Challenge of PWR new core design simulation: A focus on uncertainties due to nuclear data and reflector modelling

• Different types of uncertainties affect the calculation of the core power distribution.
• Uncertainty of basic nuclear data is an important source of uncertainty.
• Compared to UOX, nuclear data uncertainty more than doubles in presence of MOX assemblies.
• Different numerical methods of the core simulators show small discrepancies.
• Adjustment of reflector cross sections to approach the reference result.

This paper presents a reactor core uncertainty analysis in the framework of the OECD/NEA UAM Benchmark. Three types of uncertainties affecting the predictions of power distribution in the core of a nuclear reactor are discussed: the uncertainties of basic nuclear data, the uncertainties resulting from the use of different simulation tools and those due to approximations in reflector modelling. The contribution of nuclear data uncertainty on the power distribution of a UOX and a MOX core is assessed with the XSUSA tool. Overall, the results obtained with different tools in both institutions are in good agreement, showing that the power distribution uncertainty due to the use of different simulation tools is much lower than the one due to nuclear data, which is a large contributor. Lastly, the paper presents preliminary work showing the relevance of reflector modelling on the uncertainty of the power distribution at nominal conditions as well as on an asymmetrical case representative of accidental conditions.