Sodium-cooled Fast Reactors (SFR) are investigated as future Generation IV reactor concepts. In SFR, the core configuration is not the most reactive during the nominal reactor operation, its geometry change or coolant voiding can induce a reactivity insertion. Within these considerations, mitigation devices should be implemented into the core in order to limit the thermal energy released into the fuel during a severe accident and thereby the possibility to induce mechanical loadings of the reactor structure when vaporized materials expand. Complementary safety devices called Transfer Tubes (TT) are studied in a French concept as an effective mitigation measure of severe accidents to reach a final reactor safe state, even in case of a failure of all shutdown systems including the passive shutdown safety rods. More precisely the TT, with their upper part located in and above the core, cross the core support structure and are linked to the core catcher zone situated in the main vessel bottom. Their purpose is to extract fissile materials from the core zone and secondly to favor the transfer of the molten fuel from the core region to the core catcher in case of severe core damage. The fuel discharge out of the core zone is necessary in order to lower the core reactivity, the relocation into the core catcher is necessary to enable the stabilization, the sub-criticality and the fuel cooling. An extensive work on severe accident calculations and phenomena identification related to fuel discharge through TT was performed. These studies represent the outcome of the work performed in 2014–2019, performed with collaboration of Japanese JAEA and MFBR and Framatome in the framework of the SFR project carried out by CEA. Calculations of fuel discharge were carried out with the mechanistic calculation code SIMMER and the main results are presented in this paper. Firstly, the whole unprotected loss of flow (ULOF) accident sequence was calculated with the SIMMER code. From these calculations, the power and flow evolution were set as boundary conditions for a more detailed model with six fuel assemblies around TT. This model was used to focus on key phenomena and to check impact of design parameter as local bottom restriction inside TT on fuel discharge way.

钠冷快堆（SFR）已作为未来的第四代反应堆概念进行了研究。在SFR中，堆芯配置在正常的反应堆运行期间不是最活跃的，它的几何形状变化或冷却液排空会引起反应性插入。考虑到这些因素，应在堆芯中安装缓解装置，以限制严重事故期间释放到燃料中的热能，从而在蒸发材料膨胀时引起反应堆结构的机械负荷。在法国的概念中，对补充安全装置“转移管（TT）”进行了研究，以有效缓解严重事故以达到最终反应堆安全状态，即使在所有关闭系统（包括被动关闭安全棒）发生故障的情况下也是如此。更确切地说是TT 它们的上部位于岩心中和岩心上方，穿过岩心支撑结构，并与位于主容器底部的岩心捕集区相连。它们的目的是从堆芯区域提取易裂变材料，其次是在严重堆芯损坏的情况下促进熔融燃料从堆芯区域转移到堆芯捕集器。为了降低堆芯反应性，有必要将燃料从堆芯区域中排出，需要重新定位到堆芯捕集器中，以实现稳定，亚临界和燃料冷却。在严重事故计算和与通过TT排放燃油有关的现象识别方面进行了广泛的工作。这些研究代表了2014-2019年开展的工作的成果，与日本JAEA，MFBR和Framatome的合作在CEA开展的SFR项目框架内进行。用机械计算代码SIMMER进行了燃油排放的计算，并在本文中给出了主要结果。首先，使用SIMMER代码计算整个无保护流量损失（ULOF）事故序列。从这些计算中，功率和流量的演变被设置为一个更详细的模型的边界条件，该模型在TT周围有六个燃料组件。该模型用于关注关键现象，并检查设计参数作为TT内部局部底部限制对燃料排放方式的影响。使用SIMMER代码计算了整个未保护的流量损失（ULOF）事故序列。从这些计算中，功率和流量的演变被设置为一个更详细的模型的边界条件，该模型在TT周围有六个燃料组件。该模型用于关注关键现象，并检查设计参数作为TT内部局部底部限制对燃料排放方式的影响。使用SIMMER代码计算了整个未保护的流量损失（ULOF）事故序列。从这些计算中，功率和流量的演变被设置为一个更详细的模型的边界条件，该模型在TT周围有六个燃料组件。该模型用于关注关键现象，并检查设计参数作为TT内部局部底部限制对燃料排放方式的影响。