Regular Article

Bias estimation for irradiated assembly reactivity decrement and burnup

¹ Reactor Physics and Thermal Hydraulic Laboratory, Paul Scherrer Institut, Villigen, Switzerland
² National Cooperative for the Disposal of Radioactive Waste, Nagra, Wettingen, Switzerland

^* e-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 10 January 2026
Received in final form: 26 March 2026
Accepted: 17 April 2026
Published online: 16 June 2026

Abstract

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 UO₂ and MOX fuel assemblies, as well as UO₂ 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.