Analysis of rod-by-rod FP inventory distributions in BWR 8x8 UO₂ assemblies using lattice physics method

Fuel rod gamma-ray spectrometry was performed for one-, two-, three-, and five-cycle irradiated BWR 8x8-4 fuel assemblies, and relative rod-by-rod FP inventory distributions of ¹³⁷Cs, ¹³⁴Cs, ¹⁰⁶Ru, and ⁹⁵Zr were obtained 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 an infinite 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. (author)