Two-step pin-by-pin reactor depletion method based on the GET Plus SPH (GPS) leakage correction

Abstract

This thesis introduces two-step pin-resolved macro depletion analysis based on the burnup-dependent GPS (GET Plus SPH) methodology for PWR type reactor cores. In this study, the subject is largely divided into three parts for effectively achieving two-step pin-by-pin macroscopic depletion analysis. First, the Predictor-Corrector model applying the number density correction method in the transient analysis of xenon and samarium was implemented. Second, a burnup-dependent GPS neutron leakage correction method was established to achieve high-performance nodal diffusion depletion analysis. Finally, the developed methodology was proved by solving various benchmark problems. In the transient analysis of xenon and samarium, the fission yield of Xe-135 was corrected based on the ratio of two-group neutron spectra (Spectral Index, SI) that changes with burnup, and macroscopic absorption cross-sections were modified by integrating nuclides that were excluded from evaluation in the Sm-149 decay chain. The neutron leakage effects were corrected using the burnup-dependent GPS method by generating the group-constant dependent SPH factors as a function of the current to flux ratio (CFR) and the spectral index (SI) along with burnup with a color-set model using fresh fuels and depleted fuels. The burnup-dependent GPS function methodology was optimized, then validated with benchmark problems for the UOX nuclear fuel loaded small reactor and its variable reactor models after its optimization process. For the two-step pin-wise macroscopic depletion analysis, the group constants for the lattice models were generated through the DeCART2D transportation calculation, and a series of nodal diffusion analysis were performed based on the NEM (Nodal Expansion Method) in-house code with HCMFD (Hybrid Coarse-Mesh Finite Difference) solver. The two-step pin-by-pin reactor core depletion calculation methodology based on the burnup-dependent GPS neutron leakage correction method is verified to achieve a significantly improved nodal equivalence in terms of the reactivity error and the power distribution.