The onsite investigations of Fukushima Daiichi Nuclear Power Plant (1F) Units-2 and 3 indicate possibilities of multiple breaches of the Reactor Pressure Vessels (RPVs). In the meantime, some analytical works indicate possibilities that the core materials of 1F Units-2 and 3 were once relocated to the lower plenum of the RPVs and cooled (quenched) before the water inventory boiled-off (dry out) and the once-cooled debris re-melted. This study utilizes Lagrangian-based Moving Particle Semi-implicit (MPS) method to investigate such complex solid-liquid multiphase re-melting and the debris-vessel wall interactions to obtain new insight on 1F Units-2 and 3 RPV failure modes to help the future debris retrieval from these reactors. Two major modifications/developments have been carried out based on the previously developed MPS method. Namely, stabilization of rigid-body contact model and the further improvement of the speedup algorithm to enable large and long scale debris-bed re-melting analyses of the real plant scale. The numerical accuracy of the pressure boundary condition at fluid-wall boundary has also been improved. Sensitivity analyses have been carried out with the developed new MPS method. The results indicate the possibility that the lateral part of the RPV may have been subject to not only convective heat transfer of metallic melt pool, but also by conductive heat transfer by oxidic debris conglomerate. The results also indicate that following the initial vessel breach, discharge of metallic melt induces relocation of oxidic debris conglomerates, leading to concentration of the heat source around the central (bottom) part of the lower head. However, large uncertainties associated with 1F are acknowledged and further model validations are necessary before drawing further insights.

福岛第一核电站2号和3号机组的现场调查表明，有可能多次违反反应堆压力容器（RPV）。同时，一些分析工作表明，将1F单元2和3的核心材料曾经重新安置到RPV的下增压室并冷却（淬火），然后将水库存蒸发掉（干and），然后一次将其冷却。冷却后的碎片重新融化。这项研究利用基于拉格朗日的移动粒子半隐式（MPS）方法研究这种复杂的固液多相重熔和碎片-容器壁相互作用，从而获得有关1F Units-2和3 RPV破坏模式的新见解，以帮助实现将来从这些反应堆中回收碎片。基于以前开发的MPS方法已进行了两个主要的修改/开发。即 刚体接触模型的稳定性和加速算法的进一步改进，以实现对真实工厂规模的大规模和大规模碎片床重熔分析。流体壁边界处压力边界条件的数值精度也得到了提高。灵敏度分析已使用新开发的MPS方法进行了。结果表明，RPV的侧部不仅可能受到金属熔池的对流传热，而且还可能受到氧化性碎屑团块的传导性传热的影响。结果还表明，在最初的容器破裂之后，金属熔体的排出会引起氧化碎屑砾石的重新定位，从而导致热源集中在下压头的中央（底部）周围。然而，