Understanding of the rewetting behavior of heated vertical fuels pins is an important part of establishing reliable cooling of a PWR core following a loss of coolant accident (LOCA). Ahead of the quench front, complex and chaotic processes occur over a very small axial range, where high temperature gradients exist. In particular, repeated cyclic wetting and explosive evaporation at frequencies in the range 0.1 to 1.0 kHz is observed experimentally [1]. Despite extensive experimental and theoretical studies, the heat transfer mechanism in this region is still not well understood. In this paper we propose a mechanism, based on these observations, to explain the cyclical behavior. This postulated mechanism is transient near-surface cooling, followed by temperature recovery, of the metal substrate, with explosive vaporization when the homogeneous nucleation temperature is restored at the metal-water interface. A one-dimensional model of this cyclical process is constructed. The model indicates that the mechanism is plausible, predicting the observed periodic behavior, with predicted frequencies consistent with those observed in experiments.

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