To retrieve the fuel debris in Fukushima Daiichi Nuclear Power Plants (1F), it is essential to infer the fuel debris distribution. In particular, the molten metal spreading behavior is one of the vital phenomena in nuclear severe accidents because it determines the initial condition for further accident scenarios such as molten core concrete interaction (MCCI). In this study, the fundamental molten metal spreading experiments were performed with different outlet diameters and sample amounts to investigate the effect of the outlet for spreading-solidification behavior. In the numerical analysis, the moving particle full-implicit method (MPFI), which is one of the particle methods, was applied to simulate the spreading experiments. In the MPFI framework, the melting-solidification model including heat transfer, radiation heat loss, phase change, and solid fraction-dependent viscosity was developed and implemented. In addition, the difference in the spreading and solidification behavior due to the outlet diameters was reproduced in the calculation. The simulation results reveal the detailed solidification procedure during the molten metal spreading. It is found that the viscosity change and the solid fraction change during the spreading are key factors for the free surface condition and solidified materials. Overall, it is suggested that the MPFI method has the potential to simulate the actual nuclear melt-down phenomena in the future.

为了回收福岛第一核电站 (1F) 的燃料碎片，推断燃料碎片的分布是必不可少的。特别是，熔融金属扩散行为是核严重事故中的重要现象之一，因为它决定了进一步事故场景的初始条件，例如熔融核心混凝土相互作用（MCCI）。在这项研究中，使用不同的出口直径和样品量进行了基本的熔融金属铺展实验，以研究出口对铺展-凝固行为的影响。在数值分析中，采用粒子方法之一的运动粒子全隐式方法（MPFI）来模拟扩散实验。在 MPFI 框架中，熔化-凝固模型包括传热、辐射热损失、相变、开发并实施了与固体分数相关的粘度。此外，在计算中再现了由于出口直径而导致的扩散和凝固行为的差异。模拟结果揭示了熔融金属扩散过程中的详细凝固过程。发现在铺展过程中粘度变化和固含量变化是自由表面状态和固化材料的关键因素。总体而言，表明 MPFI 方法具有模拟未来实际核熔化现象的潜力。模拟结果揭示了熔融金属扩散过程中的详细凝固过程。发现在铺展过程中粘度变化和固含量变化是自由表面状态和固化材料的关键因素。总体而言，表明 MPFI 方法具有模拟未来实际核熔化现象的潜力。模拟结果揭示了熔融金属扩散过程中的详细凝固过程。发现在铺展过程中粘度变化和固含量变化是自由表面状态和固化材料的关键因素。总体而言，表明 MPFI 方法具有模拟未来实际核熔化现象的潜力。