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Fiy. 4.1. Calculated Epithermal Neutron Spectra in the 233U-Fweled MSRE and in the MSBR.
fertile salt volume fractions of 16.48 and 5.85%
respectively. The fuel salt contained approximately
2.84 mole % UF4 (-64% of total IJ is 23iU
and -5.9% is 235Uj, and the fertile salt contained
27 mole % ThF,. By comparison, the model of the
Msm core used for this analysis was a cylinder
with effective dimensions of 58 in. in diameter by
78 in. in height. The fuel salt volume fraction is
2236, and there is no fertile salt. The uranium
content of the fuel salt was approximately 0.125
mole "/o UF4 (.~,91.4% 2331J), based on calculations
summarized in the following section.
As described in ref. 1, the epithermal and thermal
neutron spectra calculations were made with the
GAM and TIIEl?MOS programs respectively. In the
choice of an effective "thermal cutoff" energy
separating these spectra, there is a considerable
degree of latitude, with the single restriction that
the cutoff energy be high enough that important
calculated neutronic properties at operating temperature
are not strongly influenced by neglect of
upscattering corrections above the cutoff energy.
In previous studies of the MSRE, lS3 we used
0.876 ev (for which the present energy group
structures for the GAM and THEICiiOS programs
coincide). This value was found to be sufficiently
high to satisfy the preceding criterion. However,
because current design calculations for the MSBR
are based on the higher value of 1.86 ev, to make a
valid comparison we calculated the MSKE thermal
spectra for this higher cutoff energy.
The results of GAM-I1 calculations of the epithermal
neutron spectra in the two reactors are
shown in Fig. 4.1. As in ref. 1, use is made of the
lethargy variable, proportional to the logarithm of
the neutron energy, in plotting the results. This
relation is
lethargy : - In (energy/lO MeV) .
For a unit fission source the calculated fluxes per
unit 1et.hargy for each GAM-I1 neutron group are
3P. N. Haubenrelrh et a1 , MSRE, Deslgn and Opera
tions Report Part III Nrzcfear Analysis, ORNL TM
730, pp. 38-40.