在核电设计中仅考虑设计基准事故是不充分的，不足以确保人员、公众和环境安全。福岛事故表明，极端条件下核电厂发生了堆芯熔化和放射性物质泄漏的重大灾难。传统事故缓解策略中，通过反应堆压力容器（RPV）外部冷却将衰变热导出，实现堆芯熔融物在RPV内滞留（IVR），采用热失效准则，认为只要RPV内产生的热流密度小于外部带走的临界值，就不会产生失效破裂，但它没有考虑强温差应力与一次应力下RPV蠕变极限。实际上，高温度梯度下RPV器壁存在不同失效模式和多种因素作用，其相互作用机制十分复杂。极端条件下，传统策略不能确保RPV在72小时内保持压力边界完整性。为此，本项目试图对RPV结构以超高温蠕变和超应变变形为主的多失效模式及相应的极限状态等进行理论探索和试验研究，揭示大温度梯度下分层失效模式耦合作用，为极端条件下RPV结构完整性设计、分析、评估提供理论支撑与技术保障，提升我国核电厂在严苛事故下的安全性。

In the design of nuclear power plant(NPP),the mere consideration of design basis accident is inadequate, so the health and safety of the staff, public and environment can't be ensured. Fukushima accident showed that the core meltdown occurred, resulting in the disaster of radioactive material leak. In the traditional mitigation strategy of severe accident, the In-vessel retention (IVR) of molten core is assumed to be achieved by using the external cooling on decay heat within the RPV. In the traditional concept of IVR, thermal failure criterion is adopted, assuming that if the local heat flux within the RPV were lower than the critical heat flux (CHF) of the external coolant, the RPV rupture can be avoided. However, the creep limit of RPV isn't fully considered with the joint action of strong thermal stress and primary stress. In fact, the various failure modes exist within the RPV wall under the condition of high temperature gradient, and furthermore interaction among the various failure mechanisms is extremely complex. Under severe accident condition, the traditional IVR technology is unable to maintain the integrity of RPV pressure boundary within the first 72 emergency hours, that is to say, the RPV rupture is unavoidable accordingly. In solving it, the failure mechanisms and interaction among them are going to be explored regarding to the ultra temperature creep, ultra plastic strain, quenching brittle fracture, and corresponding load capability by using theoretical and experimental methodology. Through the research works, it will provide theoretical and technological support to the design, analysis, assessment on RPV structural integrity, and improve the RPV safety under severe accident scenario.