Abstract In-Vessel Retention (IVR) of corium is one of the possible strategies for Severe Accident (SA) mitigation. Its main advantage lies in the fact that, by maintaining the corium within the vessel, it preserves the last containment barrier from corium aggression. One of the issues for the demonstration of the success of this strategy is the evaluation of the behaviour of the corium relocated in the lower head and how it stabilizes and affects the integrity of the vessel wall. The first modelling was developed in the nineties and assessed the heat transfers in a stratified corium pool with a top metal layer made of steel and Zirconium only. About 10 years later, the results of the MASCA program highlighted the possibility of having more complex stratified configurations, including a dense metal layer. Addressing thermochemical effects in the stratification makes the modelling of the corium pool in the lower head more difficult and, in addition, knowledge of the associated kinetics is still limited. As a consequence, available SA codes, either integral or dedicated to the lower head, can differ significantly in their models, which leads to discrepancies in the results when evaluating the IVR strategy. In order to identify the main modelling issues and to assess the capabilities of the codes, a benchmark exercise for code validation was made in the scope of the European H2020 project IVMR (In-Vessel Melt Retention). It is based on the definition of different IVR configurations at reactor scale with an increase in the complexity of the phenomena involved: starting from a steady-state stratified pool with metal on top, up to consideration of corium phase separation at thermochemical equilibrium and progressive ablation of metallic structures and vessel wall. As a last step, the more challenging transient configuration, with formation of a metal layer heavier than the oxide followed later on by stratification inversion, was also studied. It should be noted that mechanical resistance of the ablated vessel wall and cooling conditions out of the RPV in the cavity are not evaluated and considered out of the scope of this benchmark focused on the thermal load transferred from the corium pool through the vessel wall. Six organisations took part in this benchmark (CEA, EDF, GRS, IBRAE, IRSN, NRC-KI) and 6 different codes were used (ASTEC, ATHLET-CD, MAAP_EDF, PROCOR, HEFEST_URAN and HEFEST – stand-alone version of the corresponding module of the SOCRAT code). The main results and outcomes obtained are presented and discussed in this paper. Sensitivity studies have also been performed and allow obtaining a more consolidated range of results. Thanks to this benchmark exercise and to the approach followed with a progressive increase of complexity, the capabilities of codes to evaluate the heat flux profile applied to the vessel wall in steady-state are demonstrated. Then, for transient configurations, it is shown that different modelling approaches give rather consistent results since dispersion remains limited between code predictions, even in the most challenging configuration with stratification inversion: ±40% for the minimum vessel thickness (2.5 cm ± 1 cm). In addition, all codes predict that transient effects lead to more vessel ablation than in the final steady-state. Hence, the importance of the consideration of the progressive molten steel incorporation in the pool and of the consideration of thermochemical equilibrium to calculate the oxide and metal phases composition was highlighted in the benchmark. Regarding the dispersion of the results obtained, analysis shows that uncertainties on metal layer properties have a significant impact. In addition, differences in modelling assumptions were identified and discussed. The main issues are related to the following physical phenomena: (i) The interaction of the oxide crust with the molten steel; (ii) The kinetics of stratification inversion; (iii) The heat transfer in thin metal layer.

中文翻译：

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在欧洲 IVMR 项目中执行的 IVR 代码基准测试的主要结果

摘要 血管内保留 (IVR) 是缓解严重事故 (SA) 的可能策略之一。它的主要优点在于，通过将真皮层保持在容器内，它可以保护最后的密封屏障免受真皮层的侵袭。证明该策略成功的问题之一是评估重新定位在下头部的真皮的行为以及它如何稳定和影响血管壁的完整性。第一个模型是在 90 年代开发的，并评估了分层真皮池中的热传递，顶部金属层仅由钢和锆制成。大约 10 年后，MASCA 计划的结果强调了具有更复杂分层配置的可能性，包括致密的金属层。解决分层中的热化学效应使得下头部的真皮层建模更加困难，此外，对相关动力学的了解仍然有限。因此，可用的 SA 代码，无论是整体的还是专用于下头的，在它们的模型中都可能存在显着差异，这会导致评估 IVR 策略时的结果存在差异。为了确定主要的建模问题并评估代码的功能，在欧洲 H2020 项目 IVMR（船内熔体保留）范围内进行了代码验证的基准测试。它基于反应堆规模不同 IVR 配置的定义，并增加了所涉及现象的复杂性：从顶部有金属的稳态分层池开始，直到考虑到热化学平衡下的真皮相分离和金属结构和容器壁的逐渐消融。作为最后一步，还研究了更具挑战性的瞬态配置，即形成比氧化物重的金属层，随后进行分层反转。应该注意的是，烧蚀的血管壁的机械阻力和腔内 RPV 的冷却条件没有被评估和考虑不在本基准的范围内，重点是从真皮池通过血管壁传递的热负荷。六个组织参与了这个基准测试（CEA、EDF、GRS、IBRAE、IRSN、NRC-KI）并使用了 6 个不同的代码（ASTEC、ATHLET-CD、MAAP_EDF、PROCOR、HEFEST_URAN 和 HEFEST——相应的独立版本SOCRAT 代码的模块）。本文介绍并讨论了所获得的主要结果和成果。还进行了敏感性研究，并允许获得更综合的结果范围。由于这个基准练习和随着复杂性逐渐增加所遵循的方法，代码在稳态下评估应用于容器壁的热通量分布的能力得到了证明。然后，对于瞬态配置，表明不同的建模方法给出了相当一致的结果，因为代码预测之间的离散度仍然有限​​，即使在具有分层反演的最具挑战性的配置中：最小血管厚度（2.5 cm ± 1 cm）为 ±40% . 此外，所有代码都预测瞬态效应会导致比最终稳态更多的血管消融。因此，基准中强调了考虑将钢水逐步掺入池中以及考虑热化学平衡以计算氧化物和金属相组成的重要性。关于所得结果的分散性，分析表明金属层特性的不确定性具有显着影响。此外，还确定并讨论了建模假设的差异。主要问题与以下物理现象有关： (i) 氧化皮与钢水的相互作用；(ii) 分层反转的动力学；(iii) 薄金属层中的传热。