This article deals with the assessment of severe accident event sequence and mitigation strategy elaboration performed in the frame of ASTRID SFR Gen IV reactor project (Le Coz et al., 2013), including the Japanese/French collaboration involving JAEA, MFBR, Framatome and CEA. In the first part of the paper, the mitigation strategy and the approach defined for the design of mitigation features, which are transverse tubes called DCS-M-TT are presented. By considering 21 DCS-M-TTs into the core as defined at the end of the conceptual design, which enable to extract the core materials and to significantly reduce the core reactivity within the order of 10 s in some typical accidental conditions, the accidental power excursions in ASTRID should not lead to a significant mechanical energy release, thus ensuring the integrity of the main vessel. Nevertheless, some loading assumptions independent from event evolutions have been defined in order to design the main vessel and the core catcher in order to cover extreme accident consequences and to get rid of event sequence dependency when designing the reactor. Then, SIMMER-IV (3D) the main calculation results of ULOF simulations whose goal was to verify the DCS-M-TT efficiency and to support a PIRT are highlighted in the paper. They confirmed that fuel discharge through discharge tubes has a large mitigating impact on power excursions. The last part of this paper deals with an approach dedicated to the description of severe accident event sequences through generic event trees (GETs), which logically summarize the representative courses of accident progression showing relationship between critical causal events and major outcomes of the accidents. A set of trees has been elaborated for the description of each event sequence able to cause severe accidents (ULOF: unprotected loss of flow, UTOP: unprotected transient overpower and USAF: unprotected sub-assembly fault). The event-tree bifurcation points towards one branch or another of the possible event evolution could be examined alternatively with a help of phenomenological event chart (PECs). Afterwards, in association with the GET analysis, a PIRT (Phenomenon identification ranking table) on ULOF has been developed conjointly by Japanese and French sides in order to prioritize R&D needs. About 100 phenomena have been retained and classified depending on the various time periods of the accident event sequences; the primary phase, the short-term relocation phase, the expansion phase and the long-term relocation and cooling phase.

本文涉及在ASTRID SFR第四代反应堆项目（Le Coz等人，2013）的框架内进行的严重事故事件序列评估和缓解策略的拟定，包括日本/法国的JAEA，MFBR，Framatome和CEA合作。在本文的第一部分中，提出了缓解策略和为缓解特征的设计而定义的方法，这些缓解特征是称为DCS-M-TT的横向管。通过在概念设计末尾考虑在芯中考虑21个DCS-M-TT，可以提取芯材料并在某些典型意外情况下在10 s的数量级内显着降低芯反应性， ASTRID中的偏移不应导致显着的机械能释放，从而确保主容器的完整性。然而，已经定义了一些独立于事件演变的载荷假设，以便设计主容器和堆芯捕集器，以涵盖极端事故后果并在设计反应堆时摆脱事件序列的依赖性。然后，论文重点介绍了SIMMER-IV（3D）ULOF仿真的主要计算结果，其目的是验证DCS-M-TT的效率并支持PIRT。他们证实，通过放电管排出的燃油对动力偏移有很大的减轻影响。本文的最后一部分介绍了一种通过通用事件树（GET）专门描述严重事故事件序列的方法，从逻辑上总结了事故发展的典型过程，表明了关键因果事件与事故主要后果之间的关系。为描述能够导致严重事故的每个事件序列，已经详细说明了一组树（ULOF：未保护的流量损失，UTOP：未保护的瞬态过功率和USAF：未保护的子组件故障）。可以通过现象学事件图表（PEC）替代地检查可能发生事件演变的一个分支或另一个分支的事件树分叉点。此后，与GET分析相结合，日本和法国方面联合开发了关于ULOF的PIRT（现象识别排名表），以优先考虑研发需求。根据事故事件序列的不同时间段，已保留并分类了约100种现象；主要阶段，短期搬迁阶段，扩张阶段以及长期搬迁和冷却阶段。