Light water lattice studies

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Material Information

Title:
Light water lattice studies paper presented at the Reactor Information Meeting at ANL, October 7-9, 1953
Series Title:
BNL ;
Physical Description:
8 p. : ; 27 cm.
Language:
English
Creator:
Kouts, Herbert J. C
Brookhaven National Laboratory
U.S. Atomic Energy Commission
Publisher:
United States Atomic Energy Commission, Technical Information Service
Place of Publication:
Oak Ridge Tenn
Publication Date:

Subjects

Subjects / Keywords:
Light water reactors -- Fuel -- Testing   ( lcsh )
Nuclear physics   ( lcsh )
Genre:
federal government publication   ( marcgt )
non-fiction   ( marcgt )

Notes

Statement of Responsibility:
by H. Kouts.
General Note:
Cover title.
General Note:
Originally published 1953.
General Note:
"November 5, 1953."
General Note:
"Subject category: Physics."
General Note:
"Brookhaven National Laboratory, Upton, New York."
General Note:
"Date Declassified: October 27, 1955."--P. 2 of cover.
General Note:
"Work performed under contract no. AT-30-2-Gen-16."--P. 2 of cover.

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 004703661
oclc - 432307055
System ID:
AA00012185:00001


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SSIFIED


UNCLASSIFIED


BNL-1812

Subject Category: PHYSICS



UNITED STATES ATOMIC ENERGY COMMISSION



LIGHT WATER LATTICE STUDIES- PAPER
PRESENTED AT THE REACTOR INFORMATION
MEETING AT ANL, OCTOBER 7-9, 1953

By
H. Kouts




A




November 5, 1953

Brookhaven National Laboratory
Upton, New York



Technical Information Service, Oak Ridge, Tennessee






Work performed under Contract No. AT-30-2-Gen-16.


Date Declassified: October 27, 1955.


This report has been reproduced directly from the best
available copy.

Issuance of this document does not constitute authority
for declassification of classified material of the same or
similar content and title by the same authors.

Printed in USA, Price 15 cents. Available from the
Office of Technical Services, Department of Commerce, Wash-
ington 25, D. C.


This report was prepared as a scientific account of Govern-
ment-sponsored work and is made available without review or
examination by the Government. Neither the United States, nor
the Commission, nor any person acting on behalf of the Commis-
sion makes any warranty or representation, express or implied,
with respect to the accuracy, completeness, or usefulness of the
information contained in this report, or that the use of any infor-
mation, apparatus, method, or process disclosed in this report may
not infringe privately owned rights. The Commission assumes no
liability with respect to the use of, or for damages resulting with
respect to the use of any information, apparatus, method, or proc-
ess disclosed in this report.







BNL-1812








LIGHT WATER LATTICE STUDIES PAPER PRESENTED AT
THE REACTOR INFORMATION MEETING AT AiL, OCTOBER 7-9, 1953

By H. Kouts

The Brookhaven Reactor Physics Group is making a study of pile core
parameters for light water moderated, slightly enriched uranium rod assemblies.
This information is provided by measurements in a series of exponential assem-
blies which differ in uranium enrichment, moderator-to-fuel volume ratio, and
rod diameter. The enrichment range being explored varies from 1.3% to 1%, the
rod diameters vary from .600" to .250", and the volume ratios lie in the range
from 4:1 to 1:1 (and in some cases are even smaller). The quantities being meas-
ured are f, E, p, B2, M2, and reflector savings (since the assemblies are reflec-
ted). Similar measurements reported at the last Reactor Information Meeting were
done with .750" diameter rods with 1% nominal enrichment.

I wish to discuss a few aspects of some of the more recent measurements.
Since the way we have been using to determine the Buckling is somewhat novel, a
fair-sized effort has been devoted to the two-fold job of improving the accuracy
of the results and exploring the validity of the method. We have been finding B2
by measuring the vertical neutron relaxation length as a function of the loaded
diameter of the assembly, and fitting this observed relaxation length variation
to the usual expression, assuming buckling and reflector savings to have constant
values over the entire range of loading. The range of loadings used has been
quite large, ranging from about 20% of the critical mass upwards, and a principal
criticism of the method used concerns the assumption that A does not vary over
this range. Therefore we have increased the total number of such loadings, taking
them at smaller loading differences and increasing the range up to a keff of about
.97. Thus we have been able to find separately the values of B2 predicted by low
loadings and high loadings. We have also quite recently found it possible to make
moderately accurate radial traverses in some lattices. This allows finding the
buckling in the normal way by combining the information given by radial and axial
flux plots. Radial traverses have been made on only one lattice to date the one
with 1.5:1 volume ratio, .600" diameter rods, and 1.15% fuel enrichment.


