Group Title: Innovative Nuclear Space Power and Propulsion Institute informational brochures
Title: Uranium tricarbide nuclear fuels
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 Material Information
Title: Uranium tricarbide nuclear fuels
Series Title: Innovative Nuclear Space Power and Propulsion Institute informational brochures
Physical Description: Archival
Language: English
Creator: Innovative Nuclear Space Power and Propulsion Institute, University of Florida
Publisher: Innovative Nuclear Space Power and Propulsion Institute, University of Florida
Place of Publication: Gainesville, Fla.
 Record Information
Bibliographic ID: UF00091281
Volume ID: VID00002
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.


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"Space Exploration is the ultimate
investment in America's Future"


An International Leader
in Space Applications

Contact Information
General: Ms. Lynne Schreiber,
Research: Dr. Travis Knight,
Academic: Ms. Ines Aviles-Spadoni,
P.O. Box 116502
Gainesville, FL 32611-6502 USA
Phone: (352) 392-1427
FAX: (352) 392-8656



Advanced ultrahigh temperature nuclear fuels are
recognized as an enabling technology for lower cost,
high performance nuclear thermal propulsion (NTP)
systems for use in
*future manned missions to the moon or mars
*cargo transport to the moon or mars
*unmanned explorations of the outer planets
*earth orbit transfers of satellites
Uranium bearing, solid-solution tri-carbide fuels
such as (U, Zr, X)C with X = Nb, Ta, Hf, or W offer
many advantages for high performance, advanced
space power and propulsion applications. Binary
carbide fuels of (U, Zr)C were first studied for nuclear
thermal propulsion (NTP) during the Rover/NERVA
program of the 1960s and early 1970s. These
advanced fuels evolved from earlier designs and
represented the most promising space nuclear fuel at
the time the program was cancelled in 1973.
The Innovative Nuclear Space Power and
Propulsion Institute (INSPI) at the University of Florida
has focused on improvements in the processing and
fabrication of these fuels with the goal of producing net-
shape fuel elements.






The chief physical characteristics of
mixed uranium/refractory metal
carbide fuels, referred to as tri-carbide
nuclear fuels include:
* Significantly higher melting points
nearer to those of the constituent
refractory carbides (-3800K vs. 2800
K for UC) depending on the metal
mole fraction of uranium.
* Significantly higher thermal
conductivity than conventional oxide
nuclear fuels (~30 to 70 W/m K versus
about 3 W/m K for uranium dioxide).
* Greater stability especially at high
temperatures in comparison with the
monocarbide UC or other prime
candidate nuclear fuels such as UO2
and UN. Their thermochemical
stability in a flowing hot hydrogen
propellant is important to the reliability
and long lifetime of a NTP system.


These physical properties lead to
improvements in design and operation
for greater efficiency and reduced cost
* Higher operating temperatures (as
high as 3000 K) provide for greater
specific impulse (more efficient use of
propellant) for NTP.
* Reduced nuclear fuel requirement
leading to reduced launch cost owing
to smaller more compact cores as a
result of the greater power density and
reduced mass losses owing to greater
thermochemical stability of the fuel
and propellant.
* Reduced propellant requirement
leading to reduced launch cost and
reduced refrigeration and thus power
necessary to provide for propellant
storage cooling.

100 pm SEM with compositional contrast 100 Im

Hypostoichiometric tri-carbide, (Uo 1, Zro 77, Nb 13)C0 95
sintered for 4 Min. at > 2800 K and 128 Min. at > 2500 K

High solid-phase solubility of
uranium carbide in zirconium and
niobium carbides provides for high
flexibility in using very low to very
high uranium fractions in the fuel.
For requirements of compactness,
high performance, and long life,
space power reactors require low
uranium fractions in the mixed
carbide fuel and higher enrichments
of uranium.
The presence of non-uranium
carbides in the fuel allows for
gradient coating of fuel pellets and
particles with refractory metal
carbides, which act as a robust
barrier for containing fission
products. No additional coating is
necessary as with earlier graphite
matrix and composite fuels, which
lead to cracking of the coating and
mass losses due to corrosion by the
hot hydrogen propellant.
Some problems identified for
these fuels during the Rover/NERVA
program include a susceptibility to
fracture and difficulty fabricating
more complex fuel element
geometries used in earlier NTP
studies. The mixture of carbide
powders is difficult to extrude and are
very hard leading to excessive wear
on the dies. INSPI has focused on
improvements in the processing and
fabrication of these fuels with the
goal of producing net-shape fuel

Processing and fabrication efforts
at INSPI have developed methods of
cold pressing and UC liquid phase
sintering of near-stoichiometric and
hypo-stoichiometric tri-carbides to
produce high-quality, single-phase,
solid-solution samples with less than
5% porosity. In light of the difficulties
in extruding solid solution binary
carbide fuel elements in the
Rover/NERVA experiments, an
innovative space reactor fuel
geometry, square-lattice honeycomb
(SLHC), was developed that would
better lend itself to net-shape
fabrication methods available to tri-
carbide fuels.
Efforts at testing and characterizing
their performance under extreme NTP
conditions are currently underway.
The development and characterization
of these fuels could lead to advanced
NTP systems with specific impulse of
greater than 1000 seconds making
future long term, power rich space
missions to Mars or other destinations

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