Abstract In this paper, a robust, accurate and efficient coupling between the PLEIADES/ALCYONE 2.1 fuel performance code and the OpenCalphad thermo-chemical solver is presented. A challenging 3.5D simulation of a power ramp, consisting of the 3D simulations of all the fuel pellets within a complete rodlet, serves as a base case to illustrate the difficulty inherent to extensive thermochemical equilibrium calculations (more than 1.5 million in this example) within fuel performance simulations, related both to the wide temperature and pressure range encountered in irradiated nuclear fuels. In the first part of this paper, a detailed analysis of the main quantities of interest (i.e. temperature, hydrostatic pressure, fission products concentration) is provided to illustrate the couplings at hand during a power ramp and, above all, to demonstrate the good agreement of the results with those already published in the literature and with available measurements of xenon, caesium and iodine releases. In the core of this paper, several numerical strategies are proposed to reduce the overall time spent in the thermochemical solver by defining at each node and for each time step a good initial estimate of the set of stable phases that are likely to form. The first one, referred to as the spatial strategy, relies on an intelligent reorganization of the mesh nodes as a function of temperature or hydrostatic pressure with an appropriate initialization of the thermochemical equilibrium calculation at a node by the phase composition calculated at the node with the closest thermo-mechanical conditions. The second strategy, referred to as the hybrid strategy, combines the spatial strategy during the power transient (where large power variations take place at each time step) with an initialization of the nodal equilibria from the solution obtained at the previous time step in case of small power variations. The 3.5D simulation has been run several times in order to assess and compare the spatial and hybrid strategies. The simulations show that the spatial strategy is the most efficient. It leads to an overall increase of the calculation time related to the incorporation of thermochemistry in the 3.5D fuel performance simulation of less than 2.25%, showing the feasibility of large thermal-chemical–mechanical simulations within the PLEIADES/ALCYONE 2.1 fuel performance code. Finally, as a first step towards more complicated simulations (i.e. including a greater number of phases and/or performed with a finer spatial discretization), a primitive parallelization algorithm based on a multiprocess approach is proposed. Combined with the most efficient spatial strategy, it leads to a substantial reduction of the extra (real) calculation time related to thermochemistry which become less than 0.65% of the total computation time when the thermochemical equilibrium calculations are distributed over 4 cores.

中文翻译：

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在 PLEIADES/ALCYONE 2.1 燃料性能代码和 OpenCalphad 热化学求解器之间开发强大、准确和高效的耦合

摘要 在本文中，提出了 PLEIADES/ALCYONE 2.1 燃料性能代码和 OpenCalphad 热化学求解器之间的稳健、准确和有效的耦合。一个具有挑战性的功率斜坡 3.5D 模拟，由一个完整的小棒中所有燃料芯块的 3D 模拟组成，作为一个基本案例来说明广泛的热化学平衡计算（在本例中超过 150 万）固有的困难燃料性能模拟，与辐照核燃料中遇到的宽温度和压力范围有关。在本文的第一部分，提供了对主要关注量（即温度、静水压力、裂变产物浓度）的详细分析，以说明在功率上升期间手头的耦合，最重要的是，证明结果与文献中已发表的结果以及氙、铯和碘释放的可用测量结果非常吻合。在本文的核心中，提出了几种数值策略，通过在每个节点和每个时间步长定义可能形成的一组稳定相的良好初始估计来减少在热化学求解器中花费的总时间。第一种称为空间策略，依赖于作为温度或静水压力函数的网格节点的智能重组，并通过节点处计算的相组成适当初始化节点处的热化学平衡计算最接近的热机械条件。第二种策略，称为混合策略，将功率瞬变期间的空间策略（在每个时间步长发生大功率变化）与在小功率变化的情况下从前一时间步长获得的解中初始化节点平衡。3.5D 模拟已运行多次，以评估和比较空间策略和混合策略。模拟表明空间策略是最有效的。它导致与在 3.5D 燃料性能模拟中加入热化学相关的计算时间总体增加不到 2.25%，表明在 PLEIADES/ALCYONE 2.1 燃料性能代码中进行大型热化学机械模拟的可行性。最后，作为迈向更复杂模拟的第一步（即 包括更多数量的阶段和/或使用更精细的空间离散化执行），提出了一种基于多进程方法的原始并行化算法。结合最有效的空间策略，当热化学平衡计算分布在 4 个核心时，它导致与热化学相关的额外（实际）计算时间显着减少，这些时间少于总计算时间的 0.65%。