Development of the minimum film boiling temperature model during the reflood phase with the consideration of the hysteresis effect

Abstract

If a nuclear power plant enters a transient, the heat transfer regime at the nuclear fuel rod is altered to the inefficient heat transfer regime instead of the efficient heat transfer regime such as single-phase liquid convection and nucleate boiling regime. In particular, when the nuclear reactor undergoes a loss-of-coolant accident, the reactor core can be uncovered and leading to a significant temperature rise in fuel rods. Accordingly, the nuclear reactor has various strategies to reflood the reactor core. During the reflood phase, wall temperature should be suppressed under minimum film boiling temperature as fast as possible to maintain the efficient heat transfer regime. The accurate prediction of minimum film boiling temperature is imperative to predict the accident progression, so various correlations have been proposed to predict the minimum film boiling temperature. Both hydrodynamic models and empirical models were used to predict the minimum film boiling temperature. Since there is no consensus on which of these models is most accurate, each nuclear reactor safety analysis code uses a different correlation. Existing models predict the minimum film boiling temperature only with the state of fluid and wall at the moment of reaching the minimum film boiling temperature. These models do not consider the hysteresis in the Post-CHF heat transfer regime which was proven to be occurred by various studies. Within the Post-CHF heat transfer regime, the amount of heat transfer not only depends on the state of the fluid and the wall at that time but also on what state it has been through before. Therefore, the effect should be considered to predict minimum film boiling temperature. In this thesis, a minimum film boiling temperature model with the consideration of the hysteresis effect was developed. The model reflected the effect of the surface condition and the behavior of the vapor layer near the wall during the reflood. The developed minimum film boiling temperature model was validated with the reflood experiments using the nuclear safety analysis code, and it was confirmed that the reflood phenomenon is accurately predicted with the new model.