Study on Molten Core Behavior in Severe Accidents using Particle Method

采用颗粒法对熔融核-混凝土相互作用（MCCI）试验SWISS-2和MACE-MO进行了分析。在这两种情况下，熔池顶部都形成了地壳，而向上层水池的热量排出受到抑制。在两种情况下，池内的放热量和混凝土内的烧蚀速度计算值与实验值吻合较好。还分析了无限几何。上地壳形成，热释放也被抑制。然而，当地壳被溶解气体释放而破裂，旧地壳沉入熔池时，热量的释放增强。采用颗粒法对熔芯碎屑的形成进行了分析。这是一个在最初的研究计划中没有描述的附加主题。研究了单液滴熔体在水中的行为。忽略气膜，我们可以在不发生相变的情况下再现实验中得到的临界韦伯数。如果考虑气膜，则临界韦伯数增加，液滴尺寸增大。碎石床的临界热流密度为IMW/m^2，显示出足够的衰变排热能力。采用考虑水、蒸汽、熔体液滴和破片的一维有限体积法分析了蒸汽爆炸中压力波的传播过程。在以往颗粒法研究的基础上，建立了熔体液滴的热破碎模型，并将其纳入程序中。热破碎时的机械能转化率达到5%，而水动力破碎时的机械能转化率仍低于1%。在大规模蒸汽爆炸中，水动力破碎占主导地位，因此采用1%作为转化率比较合适。如上所述，目前的研究进展超出了最初的目标，并达到了目标。

Test experiments, SWISS-2 and MACE-MO, of molten core-concrete interaction (MCCI) were analyzed by a particle method. In both cases, crust was formed at the top of the melt pool and the heat removal to the upper water pool was suppressed. The heat removal to the water pool and the ablation speed in the concrete agreed well between the calculations and the experiments in two cases. An unlimited geometry was also analyzed. The upper crust was formed and the heat removal was suppressed as well. However, the heat removal was enhanced when the crust was broken by the dissolved gas release and the old crust sank in the melt pool.Formation of the molten core debris was analyzed by the particle method. This is an additional subject which had not been described in the initial research plan. Behavior of a single droplet of the melt in water was investigated. Neglecting the vapor film, we could reproduce the critical Weber number which had been obtained in experiments without the phase change. If the vapor film was considered, the critical Weber number increased and the droplet size was enlarged. The critical heat flux of the debris bed was assessed as IMW/m^2, which showed enough capabilities of the decay heat removal.Pressure wave propagation processes in vapor explosions were analyzed using a one-dimensional finite volume method considering water, steam, melt droplets and fragments. Based on the past studies using the particle method, a thermal fragmentation model for the melt droplets was developed and incorporated into the code. The mechanical energy conversion ratio achieved 5 % when the thermal fragmentation took place, while it remained less than 1% when the hydrodynamic fragmentation took place. In large-scale vapor explosions, the hydrodynamic fragmentation becomes dominant, so that 1 % is proper to employ as the conversion ratio.As described above, the present study progressed beyond the initial goal as well as reached it.