pellet 2 was lost before it could be characterized. Pellets 3 28 were fabricated in a more streamline process once the resea rchers mastered the loading process. 26 pellets were fabricated before it was determined that sufficient supply was attained. These pellets are described i n T able 8 2. 18 of the 26 pellets passed the qualification tests. Three pellets were fabricated with a densi ty of under 93%. These pellets we re also longer than the rest. It is believed that the graphite insert failed to freely move during sintering of these pellets. In that case, one of the pellets does not receive pressure during the hold and fails to fully densify. Two pellets achieved high density but had surface flaws, which were deemed to be due to poor tooling maintenance. An additional three pellets fracture immediately upon removal from the die
104 and could not be characterized. Excluding pellets 1 and 2, which were associated with obvious human errors, the success rate was 70%. The BASE pellets were undoubtedly less successful. Initial pellets fabricated via stacking produced pellets around 90% TD. Additionally, the loading process was judged to be too difficult and too much powder was being lost. This was acceptable for depleted UO2 but not for special nuclear material. For these reasons, the stacking setup was scraped and pellets were fabricated as singles. This decision also include d all composit e pellets. Fabricating Base pellets in the single pel let setup still did not initially produce high dens ity pellets. Pellets were achieving only 92 93% TD. It appeared that the enrich powder sintered differently from the depleted powder. So for the rest of the study, the temperature was increased to 1100 C from 1050 C for BASE pellets. This worked out much better. Twenty four Base pellets were produced at 1100 C in the single pellet s setup. They are described in T able 8 3 Twenty of the twenty four pelle ts based all qualification testing. The four pellets that failed all failed for structural integrity issues. Pellet s 15 and 23 had issues with the punch end reacting with the pellet. Pellet 1 and 16 had significant chipping. The average density of the BASE pellets was 96.7 % TD. This density is typical of depleted UO2 pellets but the Base pellets were sintered at 1100 C instead of 1050 C. Some property of the powder led to a required temperature increase of 50 C. No characterization studies were performed on the enriched powder at UF due to its status as special nuclear material. The Base pellets success rate was 83 %. The 31 diamond pel lets produced are described in T able 8 4 Of the 31 diamond pellets, only 15 passed pellet qualification for a success ra te of 48%. The common
105 thread with the diamond pellets was a failure to reach the 94.5% minimum density. Ten pellets failed to reach this minimum. This ran counter to previous experiment with diamond mixed with depleted UO 2 powder. The enriched UO 2 powder was more difficult to sinter. The cause of this difficulty is unknown. After pellet 12, it was determined to start using graphite foil in attempt to get higher densities. This worked a small amount. The average density of the twelve pellets without foil wa s 93.8%. The average density of the 16 pellets with graphite foil was 94.9%. There were three pellet failures with the foil due to foil inclusions into the pellet exterior. The SiC pellets were worse than the diamond pellets. Th e SiC pellets are describ ed in T able 8 5. Twenty eight pellets were produced but only nine passed pellet qualification. SiC pellets repeatedly could not achieve high density eve n though the graphite foil was u s ed to produce