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A power reactor fuel cycle code must compute the isotopic composition of the fuel as a function of both time and position. The most difficult part of such a calculation is computing the self-shielded cross sections and fluxes for the wide range of compositions seen over the power history of the fuel. The EPRI-CELL and EPRI-CPM codes use the background cross section method for self-shielding. The paper reviews the background cross section method with special emphasis on the interaction between the approximations used in CELL and CPM and the methods used for processing multigroup constants. Specific examples using data processed with NJOY are used to illustrate the effects of non-1/E flux, intermediate resonance parameters, elastic scattering self-shielding, transport corrections, and heterogeneity methods. (ERA citation 04:003295)