Wigner-Type Continuous Slowing Down Theory for the Fast-Neutron Spectrum

The Wigner-type continuous slowing down theory is derived from the physical point of view, considering the neutron balance in lethargy space, and is applied to the calculation of neutron spectra in fast-reactor compositions, where the moderating effect of inelastic scattering is very important. The present theory corresponds to the macroscopic representation of the moderating process of neutrons. Its single moderating parameter, (u), is defined as the ratio of slowing down density, q(u), to collision integral, B(u), i.e., This parameter has the physical meaning of “mean-free-path” in lethargy space and is numerically calculated by an iterative technique. The validity of the present formalism is tested by comparing numerical calculations of neutron spectra for some fast-reactor compositions with neutron spectra computed by Monte Carlo simulation.