In a severe accident of light water reactor (LWR), molten core material (corium) can be released into the wet cavity, and a fuel-coolant interaction (FCI) can occur. The molten jet with high speed is broken and fragmented into small debris, which may cause a steam explosion or a molten core concrete interaction (MCCI). Since the premixing stage where the jet breakup occurs has a large impact on the severe accident progression, the understanding and evaluation of the jet breakup phenomenon are highly important. Therefore, in this study, the jet breakup simulations were performed using the Smoothed Particle Hydrodynamics (SPH) method which is a particle-based Lagrangian numerical method. For the multi-fluid system, the normalized density approach and improved surface tension model (CSF) were applied to the in-house SPH code (single GPU-based SOPHIA code) to improve the calculation accuracy at the interface of fluids. The jet breakup simulations were conducted in two cases: (1) jet breakup without structures, and (2) jet breakup with structures (control rod guide tubes). The penetration depth of the jet and jet breakup length were compared with those of the reference experiments, and these SPH simulation results are qualitatively and quantitatively consistent with the experiments.

在轻水反应堆 (LWR) 的严重事故中，熔融堆芯材料 (corium) 会释放到湿腔中，并可能发生燃料-冷却剂相互作用 (FCI)。高速的熔融射流被破碎并破碎成小碎片，这可能导致蒸汽爆炸或熔融核心混凝土相互作用（MCCI）。由于射流破裂发生的预混阶段对严重事故的进展有很大影响，因此对射流破裂现象的理解和评估非常重要。因此，在本研究中，使用平滑粒子流体动力学 (SPH) 方法进行射流破碎模拟，该方法是一种基于粒子的拉格朗日数值方法。对于多流体系统，将归一化密度方法和改进的表面张力模型 (CSF) 应用于内部 SPH 代码（基于单 GPU 的 SOPHIA 代码），以提高流体界面处的计算精度。在两种情况下进行射流破碎模拟：（1）没有结构的射流破碎，和（2）有结构（控制棒导管）的射流破碎。射流的穿透深度和射流分裂长度与参考实验进行了比较，这些SPH模拟结果与实验在定性和定量上一致。