The purpose of this research is to examine the sensitivity and quantify the systematic uncertainty of reactor operating parameters on isotope ratios in spent fuel rods. 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 the 240Pu/239Pu ratio and the 137Cs/135Cs ratios. The model-predicted isotope ratios are also compared to experimentally measured isotope ratios for a given rod of interest.
An assembly-level model of the reactor of interest is created in MCNP. Four test cases of the rod position and four test cases of the boron concentration are created. Response functions are created for the final isotope ratios as function of input parameters. An uncertainty analysis is performed using a variance-covariance matrix for the response function of the isotope ratios. The uncertainty analysis found 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 is 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 also show that the rod of interest was most likely not located next to a water channel.
Because of the over-prediction of 240Pu/239Pu ratio in the assembly-level model, pellet models of the rod of interest in MCNP and SCALE are created to examine effects that could have contributed to this over-prediction. Perturbations in the input parameters are tested and found to have little effect on the isotope ratios. This research has evoked questions about the predictions of isotope concentrations in MCNP for criticality/depletion calculations.