Treatment of Scattering Anisotropy of Neutrons Through the Boltzmann-Fokker-Planck Equation

Handling the highly anisotropic scattering of fast neutrons with conventional methods usually means that high-order Legendre expansions can be necessary to obtain correct angular fluxes. This drawback in standard transport calculations is avoided by applying the Boltzmann-Fokker-Planck (BFP) method, already used in transport of charged particles, to neutrons. Two methods are described to obtain the relevant input data for the one-dimensional BFP-1 code, one using basic differential scattering cross sections and the other using existing standard multigroup libraries. Numerical results for both methods are produced, revealing BFP as a powerful method when solving transport problems dealing with very fast neutrons. It is found that high accuracy, even for extreme cases of anisotropy, is achieved without increase of the computational effort.