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15
Initial scoping studies showed that the self-shielding of thorim and
238~ has a most critical effect on the system neutronic%, while that og
the other uranium nuclides was comparatively less. Concentrations of
protactinium, neptunium, and plutonium remained small enough to make self-
shielding treatment of those nuclides necessary. The effest of P~SQ~BIEC~
overlap between 232Th and 238U was of particular interest and was studied
in some depth using the ROLAIDS module of the AMPX code system. Il '%he
conclusi~ns were that thPs effect could be ignored safely in the present
study and that treatment of the effect would have been burdensome had it
been required.
Statics. A set sf cross sections for the more signifisant nuclides
(Table 4 ) was prepared based ow the ENDF/B Version 4 set of standard cross
sections.12
A total of 122 energy groups was rased, with boundaries as
listed in Table 5. Downscatter from any group t o any other was allowed,
and upscatter between all groups below 1.86 eV was allowed. The 123-gromp
set was then repr~cessed to enforce strict neutron ~onser~ati~n. This was
especially important in the case of graphite.
Table 4. Nuclides in library
of 123 energy groups used
for DMSR study
Self-shielding of thorium and uranium nuclides was treated using the
NITAWL module of the AMPX socle system.
was selected in each case. The geometric parameter applicable t~ the tsi-
cusp fuel area between the logs was determined by a special Monte Carlo
computer sode devised by J. R. Knight of ORNL.13
The Nordheim integral treatment
Figure 3 illustrates