Ten-Group Calculated Equilibrium Neutron Spectrum and Diffusion Length in Natural Uranium

The degradation of neutron energies in a fast reactor is largely due to inelastic scattering. In a dilute fast system (large U²³⁸ to U²³⁵ atomic ratio) the neutron spectrum is then primarily determined by a fission spectrum distribution modified by inelastic scattering in U²³⁸. In this investigation a set of ten-group fast cross sections for U²³⁸ have been prepared with the inelastic cross cross sections below about 1.35 Mev based upon levels at 45, 150, and 700 kev. The inelastic transfer contributions from unknown higher levels were chosen to be consistent with the gross measurements of Bethe, Beyster, and Carter, having the three-group energy division consisting of above 1.4 Mev. between 0.4 and 1.4 Mev, and below 0.4 Mev. The ten-group fast cross sections were tested by comparing the calculated equilibrium spectrum, diffusion length, and detector responses in natural uranium with reported experimental values found in the blanket of the Zephyr reactor and in the Snell experiments.