Representativity studies of ZEPHYR fast/thermal coupled cores for SFR-like reactivity effects during core meltdown

The aim of the present paper is to present first representativity studies of local reactivity effects in a fast lattice during severe core accident (SCA) in Liquid Metal-cooled Fast Breeder Reactors (LMFBRs) sequence. The representativity study is based on sensitivity analysis of reactivity coefficients and local flux distributions against core degradation being analyzed in the ASTRID CFV-V0 inner core. During severe accident sequence, voiding and fuel meltdown can have pronounced influence on the neutronic characteristics of the core, and can in particular lead to prompt/superprompt recriticality. In order to predict the core behavior during such disruptions it is necessary to develop accurate and precise computational and experimental tools and methodologies. The assessment of reactivity behavior that represents different stages of the disrupted core configurations is one of the challenges associated to ZEPHYR, and requires a new way of performing measurements in critical assemblies. Serpent Monte Carlo sensitivity and uncertainty analysis of a CFV-like assembly degradation, from voiding to fuel meltdown, is performed in the ZEPHYR fast zone, using the MASURCA fast ZPR fuel materials. The SFR assembly is modelled by MASURCA MOX fuel and sodium rodlets for normal situations, and by metal fuels and stainless steel plates in case of meltdown. The sequence covers full meltdown, from partial to the collapsing of fuel corium in the upper and lower assembly parts. The sequence focuses on sensitivity profiles similarities, and hence establishes a transposition of the awaited ZEPHYR experimental results to the ASTRID case. (author)