To prevent HPME and DCH, SADV is proposed to be added to the pressurizer for Chinese improved 1000？MWe PWR NPP with the reference of EPR design. Rapid depressurization capability is assessed using the mechanical analytical code. Three typical severe accident sequences of TMLB’, SBLOCA, and LOFW are selected. It shows that with activation of the SADV the RCS pressure is low enough to prevent HPME and DCH. Natural circulation at upper RPV and hot leg is considered for the rapid depressurization capacity analysis. The result shows that natural circulation phenomenon results in heat transfer from the core to the pipes in RCS which may cause the creep rupture of pipes in RCS and delays the severe accident progression. Different SADV valve areas are investigated to the influence of depressurization of RCS. Analysis shows that the introduction of SADV with right valve area will delay progression of core degradation to RPV failure. Valve area is to be optimized since smaller SADV area will reduce its effect and too large valve area will lead to excessive loss of water inventory in RCS and makes core degradation progression to RPV failure faster without additional core cooling water sources. 1. Introduction In severe accidents, the reactor core is damaged and the molten core can relocate to the lower head of the reactor pressure vessel (RPV) while the pressure in the reactor coolant system (RCS) remains relatively high. With the failure of the lower head under these conditions, the molten core material is ejected to the cavity of the reactor vessel, which is generally called high pressure melt ejection (HPME). Furthermore, the temperature and pressure of the containment atmosphere will increase rapidly, which is generally called direct containment heating (DCH) [1, 2]. DCH could induce the containment atmosphere pressure and temperature increase rapidly, which would challenge the structure integrity of the containment [3]. It is necessary to ensure depressurization before primary system failure by a hardware modification or a procedural measure as prevention of HPME and DCH [4]. Safety Depressurization Systems (SDS) are adopted to enable feed and bleed operation to mitigate severe accidents for advanced light water reactors, such as ABB-CE System 80+, and the Korean standard nuclear power plants of Yonggwang 3 and 4 and Ulchin 3 and 4 [5], Korean Optimized Power Reactor (OPR) 1000 [6]. In Chinese 600？MWe PWR NPP, the extended function of the existing safety valves is planned to implement the intentional RCS depressurization as one severe accident management
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