Bias estimation for irradiated assembly reactivity decrement and burnup

In this paper, we are estimating biases for the reactivity decrement and burnup of irradiated fuel assemblies, based on differences between measured and calculated local in-core power measurements (flux maps). The simulations are performed with validated CASMO5 and SIMULATE5 cycle follow-up models for three Pressurized Water Reactor (PWR) cores. A total of more than 70 cycles of operation and 1000 in-core flux maps at various core burnup steps are used, including both UO 2 and MOX fuel assemblies, as well as UO 2 alone, with relatively high assembly average burnup at the end of life (greater than 60 MWd/kg). The effect of nuclear data uncertainties is also calculated and quantified on both bias types. The main findings are that while the burnup biases are globally constant as a function of the assembly burnup, the bias for the reactivity decrement tends to increase, both in terms of average values and tolerance intervals.