An analysis of initiating and transition phases for an unprotected loss-of-flow accident in an axially heterogeneous fast breeder reactor core

An unprotected loss-of-flow (LOF) event has been analyzed for a 1000-MW (electric) axially heterogenous core (AHC) at the end of an equilibrium cycle, using a realistic model to evaluate the AHC safety potential. The SAS3D code was used for the initiating phase analysis, while a phenomenological approach was employed for the transition phase. The SAS3D results showed that the system rapidly approached subcriticality after experiencing a benign power burst, because of axially flattened fuel worth distribution and reduced sodium-void worth particularly around the core center. During the transition phase, fuel-steel discharge into the interassembly gaps, coupled with engagement of the upper axial blanket material in the core region, was found to result in permanent subcriticality and nonenergetic termination of the LOF event