Study on Molten Core Behavior in Severe Accidents using Particle Method

采用质点法对SWISS-2和MACE-MO熔融芯-混凝土相互作用试验进行了分析。在这两种情况下，外壳形成在熔池的顶部和热量转移到上层水池被抑制。在两种情况下，计算结果与实验结果吻合较好。还分析了无限几何。上地壳形成，热量的排出也受到抑制。溶解气体的释放使地壳破裂，老地壳沉入熔池中，增强了热量的排出。用粒子法分析了熔融核碎片的形成。这是一个额外的主题，在最初的研究计划中没有描述。研究了单滴熔体在水中的行为。在忽略汽膜的情况下，我们可以重现在没有相变的情况下实验得到的临界韦伯数。如果考虑汽膜，临界韦伯数增加，液滴尺寸增大。采用一维有限体积法，考虑水、蒸汽、熔滴和碎片等因素，分析了蒸汽爆炸中压力波的传播过程。在以往研究的基础上，采用颗粒法，建立了熔滴的热破碎模型，并将其纳入程序。当发生热破碎时，机械能转换率达到5%，而当发生水动力破碎时，机械能转换率保持在1%以下。在大规模蒸汽爆炸中，流体动力学破碎成为主导，因此1%作为转化率是合适的。如上所述，本研究的进展超出了最初的目标，并达到了它。

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.