Monte Carlo calculations with nuclear point data based on JEF-2.2 and JEFF-3.1 for the VENUS-7 critical benchmarks

Abstract
Currently, the VENUS-7 series of critical experiments is evaluated within an OECD/NEA benchmark activity for the validation of nuclear data and codes for MOX fuel. Measurements are available for various cold square lattice cores, consisting of an inner MOX and an outer UO₂ zone, moderated and reflected by light water. In addition to the multiplication factors, reactivity changes for substitution or removal of a small number of fuel pins, as well as radial pin-wise fission rate distributions are known from the experiments. Results obtained with the MCNP code, using point-wise cross section libraries processed from the JEF-2.2 and JEFF-3.1 evaluated data, are presented. Concerning the reactivity differences and fission rate distributions, satisfactory agreement between the calculated and measured results is obtained. The absolute values of the multiplication constants are underestimated with the JEF-2.2 data by approximately 700 pcm, which is consistent with the results of earlier calculations for the KRITZ-2 LWR lattice UO₂ cores. When using the new JEFF-3.1 data available from the NEA data bank, the calculated multiplication constants increase by a statistically significant amount of about 200 pcm. The results supplement a series of benchmark calculations with the Monte Carlo method on critical experiments performed for the validation of evaluated nuclear point data libraries.