Resonance absorption calculations in thermal reactors

Abstract

Publisher Summary The calculation of resonance absorption rates in nuclear reactor cores has long been recognized as one of the most complicated parts of reactor analysis on account of the large number of nuclides with pronounced resonances that are present in the fuel and the drastic cross-section changes that occur in each resonance, sometimes over very narrow energy ranges. The difficulties arise both from the mathematical complexity involved in the accurate description of the cross-section shapes and their changes with temperature, and from the flux changes produced by the resonance absorption. The resonance absorption depends on the fine structure of the flux that has pronounced dips near the energies of the cross-section peaks. An accurate calculation aims at evaluating the resonance reaction rates directly after the energy dependence of the flux has been determined in sufficient detail, particularly at energies in the vicinity of the resonance peaks. Such calculations are quite feasible at present on large computers, for cases in which the material composition and geometrical description of the problem under investigation are not too complicated. This chapter discusses a number of different practical methods in which the resonance absorption problem has been handled in the case of thermal reactor lattices. Lattice heterogeneity effects lead to several practical methods for the calculation of effective resonance integrals that are utilized in a number of core physics analysis codes. They are also required in codes that evaluate the resonance reaction rates directly.