Improved subgroup method for rim effect in resonance calculation

To overcome the incapability of Bondarenko Iteration Method (BIM) and Resonance-Interference-Factor Method (RIFM) in solving rim effect problems and break the limit that Pseudo-Resonant-Nuclide Subgroup Method (PRNSM) cannot solve problems with number density spatial distribution, the subgroup method combined with Improved-Pseudo-Isotope-Model (IPIM) is proposed in this paper. This method generates subgroup parameters for fuel nuclides mixture in a reasonably chosen "base state" and the subgroup fixed source problem of pseudo-isotope is solved afterwards. Subsequently, the effect cross sections of all resonant nuclides in pellet with non-uniform number densities can be obtained by introducing a number density perturbation factor, which is stored in prebuilt tables interpolated by temperature and background cross sections. The numerical results show that subgroup method with IPIM hold an obvious advantage over BIM and RIFM in dealing with resonance interference and rim effect with number density spatial distribution.