Thermodynamic approach for determination of fuel relocation condition in severe accident progression

Abstract

In a severe accident, a rapid temperature increase leads to thinning of the ZrO2 layer and the both of the UO2 dissolution by molten Zr, and relocation could take place within very short period. This paper considered such situation and proposed the relocation condition of “the quick dissolution scenario” with the melt composition at relocating temperature under a quasi-equilibrium assumption. The calculated values were compared with the UO2 solubility observed in the UO2–Zr dissolution tests representing “the slow dissolution scenario” in which the melt is held for a while by the ZrO2 layer. Two phases were recognized in the experiments with latter scenario: ceramic (U,Zr)O2−x phase and metallic liquid phase. The melt composition in each scenario was also converted from the U–Zr–O system to the UO2–ZrO2–Zr system as an example of user-defined criteria of dissolution which can be applied to the severe accident code analysis. The difference between two scenarios (“quick” and “slow”) can result in differing times of lower head failure through the high liquidus temperature of oxide solid phase or chemical activity of metallic liquid phase. Providing the conditions for different mechanisms in the consistent manner for the thermodynamic analysis enabled evaluation of the fuel dissolution behavior in the context of integral accident progression.