In-Pile Observations of Fuel and Clad Relocation During LMFBR Core Disruptive Accidents in the STAR Experiments

The reactivity effects of the initial fuel and clad motion during unprotected loss-of-flow accidents (ULOFs) strongly influence the accident progression in liquid-metal fast breeder reactors. To study these phenomena, a series of in-pile experiments (the STAR experiments) is being performed in which clad motion and fuel dispersal are observed in small pin bundles with high-speed cinematography. The major parameters varied in the series are power level, fuel type (fresh versus preirradiated), and number of pins. The results of the first four experiments are presented. Overall, the observations made in these experiments show that ULOFs tend to have early fuel disruption caused by fission products, followed almost immediately by fuel sweepout occurring simultaneously with axial clad relocation.