New aspects in the analysis of loss-of-flow transients for homogeneous and heterogeneous LMFBR cores

This paper presents the results of analyses of unprotected loss-of-flow (LOF) transients which have been performed to date using the new SAS4A code system. Accident histories for homogeneous and heterogeneous demo-sized cores (300 MWe) are compared and emphasis is placed on phenomena occurring after the initiation of fuel motion as described by LEVITATE. LEVITATE is the SAS4A model for the analysis of fuel and cladding dynamics under loss-of-flow (LOF) conditions and is believed to be the most-sophisticated computational tool currently available for fuel-motion analysis. The results of this analysis indicate that the initiation phase of an unprotected loss-of-flow accident has a considerably lower energetics potential in a heterogeneous core than in a homogeneous core. The difference is larger than previously indicated by SAS3D. Better phenomenological models implemented in SAS4A provide increased confidence in this aspect of safety evaluation of LMFBR cores