The Phenomenology of Packed Beds in Heavy Liquid Metal Fast Reactors During Postaccident Heat Removal: The Self-Removal Feedback Mechanism

The phenomenology for a particular behavior of packed beds in heavy liquid metal (HLM) fast reactors during postaccident heat removal is proposed. Because of the similar densities of the fuel and the HLM, an inherent passive safety self-removal feedback mechanism due to buoyancy forces is developed, which propels the packed bed away from the wall, thus preventing temperatures that can jeopardize the vessel’s structural integrity and also reducing the recriticality potential by limiting the allowable bed depth. This identified mechanism will have somewhat compensatory tendencies in the self-leveling behavior of debris beds, which are crucial for sodium-cooled reactors, but unfortunately, it is not operative for HLMs because of the absence of boiling of the coolant. By means of a simplified geometrical model, a preliminary analysis of the potentiality of the phenomenon has been performed.