all of them. The final eight pellets used an increased s intering temperature of 1450 C. This led to dense pellets but the punches became single use and many pellets require significant post processing to remove graphite from the dish. The average density of the first twenty pellets was 93.1%. The average densit y of the last eight pellets was 96.1% TD. 8.4 Conclusions Previous work on depleted UO 2 powder did not match up with results on the enriched powder. Elementally these powders are indistinguishable. The isotopic difference should not have any effect on the sintering of the powder. Th e only clear difference is the M /M ratio. The depleted powder had a U/M ratio of 2.11; whereas, the enriched powder had a U/M ratio of 2.038. It is unknown if this would make such a large difference. In conventional sintering hyperstoichiometric UO 2 sinters easier than 2.00
106 UO 2 Many fuel fabricators add U 3O8 to their fuel to improve si n t erability. There may be some morphological differences. The sintering process is highly dependent on the shape, contact and surface featur es of the powder particles. More invasive characterization of both powders should be performed in the future. This may actually be beneficial. Further powder processing may be able to reduce sintering temperatures in the future. The loading process for st acking pellets proved to be very difficult in a more industrial setting. A utomating that process would help, but a method to guarantee the insert glides freely must be uncovered. A process of pre pressing the powder may prevent powder from falling in betw een the insert and the die wall; thus, allowing the insert to move freely. Conventional sintering uses cold pressing paired with sintering. The effect of pre pressing the powder before insertion into the SPS furnace should be investigated. Table 8 1 Planned ATR pellet sintering parameters. Pellet Type Peak Temperature Pressure Blanket 1050 C 40 MPa Base 1050 C 40 MPa Dia 1400 C 40 MPa SiC 1400 C 60 MPa
107 Table 8 2 ATR Blanket pellet results Pellet # Diameter (in) Length (in) Mass (g) D ensity (%TD) D&C (Pass/Fail) 1 no insp 3 0.3226 0.424 5.5675 90.4 P 4 0.3228 0.391 5.5172 97.0 P 5 0.3227 0.393 5.5349 96.7 P 6 0.3230 0.398 5.5417 95.4 F 7 3.3223 0.384 5.4507 97.4 P 8 0.3231 0.374 5.2662 96.4 P 9 0.3229 0.387 5.4502 9 6.7 P 10 0.3225 0.407 5.4316 91.8 P 11 0.3229 0.387 5.4329 96.4 P 12 0.3224 0.375 5.2446 96.7 P 13 0.3223 0.390 5.3990 95.2 P 14 0.3227 0.403 5.4599 93.0 P 15 no insp 16 no insp 17 0.3230 0.384 5.4467 97.3 P 18 0.3221 0.386 5.3746 96.9 P 19 0.3226 0.383 5.4366 97.6 P 20 0.3231 0.384 5.4401 97.1 P 21 0.3224 0.398 5.6135 97.1 P 22 0.3221 0.384 5.4280 97.4 P 23 no insp 24 0.3233 0.388 5.5162 97.3 P 25 0.3231 0.386 5.4558 96.9 P 26 0.3230 0.385 5.4565 97.2 P 27 0.3235 0.377 5.3829 97.4 F 28 0.3222 0.380 5.3551 97.1 P
108 Table 8 3 ATR Base pellet results Pellet # Diameter (in) Length (in) Mass (g) Density (%TD) D&C (Pass/Fail) BE01 5.4046 no insp BE02 0.3226 0.391 5.4629 96.2 P BE03 0.3232 0.386 5.4527 96.7 P BE04 0.3233 0.383 5.4396 97.1 P BE05 0.3231 0.380 5.4462 97.1 P BE06 0.3227 0.386 5.4389 96.9 P BE07 0.3228 0.385 5.4495 97.1 P BE08 0.3229 0.388 5.4493 96.5 P BE09 0.3229 0.386 5.4562 97.1 P BE10 0.3231 0.382 5.3966 96.9 P BE11 0.3231 0.382 5.40 65 96.9 P BE12 5.4290 no insp BE13 0.3226 0.390 5.4439 96.0 P BE14 5.4800 F BE15 0.3230 0.385 5.4188 96.4 P BE16 0.3230 0.388 5.4685 96.6 P BE17 5.2522 P BE18 0.3231 0.388 5.4370 96.0 P BE19 0.3226 0.344 4.8309 96.4 P BE20 0.3225 0.382 5. 3922 97.1 P BE21 0.3227 0.386 5.4477 97.1 P BE22 0.3230 0.386 5.4702 97.1 P BE23 0.3225 0.385 5.4173 96.9 P BE24 0.3230 0.390 5.4585 96.1 P
