Analysis of Rod-by-Rod FP Inventory Distributions in BWR 8 × 8 UO2 Assemblies Using Lattice Physics Method

Abstract

Fuel rod gamma-ray spectrometry was performed for one-, two-, three-, and five-cycle irradiated BWR 8 × 8-4 fuel assembles, and relative rod-by-rod FP inventory distributions of 137Cs, 134Cs, 106Ru, and 95Zr wereobtained for the upper and lower axial height nodes of the assemblies. The measured data was corrected for the difference in gamma-ray transmission between UO2 and Gd2O3-UO2 rods. These distributions were compared with those calculated using the collision probability method, Pij, of the SRAC code system with aninfinite lattice model of the fuel assembly. The calculated results generally well reproduce the measured distributions, and the accuracy of the analysis method of the present study was evaluated to be 1 to 2% for the inventory distributions of 137Cs, 2 to 3% for 134Cs, 2 to 3% for 106Ru, and 2 to 3% for 95Zr, which represent the distributions of the burn-up, the thermal flux multiplied by the burn-up, the buildup of 239Pu, and the fission rate at the end of the fuel discharge cycle, respectively. One of the notable results of the analysis of this study is that the FP inventories for the Gd2O3-UO2 rods were underestimated in most cases.