Study of radionuclide inventory in nuclear fuel under uncertainties in boron concentration using high-fidelity models

Uncertainties in the critical boron concentration during reactor burnup calculations strongly affect the spectrum, thus propagating to the 1-group cross sections and reaction rates used in the Bateman equations and eventually affecting the resulting nuclide densities. These, in turn, alter the calculated critical boron concentration and so on. Usually, the uncertainty due to this nonlinear feedback is overlooked since only final (ie, at the end of the cycle) radionuclide densities are considered for fuel and waste management. However, for source term analysis, an accurate estimation of the core's radionuclide inventory is required at any time during the irradiation cycle. This paper presents an in-depth uncertainty analysis on the nuclide inventory calculations by considering the nonlinear feedback due to deviation from the critical boron concentration during calculation. In particular, the physical characterization of the interrelated effects among spectrum, cross-sections, reaction rates, and boron concentration are highlighted. The results indicate that deviation from the critical boron concentration during calculation may lead to significant discrepancies in nuclide densities during the irradiation cycle, which tends to decrease towards the end of the cycle. The physical processes underlying this behaviour are studied in depth using a high-fidelity model and the Monte Carlo transport calculations. The methodology presented in this study may be used for systematic uncertainty and sensitivity analysis.