A Visual Study of Molten Metal Fuel Coolant Interactions under an Initial Phase of SFR Severe Accident using Gallium Metal vs Water or R123

Abstract

In hypothetical core disruptive accident (HCDA) of a sodium-cooled fast reactor (SFR) as a severe accident, the possibility of the severe recriticality event would increase if the molten fuel forms tight blockages within the subchannel. However, the metal fuel is known to have lower potential to reach up the HCDA compared to oxide fuel. Because of particular characteristics of the metal fuel, it can be upward dispersed without blockage even in the case of pin failures. This fuel transport introduces a substantial negative reactivity, producing a shutdown effect. Therefore, it is required to verify upward dispersion of the molten metal fuel leading to the negative reactivity feedback. There are various injection conditions of the melt with radial core positions, so it is necessary to identify whether the molten fuel is dispersed well enough with structural conditions, coolant void conditions, and the boiling conditions. In the present study, a series of experiments were conducted to clarify the fundamental behavior of the melt injected into the subchannel. Molten gallium was selected as simulant material for the metal fuel. For simulant materials of the coolant, water and R123 were used. The behavior of the molten gallium in the coolant channel was observed using a high-speed camera and visually analyzed. As a result, the driving force to move upward the melt was observed with the coolant channel conditions.