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 lOOO-MW(electric) axially heterogeneous 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 a nonenergetic termination of the LOF event.