Thin Regions In Diffusion Theory Calculations

A method for determining effective cross sections for geometrically thin absorbing regions in multigroup calculations is described. The effective absorption cross section in multigroup calculations provides a smooth transition from the usual diffusion theory cross section for low absorption slabs to the λ_tr extrapolated end-point condition for black slabs. In effect, the average flux between mesh points of the difference equation grid is related to the fluxes at the mesh points. Self-shielding effects are accounted for by material cross-section rather than difference equation modification. Application of the theory to lattice calculations is discussed, and comparisons are made with other methods for limiting cases.