-~ 44


superposition of an epithermal graphite spectrum and a thermal uranium spectrum. Now consider the heterogeneous system consisting of a central UJ-235 plasma surrounded by a graphite reflector. Due to the low uranium density, the epithermal macroscopic scattering cross section of uranium is negligible compared to that of graphite and the uranium plasma is essentially transparent to high energy neutrons. Hence, the higher energy epithermal neutrons in the core region have a spectrum determined by the reflector and experience a negligible attenuation in the uranium plasma. The low energy portion of the epithermal neutron spectrum will be determined by graphite scattering plus uranium absorption due to the U-235 resonances in the 1 eV to 100 eV range. Epithermal neutrons which are slowed down by scattering collisions in the reflector become the source of thermal neutrons in the core region. The thermal neutron spectrum emerging from the reflector is then modified by absorptions in the uranium plasma. The flux weighted thermal and epithermal multigroup cross sections of the uranium plasma region are calculated using the neutron spectrum of graphite containing a dilute quantity of homogeneously mixed U-235. This method of flux weighting includes the spectral effects of both graphite scattering and uranium absorption and thereby provides a more accurate