Control of neutron albedo in toroidal fusion reactors

Neutron albedo effects in toroidal fusion reactors are evaluated to determine how altering the albedo of blanket regions can be used to improve the neutronics characteristics of experimental or power reactors. Basic neutron albedo data for materials of interest in fusion applications are calculated with the ANISN and MCNP codes. Total albedos are shown to vary directly with material cross sections, and the angular distribution of reflected neutrons is approximately cos theta for thick slabs and flatter for thin ones. Equations describing the physics of neutron multiplication and reflection are used to construct a fast-neutron albedo model that provides the first direct calculation of total albedo. Reflected angular distributions are treated assuming that neutrons are either singly- or multiply-scattered. The calculated albedo data are combined with a simple torus-neutron source model to show that significant changes can be made to the intensity and energy spectrum of the scattered neutron current at outboard first-wall locations. Maximizing the albedo of non-breeding blanket regions increases the local tritium breeding ratio (TBR) in the breeding zones. These resutls indicate that limited-coverage breeding blankets have sufficient breeding margin to insure a global TBR greater than unity when more detailed models are considered