Sensitivity and uncertainty analysis of plutonium and cesium isotope ratios in BR3 core 4A/B fuel rod

Abstract

The purpose of this research is to examine the effects of systematic uncertainty of reactor operating parameters on isotope ratios in spent fuel rods, specifically from the BR3 reactor. The primary operating parameters of interest are position of the rod within an assembly and the boron concentration in the coolant and the ratios examined are 240Pu/239Pu and 137Cs/135Cs ratios. The model-predicted isotope ratios were also compared to experimentally measured isotope ratios for the rod of interest. An assembly-level model of the reactor of interest was created in MCNP. Four test cases of the rod position and four test cases of the boron concentration were created. The method involved the development of response functions for the final isotope ratios as function of input parameters. An uncertainty analysis was performed using a variance-covariance matrix for the response function of the isotope ratios. The uncertainty analysis revealed a high systematic uncertainty for the 240Pu/239Pu ratio and an over-prediction of approximately 30% from the experimental isotope ratio. The systematic uncertainty for the 137Cs/135Cs ratio was found to be slightly higher than that of the experimental but not as high as the 240Pu/239Pu ratio. The sensitivity analysis of the 137Cs/135Cs ratio showed that it was difficult to gain information about the rod's location within the assembly.