-1-







Figure 1 shows the results of B2 measurements made so far with 1.15%
metal. It is seen in every case that the buckling obtained from the higher load-
ings, the buckling obtained from lower loadings, and the buckling obtained from
all loadings all agree to within statistical error. Further, in the case of the
1.5:1 lattice, the value obtained from radial traverses agrees with that obtained
from axial measurements to within about 1%, and this difference is less than exper-
imental error allows. We intend to do radial traverses in several more lattices,
in order to generalize our results, but I think we can say at present that all
signs point to justification of our method of measurement. For instance, two-group
theory predicts that the reflector savings should decrease by about .15 cm. as the
loaded radius increases over the range we used with the 1.5:1 lattice. This should
make B2 as determined from the axial measurements alone about 3.5% lower than that
obtained from radial and axial measurements. The difference is much less, and in
the opposite direction. The implication, of course, is that the fine points of
homogeneous pile theory are not to be trusted when one is calculating the proper-
ties of heterogeneous systems.

We have been measuring migration areas in two ways. The first method
used was reported at the last Reactor Information Meeting; it involves measurements
of buckling and themal utilization as boron is added to the moderated water. If
it is assumed that these are the only quantities influenced by the addition of boron
poisoning, then they are related linearly, and the migration area can be found from
a straight line fit to a plot of them. Chernick had pointed out that the assumption
that 1 is not influenced by the high boron content of the water is very question-
able. The mean neutron temperature must rise as the poison is added, and one will
have the usual decrease of t7 which results. It does not stretch the imagination to
believe that an error of as much as 10-15% in the estimated migration area can re-
sult from this source; that is the value of M2 so deduced may be that much too low.

As a result, we have been exploring a second method of finding M2, which
still resembles the first in principle. This method is made possible by our being
able to make measurements of f and B2 in lattices which differ only in the enrich-
ment of the metal used. So far we have looked at only 1.3% and 1.15% enrichments,
but the results are already quite interesting. It should be noted first that the
change of neutron temperature with this small enrichment variation should be small,
because the mean uranium cross-section changes only from 10.2 to 9.3 barns. If a
temperature effect inIl is important here, its effect would be to make the experi-
mental values of M2 greater than the actual values. Thus migration areas by boron
poisonings are lower limits; migration areas by fuel enrichment variation are upper
limits. The measured values obtained the latter way should be greater than those
found the former way. This does seem to be the trend.

Figure 2 shows values of M2 found by the two methods. Considerable scat-
ter exists in both cases; the two procedures are extremely sensitive to normal er-
rors in thermal utilization measurement. For instance, a 1% error in f for one of
the 3:1 lattice measurements would push that value in line with those of the other
two lattices. By the same token, of course, errors may exist in the other values.


-2 -







We have used for all these analyses measured values of f. If for the
measurements with changing enrichment we use values of f based on simple diffusion
theory, the result of the analysis is a monatonic variation of M2 from 36 cm2 to
41 cm? as the volume ratio decreases from 4:1 to 1:1. The use of even such a sim-
ple minded calculation is probably not too bad, because only the ratio of f for
the two enrichments in involved in the analysis, and the change&in f is between .4%
and 1%. For better analysis we are redoing some f measurements, and waiting for a
set of P3 calculations which Fleck is about to have done on the N. Y. U. Univac.

The fast effect measurements we have been doing are carried out in a more
or less standard way. We measure the fraction of fission product activity produced
by U238 in a fuel rod, and assume this is equal to the fraction of fission prod-
uced in the U238. This quantity is directly related to E through the upper part of
the neutron fission cycle.

The assumption of equality of ratios of fissions and fission product ac-
tivities is pretty standard for this type of measurement, but it has never been
systematically checked. There is reason to suspect a slight error from this source.
We have observed in a set of delayed neutron studies now being carried out some
differences between the relative yields of delayed neutrons from fast-fissioning
species like U238 and Th235, and thermally-fissioning species like U235. Presum-
ably the fission product yield must differ in these cases. Moreover, all the cal-
culations I know of for lattices we have measured predict slightly higher values
of E than we observe. Presumably our values of E are then a little too low. Soon
we expect data on the relation between the fission rates and fission product ac-
tivities; our values of E may then change a little (probably upwards).

A second source of possible error is the uncertainty in knowledge of V28
We assume it to be equal to v25. If it is different, our deduced values of E will
be different. Of course, the calculated values would then change by the same factor.

For tight packed lattices such as must be used with light water, E can
get quite large. The last two figures give values measured for volume ratios rang-
ing from 4:1 to .17:1, under a variety of conditions. The very tight packed lattice
has a value of E closely equal to that measured some time ago in the Snell experi-
ment; there it was found to be 1.21.

The measurements made at varying rod loadings and relaxation lengths were
intended to show what errors, if any, were introduced in the experiment by fast
leakage of neutrons from the exponential assembly. Calculation set an upper limit
of about .1% in 1 for this difference between the measured E and that for an in-
finite lattice. As can be seen, variation of the radius and the relaxation length
simultaneously produced no observable change in E. Variation of the relaxation
length alone by boron poisoning also produced no obvious change. Since either a
change of loaded radius or relaxation length changes the leakage, we conclude that
within the ranges we used, such an effect on E is negligible.


-3-







Such large values of E as are listed here are unique to the systems we
have been studying. An interesting corollary is the relatively large fraction
of U230 which is directly burned without being turned into thermally fissioning
nuclides. For an C of 1.10, for instance, as exists at about a 1:1 volume ratio,
fast fissions in the 28 make up 15% of the total fissions.


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