109 Table 8 4 ATR Diamond pellet results Pellet # Diameter (in) Length (in) Mass (g) Density (%T D) D&C (Pass/Fail) 1 0.3230 0.389 5.014 94.7 P 2 0.3234 0.393 4.9634 92.7 P 3 0.3229 0.385 4.9261 94.1 P 4 4.7335 no insp 5 0.3218 0.407 5.1626 94.1 P 6 0.3226 0.396 5.1372 95.5 P 7 0.3232 0.399 5.14 94.7 P 8 0.3234 0.401 5.1987 95.1 P 9 0.3230 0.400 5.1591 94.8 P 10 0.3227 0.405 5.1217 93.1 P 11 0.3233 0.409 5.0841 91.2 P 12 0.3231 0.404 5.0886 92.5 P 13 0.3203 0.388 5.003 96.3 P 14 0.3221 0.385 4.9703 95.6 P 15 0.3222 0.385 4.9007 94.2 P 16 5.0391 no insp 17 0.3234 0.385 5.015 95.6 P 18 0.3222 0.388 4.9929 95.1 P 19 0.3223 0.385 5.0113 96.1 P 20 0.3234 0.391 5.1256 96.1 P 21 0.3223 0.392 5.0634 95.3 P 22 0.3229 0.386 4.8735 93.0 P 23 5.1069 no insp 24 0.3227 0.376 4.8417 94.9 P 25 0.3225 0.402 5.03 09 92.4 P 26 0.3224 0.386 4.9399 94.4 P 27 0.3229 0.379 4.9082 95.3 P 28 0.3231 0.384 4.9774 95.4 P 29 0.3195 0.372 4.6768 94.6 no insp 30 0.3228 0.375 4.8445 95.2 P 31 0.3218 0.380 4.8325 94.2 no insp
110 Table 8 5 ATR SiC pellet resul ts Pellet # Diameter (in) Length (in) Mass (g) Density (%TD) D&C (Pass/Fail) 1 0.3212 0.401 4.9881 92.7 P 2 0.3227 0.401 4.9922 92.0 P 3 0.3207 0.404 4.9769 92.2 P 4 0.3209 0.388 4.8369 93.0 P 5 0.3218 0.388 4.9899 95.7 P 6 0.3226 0.401 5.0256 92.6 P 7 0.3226 0.384 4.9530 95.4 P 8 0.3220 0.395 5.0625 95.1 P 9 0.3279 0.403 5.0999 90.7 P 10 0.3227 0.396 5.0720 94.6 P 11 0.3213 0.392 5.0315 95.6 P 12 0.3226 0.395 5.0413 94.3 P 13 0.3212 0.400 4.9250 91.8 P 14 0.3231 0.409 5.1347 92.4 P 15 0.322 2 0.398 5.0158 93.5 P 16 0.3231 0.404 5.0774 92.7 P 17 0.3240 0.400 4.9994 91.6 P 18 0.3228 0.403 5.0563 92.6 P 19 0.3211 0.406 4.9887 91.6 P 20 0.3225 0.392 4.9014 92.4 P 21 0.3234 0.379 4.9683 96.5 P 22 0.3232 0.373 4.8889 96.5 P 23 0.3231 0.377 4.9371 96.7 P 24 0.3183 0.380 4.6631 93.2 P 25 0.3230 0.365 4.7662 96.4 P 26 0.3231 0.365 4.7873 96.6 P 27 0.3225 0.370 4.8102 96.3 P 28 0.3223 0.365 4.7427 96.2 P
111 CHAPTER 9 ECONOMIC CONSIDERATIONS 9.1 Introduction The commercial fabr ication of pure UO2 pellets has been optimized over the past sixty years. As previously stated, the process includes producing the UO2 powder from feedstock, forming dense pellets from UO2 the powder, and loading pellets into a fuel assembly made up of lon g, thin fuel rods. The SPS process only affects the middle step: forming dense UO2 pellets from the powder. To form pellets from the UO2 powder, a four step process is applied. The first step is conditioning the UO2 powder for pressing and firing by adding lubricants, pore formers and other additives. The second step is pressing the conditioned powder into pellet form with a high pressure press. These pressed pellets are called green pellets. The third step involves firing the green pellets into fully dens e pellets through heating them through a series of furnaces. In the fourth step, the fully dense pellets are post processed via centerless grinding to ensure pellets meet the dimensional requirements. The SPS process can interrupt all four of these of thes e steps with as few as a single step: step 3. The SPS process intrinsically is simply the firing process. A powder is heated and forms a dense compact. In SPS, the pressing process can be incorporated into the firing process. Pellets do not need to underg o centerless grinding due to the pellets consolidating into tighter dimensional ranges. Additionally, if optimized, post processing can be drastically reduced or even eliminated. Finally, all work done during this study used unconditioned powder, so that s tep could hypothetically be removed as well. It has been theorized that conditioning the powder would still be useful. The experiments with enriched UO2 powder showed that powder behaved differently than the depleted
