The model development for the gap cooling phenomena and its application to LAVA-4 test

The coolability of hot debris in the lower head filled with water during a severe accident has been an important issue concerning the plant safety. KAERI has launched the SONATAexperimental program to resolve the in-vessel retention issue and the LAVA series experiments have been performed to study the existence of initial gap and its effect on the cooling of hot debris. The objective of this study is to simulate the rapid cooling of debris and lower vessel wall occurred in LAVA-4 experiment using the MELCOR1.8.4 code. The current MELCOR code can not simulate the LAVA-4 experiment that has a gap between debris and inside wall. Therefore a gap-cooling model has been developed and implemented into the lower plenum model in MELCOR, resulting in good prediction of the temperature behavior of the vessel wall and the debris. The basic concept of the gap cooling model assumes that if the heat fluxes from the outer surface of the bottom-most debris to the inner-most wall are large enough to vaporizes the water within gap immediately, only the excessive heat from the debris reaches the inside vessel wall. It is also assumed that the gap exists over the transient from the beginning of debris relocation. The current ''gap model'' has a capability to simulate the phenomena such as prevention of water ingression into the gap due to rapid steam generation. The Wallis correlation determines whether the water can not penetrate into a gap or not. It is assumed that the gap is filled with steam when there is no water in the gap. In this case, the heat transfer from the debris to the inside wall through a steam filled gap occurs simultaneously by radiation and conduction. In this study, the radiation effect is considered when the outer surface temperature of debris reaches 1000 K. The current gap-cooling model needs to be evaluated sufficiently using the other experimental results concerning the gap formation and its cooling effect on the debris and wall. In order to make this gap cooling model useful in the safety analysis on NPP, an actual heat transfer mechanism over the gap boundary should be more identified under the condition of wide range of thermal hydraulic conditions and debris temperatures