The rewetting of PWR fuel cladding during post-LOCA reflood: a proposed physical explanation for the micro-scale high-frequency sputtering observed

Abstract

Following a loss of coolant accident in a light water reactor, reflooding of the core to quench overheating fuel is urgent. The process of quenching hot metal with liquid water has been much studied, in large part because of its great importance in this context. There is good experimental evidence such as quenching occurs via multiple micro-scale wettings of the surface, followed by vapour-driven ejection of liquid water. It is only after some tens of such wettings that permanent wetting seems to take place. In this article, we investigate this process, and put forward a physical mechanism that seems to explain this cyclical behaviour. The mechanism involves the need for some time to elapse following wetting of a hot body for heat to be conducted from within the depths of the body into the liquid water, such as to form a layer of water hot and deep enough to accommodate a critically sized bubble and thus cause ejection of the water film. Simple one-dimensional calculations using this model produce results that are in broad accord with experimental observations.