Concrete ablation depth analysis method during top water flooding ex-vessel cooling by considering phenomena in dry and flooded conditions

This study proposes a method to analyze the concrete ablation depth during top water flooding ex-vessel cooling by considering the phenomena occurring in dry and flooded reactor cavity conditions. In the dry condition, it is assumed that there is radiation heat transfer between a melt and an upper concrete. In the flooded condition, it is assumed that dryout heat flux decreases and converges to a certain value as the thickness of porous bed in a crust increases. The thermal concrete ablation depth is obtained assuming that there is convection heat transfer to the underlying concrete and that it is an ideal insulator so no thermal conduction exists below the concrete melting surface. The present method also considers the phenomena such as decay heat, chemical reaction, formation and growth of the crust, and so on. Concrete ablation depth analyses during ex-vessel cooling are performed according to cavity flooding delay time using the present method and the CORQUENCH 3.03 code for a postulated severe accident. The code applying the method proposed in this study conservatively predicts the concrete ablation depth by 6.9 similar to 12% more than the results from the CORQUENCH 3.03 code. If the cavity flooding delay time is 1 h, the concrete ablation depth at the end of ex-vessel cooling from the present code is about 41 cm, however, the result from the CORQUENCH 3.03 code is 37 cm.