Improved Mixed Oxide Fuel Calculations with the Evaluated Nuclear Data Library JEFF-3.2

An overestimation of the k_eff values for mixed oxide (MOX) fuels was identified with Monte Carlo (TRIPOLI-4) and deterministic (APOLLO2) calculations based on the Joint Evaluated Fission and Fusion (JEFF) evaluated nuclear data library. The overestimation becomes sizeable with Pu aging, reaching a reactivity change of Δρ≈+700 pcm for integral measurements carried out with MOX fuel containing a large amount of americium. This bias was observed for various critical configurations performed in the zero-power reactor EOLE of the Commissariat à l’énergie atomique et aux énergies alternatives (CEA), Cadarache, France. The present work focuses on the improvements achieved with the new ²³⁹Pu and ²⁴¹Am evaluated nuclear data files available in the latest version of the JEFF library (JEFF-3.2). The resolved resonance range of the plutonium evaluation was reevaluated at Oak Ridge National Laboratory (ORNL), Oak Ridge, Tennessee, with the SAMMY code in collaboration with CEA Cadarache. The resonance parameters of the americium evaluation were obtained with the REFIT code in collaboration with the research institutes Institute for Reference Materials and Measurements (IRMM), Geel, Belgium, and Institut de recherche sur les lois fondamentales de l’Univers (Irfu), Saclay, France.