112 UO2 It is expected that processing th e powder before sintering would improve the product but is not strictly necessary. The major economic consideration of SPS is the costs associated with firing. These costs can be further broken up into capital costs and production costs. Capital costs are the cost of the SP S furnaces and associated equipment. Since all major fuel fabricators have large factories already, it is not expected that any capital costs except for the new furnaces are required. The production costs include the energy required to power the SPS furnaces, the tooling used in the SPS process, and personnel and maintenance costs. These are the focus of this study. It is not expected that SPS would provide a financial incentive over conventional sintering of commercial grade UO2. This is primarily due to the long history of optimizing the conventional sintering process, the capital costs already spent and the associated technical expertise the industry has acquired. During these studies, UO2 was used as a base material because it has be en the standard material used in the nuclear fuel industry. Any future material sintered via SPS may have its own unique characteristics, but the general process is not expected to change much. 9.2 Capital Costs There are a variety of larger scale SPS fu rnaces available on the market currently. The largest available standard SPS machine from FUJI is a machine with a capacity of 45,000 amps and 6,000 kN. The cost of this machine, off the shelf, is $3 million. This machine ( Figure 9 1 ) is very similar to th e Dr. Sinter 1030 located at UF. Any high volume manufacturing would require the tunnel type system ( Figure 9 2 ) The tunnel systems are all custom made but a single line with the large furnace shown in
113 Figure 9 2 would cost around $5 million. This is for a single furnace, along with a loading and pre heating stage and a cooling chamber. This system would load dies with an automated system. Such as system would heat the powder to a prescribed temperature conventionally, then send the compact to the SPS furn ace. After firing in the SPS furnace, the compact is immediately sent to the cooling chamber to cool naturally. This would make the time the compact is in the SPS furnace about 10 minutes. The SPS furnace would be the bottle neck in this process. The Dr. Sinter 1030 here at UF has a maximum capacity of 3000 amps. This much larger furnace is capable of driving 45,000 amps through a die or series of dies. Taking this scaling factor and assuming that requisite insulation could ensure the die could maintain t he require temperature, 285 pellets could be sintered in this large furnace in a single run. This number may be increased with a different set up. It is not suggested to have a single die of 285 pellets but a series of dies either stacked or in parallel. But with 285 pellets sintered in ten minutes, a single machine would have a production rate of 1710 pellets per hour. To achieve the industry required 45000 pellets per hour, 27 machines would be needed. Assuming some of these machines may require mainten ance, 30 running SPS furnaces would meet this requirement. 30 furnaces would cost $150 million. There could be added efficiency in the cooling chamber setup or the loading setup. Since the SPS furnace is the limiting step, perhaps only one loading line or one cooling chamber would be required for every two to four furnaces. Even with the best case scenario, this is a capital cost of $100 million under ideal conditions.
