Use of additional fission sources or scattering sources to model inward axial leakages in fast-reactor analysis

Abstract

When calculations of flux are done in less than three dimensions, bucklings are normally used to model leakages (flows) in the dimensions for which the flux is not calculated. If the net leakage for a given energy group is outward (positive), the buckling is positive, and buckling methods work well. However, if the new leakage for a given energy group is inward (negative), the buckling is negative and can lead to numerical instabilities (oscillations in the iterative flux calculation). This report discusses two equivalent non-buckling methods to model inward leakages. One method (the chi/sub g/ method) models these incoming neutrons by additional fission sources. The other method (the sigma/sub s/(1 --> g) method) models them by increased down-scatter sources. The derivation of the two methods is shown, and the flux spectra obtained by their use are compared with those obtained from two-dimensional (RZ) calculations.