Theoretical studies on some aspects of molten fuel-coolant thermal interaction

Abstract

Rapid generation of high pressures and mechanical work may result when thermal energy is transferred from the hot molten nuclear fuel to the coolant in an LMFBR accident. Such energetic thermal interactions are facilitated by the large heat transfer area created when molten fuel is fragmented in the coolant. Two aspects of the molten fuel coolant interaction problem are investigated: (1) the effects of gas/vapor blanketing of the fuel on post-fragmentation generation of pressure and mechanical work, and (2) the mechanism of the fragmentation of the molten fuel as it contacts the coolant. A model developed at Argonne National Laboratory to analyze fragmentation-induced energetic fuel-coolant interactions is modified to allow for gas/vapor blanketing of the fuel. The modified model is applied to a. hypothetical accident involving an FFTF subassembly. The results indicate that high shock pressures are not necessarily precluded by gas/vapor blanketing of the fuel. However, the generation of mechanical work is greatly reduced. A model is developed to simulate the dynamic growth of the vapor film around a hot spherical particle which has been suddenly immersed in a coolant. The model is applied to various cases of hot spheres in water and in sodium. A fragmentation mechanism based on the ability of the pressure pulsations of the vapor film to induce internal cavitation in the molten material is shown to predict the reported fragmentation behavior of drops of several hot molten materials in water and sodium.