114 While it is unclear if the capacity of a single machine could be increased beyond 285 pe llets, there was some indication of such possibilities in the earlier work. The ceramic punch tips seemed to cut the energy requirements of a single pellet in half. Also the added thermal insulation may make a higher capacity possible; however, these eff iciencies may just make each furnace require less energy but not increased pellet capacity. This all assumes that all the pellets are pure UO2 and will meet specifications. The ceramic composite pellets require a higher temperature, thus each machine would have a lower capacity. This may not be a drastic effect depending on the pre processing of the composite powder and the actual sintering conditions. Then again the added temperature could reduce the capacity by as much as two fold leading to a capital co st of $300 million. Any even more exotic material, would obviously change the capacity of each furnace. There are newer hybrid heating systems being built. These systems use an external heating source in addition to joule heating via the electrical curren t. No work has been performed on UO2 for these systems, so it is not known if these would produce the same products. 9.3 Production Costs The production costs associated with SPS fabrication have two main costs: materials costs and tooling costs. The ma terials costs are the costs of the powder materials being sintered. This may be limited to just raw UO2 power as currently produced in all fuel fabrication plants or may include additives such as burnable poisons, pore formers and any other desired additio ns. UO2 is comparatively cheap.
115 This is one advantage of the SP process, in that it does not require any pre processing of powder. Pre processing may be desirable but it is unnecessary. SPS will not replace conventional sintering for the production of the current UO2 fuel. The industry is too far ahead in terms of both optimizing the process and capital investment. Since, any future commercial use of SPS for the fuel industry would be for a composite material of exotic material not yet used, predicting the materials costs is just guesswork. Th e tooling cost for this work was the major expens e Custom m achined graphite punches cost about $50 each. Standard dies cost about $100 each. The large 19 pellet die cost about $500. The major cost for these tools w ere paying a machinist to custom produce the punch tips. These prices are obvious high and unsustainable if tooling is constantly replaced If you take into account the fact that during composite pellet production typically punches were single use or at best use twice, it cost between 100 and 200 dollars to make single composite pellet. Commercial pellet production is on the to make a composite pellet than a commercial UO2 pellet conventionally. The most obvious cost savings would be from improved materials. The YSZ punch tips were custom made and very expensive. The YSZ punch tips cost $500 per pair of punch tips. But these punch tips did not wear out. They could be r eused many times. Of course purchasing something like that in commercial quantities would be vastly cheaper. YZS can be purchased for under $100 per kilogram. Punch tips weigh 1 2 grams. YSZ is certainly less expensive than B4C, although B4C may eventually prove to be more desirable. The conventional process is not foreign to these types of materials. A tungsten carbide punch tip is used during the pressing of conventional
116 pellets. The major cost for all these materials is the machining or forming them with the desired pellet end geometry. A cost reduction to $25 per pair seems conservative. This would still require flat UO2 punches but those are cheaper and last through multiple runs. The major tooling consideration would then be the wear of the die which d epends on whatever barrier is used between the powder and the die body. Buying multiple dies provides a large price reduction. The 19 pellet die cost about $500 but it was custom made. This too could be acquired in bulk possibly for as low as $50. The est imate cost of a whole compact, a 19 pellet die, 38 flat graphite punches and 19 YSZ pairs of punch is about $500. These punches and dies could conservatively survive 20 fabrication runs. That would make the tooling cost $25 per run or about $1 per pellet. One kilogram of UO2 is about 200 pellets. So the tooling costs alone, in this highly speculative case, already approach the current cost of UO2. The other production cost is the actual electricity requirements. Scaling electrical requirements is also quite difficult to do with any certainty. For the 19 pellet die, 0.11 kWh per pellet was required. Compared to fabricating a single pellet which required 0.22 kWh to sinter. These numbers are skewed because they only include certain parts of the heating c ycle, namely the rise from 600C and the hold at the peak temperature. Also the 19 pellet die could be insulated better which reduce these requirements. If the preloading stage heated the powder to 600 C, this part could be skip and the SPS furnace would on ly be heating the powder up from 600 C and holding at peak temperature. A reduction in required power to 0.05 kWh per pellet is not out of the question. At the current rate of $0.12 per kWh, the electrical cost would be $1.20 per
