- 45 -


description of the physical system than does the method of weighting uranium cross sections by the unmodified neutron spectrum in graphite.

        Several calculated n'eutron spectra in a graphite moderated uranium plasma are shown in Figure 6. These spectra were calculated by the methods described in the preceding paragraph. When the uranium density is less than N235 = 1018/cc, the epithermal spectrum in the uranium plasma is equal to the spectrum in graphite, since resonance absorptions in uranium are negligible for such dilute quantities of U-235. As the uranium density is increased to N235 = 1019/cc, resonance absorptions begin to slightly depress the epithermal spectrum in the 1 eV to 100 eV range. At a density of N235 = 1020/cc, the neutron spectrum is significantly modified by thermal and epithermal absorptions in the uranium plasma.

        The spectra shown in Figures 5 and 6 were used to calculate multigroup cross sections for the reflector and uranium regions, respectively. Using the AIM-6 multigroup diffusion theory code (35), a parameter study was made to determine the range of values of uranium density, core size, flux shapes, and reflector thickness for which a critical U-235 plasma reactor can be realized. The purpose of these calculations is to determine a reasonable reference model without attempting to optimize all the