A correction method for RAPID fission matrix calculations with control rod movement

Abstract

The RAPID radiation transport code, based on a fission matrix combination method, was initially designed for spent fuel systems but has been applied on reactor core neutronics analysis. To improve the accuracy of the RAPID fission-matrix approach, a ratio correction method has been applied to the reflector region and for heterogeneous adjacent assembly types. The RAPID code has been tested on a modified BEAVRS benchmark at hot zero power condition without control rods, and reports accurate results (∼0.52% 2-D, pin-wise RMS error with ∼25 pcm keff difference compared to a Serpent 2 reference). In this paper, we apply the assembly correction to rodded assemblies on different control rod bank worth calculations, as well as separate ratios on different slices in order to solve for the partially inserted control rod problems. It is shown that RAPID is able to estimate the control rod worth for different banks (a maximum 37 pcm error and most within 20 pcm). RAPID can also perform calculations on a partially inserted control rod case, reporting a maximum control rod worth error of 17 pcm across some insertion levels of bank D. In addition, another type of correction ratio method is proposed, namely the “localized” ratio. It makes use of sets of four-assembly fission matrices from criticality calculations and allows the choice of any fission source in ratio generation. It greatly reduces the errors located near large source gradient locations, which was observed with the “multiplied” correction ratio since it always assumes a uniform source.