Midplane and off-midplane axial leakage simulation of heterogeneous subassemblies in EBR-II

Generally EBR-II XY geometry and one-dimensional (1D) cylindrical neutron flux calculations using transport theory analysis assume energy independent DB/sup 2/-type absorptions to simulate effects of axial leakages. This assumption, while generally resulting in satisfactory eigenvalues and high- and intermediate-energy flux spectra, gives large errors in the low-energy flux spectra where the flux levels are smaller. These midplane errors, and more importantly the off-midplane errors, can be reduced by using a more realistic leakage model: space and energy dependent leakage absorption cross sections. Analyses have been reported in which transport theory methods using row-wise azimuthally-homogeneous RZ-geometry boundary angular fluxes to calculate space and energy dependent leakage absorptions which were then used in subsequent 1D cylindrical simulations of RZ calculations. The present paper extends the study to include heterogeneous core loading configurations. This study contains modeling of heterogeneous XYZ loadings using heterogeneous XY geometry and space and energy dependent leakage absorptions. Because of the complexities arising from the three-dimensional analysis, the results presented here use diffusion theory. Although the actual negative leakage absorption values can be used in the CITATION diffusion theory code, it was found that the ..sigma../sub s/(1..-->..g) method gave better results in the core region of these studies.