Advanced modeling and analysis are always essential for the development of safe and reliable nuclear systems. Traditionally, the numerical analysis codes used for nuclear thermal hydraulics and safety are mostly based on mesh-based (or grid-based) methods, which are very mature for well-defined and fixed domains, both mathematically and numerically. In support of their robustness and efficiency, they have been well-fit into many nuclear applications for the last several decades. However, the recent nuclear safety issues encountered in natural disasters and severe accidents are associated with much more complex physical/chemical phenomena, and they are frequently accompanied by highly non-linear deformations. Sometimes, this means that the conventional methods encounter many difficult technical challenges. In this sense, the recent advancement in the Lagrangian-based CFD method shows great potential as a good alternative. This paper summarizes recent activities in the development of the SOPHIA code using Smoothed Particle Hydrodynamics (SPH), a well-known Lagrangian numerical method. This code incorporates the basic conservation equations (mass, momentum, and energy) and various physical models, including heat transfer, turbulence, multi-phase flow, surface tension, diffusion, etc. Additionally, the code newly formulates density and continuity equations in terms of a normalized density in order to handle multi-phase, multi-component, and multi-resolution problems. The code is parallelized using multiple graphical process units (GPUs) through multi-threading and multi-streaming in order to reduce the high computational cost. In the course of the optimization of the algorithm, the computational performance is improved drastically, allowing large-scale simulations. For demonstration of its applicability, this study performs three benchmark simulations related to nuclear safety: (1) water jet breakup of FCI, (2) LMR core melt sloshing, and (3) bubble lift force. The simulation results are compared with the experimental data, both qualitatively and quantitatively, and they show good agreement. Besides its potential, some technical challenges of the method are also summarized for further improvement.

先进的建模和分析对于开发安全可靠的核系统始终至关重要。传统上，用于核热工水力和安全的数值分析代码主要基于基于网格（或基于网格）的方法，对于定义明确和固定范围的数学和数字方法，它们已经非常成熟。为了支持它们的鲁棒性和效率，在过去的几十年中，它们已经很好地适合许多核应用。但是，最近在自然灾害和严重事故中遇到的核安全问题与更为复杂的物理/化学现象有关，并且常常伴随着高度非线性的变形。有时，这意味着常规方法会遇到许多困难的技术挑战。在这个意义上说，基于拉格朗日CFD方法的最新进展显示了巨大的潜力，可以作为很好的选择。本文总结了使用平滑粒子流体动力学（SPH）（一种著名的拉格朗日数值方法）开发SOPHIA代码的最新活动。该代码结合了基本的守恒方程式（质量，动量和能量）和各种物理模型，包括热传递，湍流，多相流，表面张力，扩散等。此外，该代码还用新的公式表达了密度和连续性方程式为了处理多相，多分量和多分辨率问题，需要使用归一化的密度。通过多线程和多流使用多个图形处理单元（GPU）将代码并行化，以减少高计算成本。在优化算法的过程中，计算性能得到了极大的提高，可以进行大规模仿真。为了证明其适用性，本研究进行了与核安全有关的三个基准模拟：（1）FCI的水射流破裂，（2）LMR堆芯熔体晃动，以及（3）气泡升力。从定性和定量的角度将仿真结果与实验数据进行比较，并显示出良好的一致性。除了其潜力之外，还总结了该方法的一些技术挑战，以进一步改进。（3）气泡提升力。从定性和定量的角度将仿真结果与实验数据进行比较，并显示出良好的一致性。除了其潜力之外，还总结了该方法的一些技术挑战，以进一步改进。（3）气泡提升力。从定性和定量的角度将仿真结果与实验数据进行比较，结果表明吻合良好。除了其潜力外，还总结了该方法的一些技术挑战，以进一步改进。