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slowed down by collisions in a surrounding graphite reflector. Since the reflector contains no uranium, the spectrum is determined entirely by scattering and absorption in graphite. The calculated neutron spectra corresponding to several reflector temperatures are shown in Figure 5. Changing the graphite temperature affects the thermal portion of the spectrum by shifting the most probable energy, E = kT, at which the thermal neutron
                  P
flux is a maximum.* Once the graphite temperature is specified, the calculation of the neutron energy spectrum and multigroup cross sections in the reflector is straightforward.

        The neutron spectrum and cross sections in the

central uranium plasma are calculated using the following argument. Consider a homogeneous mixture of graphite and a small amount of U-235. Most of the epithermal neutron scattering will be due to graphite, while thermal neutron absorption will be almost entirely due to uranium. For a sufficiently small amount of U-235, the epithermal scattering effects of uranium and the thermal absorption effects of graphite are negligible. In this case, the total neutron spectrum can be considered to be the



         The thermal energy flux i(E) has a maximum at
E = kT, while the thermal lethargy flux   (u) has a maximum at a lethargy corresponding to E = 2kT.