Development of Assessment Method for a Self-Leveling Behavior of Debris Bed and Analyses of Experiments

When core melt occurs in severe accident in Sodium Cooled Fast Reactor (SFR), molten core material moves to the lower plenum in reactor vessel and fragmented by fuel coolant interaction. These fragmented particles, so called debris, accumulate on the structure surface to form debris bed. If the thickness of the debris bed exceeds the coolable thickness of the decay heat, boiling of sodium occurs inside the debris bed. It is found from past in-pile experiments that the sodium flow and boiling inside the debris bed caused by a decay heat planarize the debris bed to lower the debris bed thickness. This mechanism is called self-leveling of debris bed. In the accident sequence of SFR, when fuel debris locally accumulates beyond the coolable thickness, fuel debris remelts with decay heat and they cannot be retained in-vessel. However, it is expected that the debris bed thickness lowers the coolable thickness with self-leveling phenomenon and they can be safely retained in-vessel. This is why an appropriate assessment for self-leveling behavior is important for safety analysis of SFR with the object of safety cooling of fuel debris. Therefore, the object of this study is to develop new analytical methods to simulate unique phenomena in self-leveling behavior and implement it to SFR safety analysis code. The characteristic of self-leveling is that when the larger external forces caused by environmental fluids are larger than a threshold value, the debris bed is fluidized. The new methods are developed with assuming that the debris bed behaves as Bingham fluid from this feature. They are categorized into two main parts. The first part is particle interaction models to model the effect of particle-particle contacts and collisions. Particle pressure and particle viscosity related to particle-particle collisions and contacts, respectively, are applied to pressure and viscosity term in the particle momentum equation. The second part is a large deformation method, which simulates Bingham fluid characteristic of debris bed. This method numerically judges a onset of debris bed fluidization which depends on a shear stress strength. An experimental study of self-leveling behavior, in which the particle bed behavior driven by bubbles inflow from the bottom of bed in gas-solid-liquid three-phase flow was observed, is analyzed to validate the new methods. Simulation results well reproduced the transient changes of particle bed, whose elevation angle and form deformation becomes gradually small and obscure, respectively. Their dependencies on particle size and density are also well simulated with new methods. The assessment results show that these methods provide a basis to develop analytical methods of self-leveling behavior of debris bed in the safety assessment of SFRs.