Pinwise Diffusion Solution of Partially MOX-Loaded PWRs with the GPS (GET PLUS SPH) Method

The generalized equivalence theory (GET) plus superhomogenization (SPH) [GET Plus SPH (GPS)] method, which is a new leakage correction method for the pin-by-pin reactor analysis of light water reactors, has been applied to benchmarks for partial loading of mixed oxide (MOX) fuel in pressurized water reactor (PWR) cores. In the GPS method, the pinwise, cross section–dependent SPH factors are parameterized as a function of normalized leakage, i.e., current-to-flux ratio. As partially MOX-loaded PWRs usually have a stiff gradient of neutron flux on nodal interfaces, the original GPS functions for UO₂ cores are slightly modified to take into account the strong spectral interaction. To determine the coefficients of the GPS function, several colorset models are considered to obtain fitting data. In this work, the two-dimensional method of characteristics–based DeCART2D code is used for both colorsets and reference core calculations. The GPS method is implemented in an in-house, pin-by-pin diffusion solver with the pinwise coarse mesh finite difference method. To evaluate the performance of the GPS method on partially MOX-loaded PWRs, the Korea Advanced Institute of Science and Technology (KAIST) 1A benchmark is analyzed in this work. In addition, various small and large variants of the KAIST 1A benchmark are also analyzed using the same GPS functions to demonstrate the general applicability of the predetermined GPS functions. Based on the comprehensive results of this work, it is concluded that the GPS method can clearly improve the accuracy of the conventional GET-based, two-step, pin-by-pin core analyses.