The accuracy of the diffusion theory component of removal-diffusion theory

Abstract

The neutron fluxes in five neutron shields consisting of water, concrete, graphite, iron and an iron-water lattice respectively, have been calculated using P1 theory, diffusion theory with the usual transport correction for anisotropic scattering (DT), and diffusion theory with a diagonal transport correction (DDT). The calculations have been repeated using transport theory for the flux above 0.5 MeV and the diffusion theories for lower energies. Comparisons with transport theory calculations reveal the accuracy of each diffusion theory when it is used for flux evaluation at all energies, and also its accuracy when used for flux evaluation below 0.5 MeV given the correct flux above 0.5 MeV. It is concluded that the diffusion component of removal-diffusion theory has adequate accuracy unless the high energy diffusion flux entering the shield is significantly larger than the removal flux. In general, P1 and DT are more accurate than DDT and give similar fluxes except for shields having a large hydrogen content, in which case DT is better. Therefore it is recommended that DT be used in preference to P1 theory or DDT.