Abstract The strategy for In-Vessel Retention (IVR) of corium is currently considered for several new reactor designs in various countries. One of the issues for the demonstration of the success of this strategy is that there are significant uncertainties in physical modelling of corium in the lower plenum and its transient chemical and thermal interactions with the vessel leading to its significant ablation. Since the initial approaches developed in the nineties for the AP600 and Loviisa VVER-440 plant, knowledge about corium pools (thermochemistry and heat transfer characteristics) and mechanical behaviour of the vessel has improved and it is now possible to model more accurately this phenomenon. The H2020 European project IVMR (In-Vessel Melt Retention) addresses the issue of selecting and improving IVR models for safety evaluation. As a first step, a Phenomena Identification Ranking Table (PIRT) involving the relevant physical processes was developed. The methodology followed to build this PIRT results from the consideration of a few principles: (a) Identifying and separating the risks with respect to which the importance of a physical process is evaluated. (b) Defining physical processes or parameters which can be considered as independent of the other ones. (c) Avoiding expert judgement, as much as possible, and using, instead, the results of previous sensitivity studies to estimate the impact of each physical process or parameter. In order to obtain results representing the currently “shared” state of knowledge, code developers of the most widely used codes were consulted. It includes ASTEC, MELCOR, MAAP (EDF version), SOCRAT, ATHLET-CD integral codes and PROCOR, SIMPLE, HEFEST_URAN, and IVRSYS dedicated codes (specific to IVR calculations). One of the positive outcomes of this PIRT is that the results show significant tendencies, with distinct groups of phenomena or parameters. This allows the identification of the uncertainties and of the phenomena/variables with the highest (or lowest) importance. Some dispersion in the results could also be noticed but it can be understood either because of the variability due to reactor design or the impact of models chosen by experts in the codes they have developed. When the dispersion cannot be explained, it indicates that the phenomenon is poorly understood and may deserve further research. Among the phenomena with highest importance, the heat transfers in the top metal layer and the chemical/thermal interaction with the oxide crust have been identified, as well as the transient formation and stratification of metal and oxide layers, including thermochemical peculiarities of the (U,Zr,Fe,O) system. Another phenomenon of highest importance is the mechanical behaviour of the thin ablated vessel wall, where elasticity, plasticity and creep all play a role. It is concluded that codes should include improved models for those phenomena in order to be able to provide a reasonable assessment of potential for IVR.

中文翻译：

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为容器内滞留建模制定现象识别排名表 (PIRT)

摘要 目前，各国正在考虑在几种新的反应堆设计中采用容器内保存 (IVR) 的策略。证明该策略成功的问题之一是，下腔室中的真皮及其与容器的瞬态化学和热相互作用导致其显着消融的物理建模存在很大的不确定性。自从 90 年代为 AP600 和 Loviisa VVER-440 工厂开发的初始方法以来，关于真皮池（热化学和传热特性）和容器机械行为的知识得到了改进，现在可以更准确地模拟这种现象。H2020 欧洲项目 IVMR（容器内熔体保留）解决了选择和改进 IVR 模型以进行安全评估的问题。作为第一步，开发了一个涉及相关物理过程的现象识别排名表 (PIRT)。构建该 PIRT 所遵循的方法是基于以下几个原则的考虑： (a) 识别和分离评估物理过程重要性的风险。(b) 定义可被视为独立于其他过程或参数的物理过程或参数。(c) 尽可能避免专家判断，而是使用先前敏感性研究的结果来估计每个物理过程或参数的影响。为了获得代表当前“共享”知识状态的结果，咨询了最广泛使用的代码的代码开发人员。包括ASTEC、MELCOR、MAAP（EDF版）、SOCRAT、ATHLET-CD积分码和PROCOR、SIMPLE、HEFEST_URAN 和 IVRSYS 专用代码（特定于 IVR 计算）。该 PIRT 的积极成果之一是结果显示出显着的趋势，具有不同的现象或参数组。这允许识别不确定性和具有最高（或最低）重要性的现象/变量。也可以注意到结果中的一些差异，但可以理解这是由于反应堆设计引起的可变性或专家在他们开发的代码中选择的模型的影响。当色散无法解释时，表明对该现象知之甚少，可能值得进一步研究。在最重要的现象中，顶部金属层中的热传递以及与氧化物外壳的化学/热相互作用已被确定，以及金属和氧化物层的瞬态形成和分层，包括 (U,Zr,Fe,O) 系统的热化学特性。另一个最重要的现象是薄消融血管壁的机械行为，其中弹性、可塑性和蠕变都起作用。得出的结论是，代码应包括这些现象的改进模型，以便能够对 IVR 的潜力进行合理的评估。