117 kilogram of UO2. This appe ars to be much smaller than the potential tooling costs of the SPS p rocess. This is not unexpected since one of the overall benefits of SPS should be increased efficiency. 9.4 Conclusions The economics of large scale SPS production are highly speculative A complete large scale proof of concept is required to show the maximum number of pellets that can be fabricated with the largest available furnace. The trend implied by this research would be around 285 pellets in a 45,000 amp machine. Further optimizat ion of materials and tooling structure may be able to drive that number closer to 500 pellets. These optimizations would be highly dependent on the material being sintered. A large production line with a single SPS furnace costs around $5 million. The act ual automation of loading pellet quantity powder into dies has not been achieved currently. If the die remained in s ingle or possibly a double layer of pellets, the automation would be trivial. The largest cost uncertainty is in the tooling costs. Since no set of tooling meets all the requirements of the nuclear industry yet, this would be determined by the specific material system. Separating as many part s as possible seems like an efficient idea. Punch tips could be replaced without replacing the punc h body. The current costs of nuclear grade powder are well known but punch and die costs are not. Producing commercial quantities of the tooling is not currently available but the materials are not so foreign as to inhibit the possible of SPS for commercia l production. B4C is the only material that would be difficult to acquire and this is only due to the pellet end geometry requirement. Most of these tooling concerns would be greatly minimized if flat pellets
118 could be produced. Stacking of flat pellets wou ld be much simpler than optimized pellets. Any future material that did not require such a detailed geometry would be a better candidate for the use of SPS. The power requirements are also highly speculative. An electricity cost of $1.00 per kilogram of U O2 is proposed This may be higher or lower an order of magnitude depending on the specific requirements. Figure 9 1. Dr. Fuji Dr. Sinter largest standard furnace with a capacity of 45000 amps and 6000kN. Reproduced with p ermission from FUJI ELECTRONIC INDUSTRIAL CO., LTD. (Kawasaka, Japan 2010).
119 Figure 9 2. Dr. Fuji Dr. Sinter example tunnel furnace with pre sintering and cooling areas Reproduced with p ermission from FUJI ELECTRONIC INDUSTRIAL CO., LTD. (Kawasaka, Japan 2010).
120 CHAPTER 10 CONCLUSIONS AND FUTURE WORK While the feasibility of SPS for production of commercial grade fuel pellets has been shown, further improvements must be made for scale up to manufacture 400 million of pellets per year, which is about to ~ 45 ,000 pellets/ hour for 24 hour operation. It is also essential to further explore new die design configurations and materials to accommodate thousands of pellets in each run. New tooling materials must be developed to ensure that the dies can be used witho ut unacceptable wear. While graphite is the material of choice based on electrical properties, its wear properties have been found to be insufficient to meet the rigorous industrial demands. YSZ and B4C both show some promise as punch tip replacements. The se new materials show that other materials can be integrated into the established graphite tooling to achieve more desirable results. These materials should also be tested with the composite materials to determine if more desirable results can be achieved. The composite pellets proved more difficult than pure UO2. This is primarily due to the higher sintering temperatures required, along with the incapability of graphite tooling at these elevated temperatures. The pellets can still be fabricated and were p roduced at a level high enough to be accepted for entry into the ATR. The further scaling of composite pellet fabrication requires new materials in contact with the powder. B4C or YSZ may make this possible. Finally, to further assess the economic viab ility, a large r scale proof of concept should be explored to determine the maximum number of pellets that can be successfully produced within a single run. An example was performed showing
121 temperature could be achieved with a 19 pellet die in the Dr. Sinte r 1030. These pellets were of poor density but the electrical and thermal aspects of the process were successful. Further scaling requires optimized pressure application and further finite element work to determine thermal and electrical gradients within the SPS compact.
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125 BIOGRAPHICAL SKETCH Timothy Ironman, born 1986, received h is B.S. (2009) and M.Eng (2010) degrees in m aterials s cience and e ngineering from Cornell University. focusing on nuclear fuels advanced manufacturing and advanced materials. In the fall of 2017, he received his doctorate in n uclear e ng ineering s ciences under the guidance of Professor James Tulenko. He is the son of Paul and Felicia Ironman. He has an older sister, Kerrie and an older brother, Sean.