Regular Article

Bias and uncertainty estimates for in-core LWR reaction rate distributions

Reactor Physics and Thermal hydraulic Laboratory, Paul Scherrer Institut, Villigen, Switzerland

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Received: 10 January 2026
Received in final form: 23 March 2026
Accepted: 7 April 2026
Published online: 19 May 2026

Abstract

We analyze in-core reaction rate measurements coming from three Pressurized Water Reactors and two Boiling Water Reactors (named E) and compare them with CASMO5 and SIMULATE-3 (and -5) calculations (named C). A total of almost 1 million experimental values were considered, covering various cycles, detector systems and locations, as well as UO₂ and MOX fuel types. Biases and root mean square values (rms) were extracted based on (E − C)/C ratios, leading to the finding of outliers, most likely due to detector issues and possible modeling effects for locations with low power. The average rms value is 2.9%, once the outliers are removed, but dissimilar values are obtained for different reactor types. The effect of nuclear data is also quantified for one PWR, leading to an absolute average rms uncertainty of 0.3%. We conclude, based on this study, that an improvement of the local rms values can be achieved by a better knowledge of measurements (i.e. uncertainties) and a possible update on specific parts of the involved models.