Since first Nuclear Power Plant (NPP) in operation in early 1990s, China has built dozen of Light Water Reactor (LWR). Originated from Pressure Water Reactor (PWR) technology, almost all these plants, such as Qinshan, Dayawan, and Tianwan, were designed with a Pre-stressed Concrete Containment Vessel (PCCV). With the application of Third Generation (GEN III) reactor technology in recent years China is designing and building more Gen III NPPs. As the first kind of passive safety GEN III + technology, AP1000 uses double-layered containment structures comprising steel containment and shield building. GEN III EPR use similar double-layered containment structures, the outer layer is reinforced concrete shield structure, the inner layer is PCCV. In the traditional containment design, the design load and load combinations are determined based on different NPP operation conditions. This means the basic design principles of defense-in-depth and single failure criteria are incorporated into containment structure design through the consideration of all operation-related design basis loading conditions. However The Indian Ocean tsunami and the Fukushima earthquake and tsunami followed by the Fukushima Daiichi accident illustrated the hidden potential vulnerability in current NPP design when subjecting to Beyond Design Basis Event (BDBE) loading conditions. Containment, as the final physical shielding barrier to protect plant from potential radioactive releasing in accidental events, shall be designed to be not only functional during design basis accident conditions, but also be able to perform its containment and shielding functions, maintain the integrity of pressure boundary during BDBE and their loading conditions.
Aiming on containment and shield structure design for GEN III NPPs, this paper summarized preliminary research results on BDBE design. Considering the occurrence frequency and impact severity to containment structure, Beyond Design Basis Seismic (BDBS) is taken as the investigation focus in this paper. The summary is based on an investigation of the BDBS loadings and their impact to safety-related nuclear containment and shielding structures of the NPPs. The purpose of this investigation is to understand the failure pattern and modes under BDBS loads; and how such failure impact the safety features of the plant and the safety functions of the systems (e.g. the impact to passive safety functions). This paper also covers ways to reduce the impact and mitigate the consequences of BDBS impact by pre-incorporated measures in the structural design.