Abstract
Abstract In-vessel retention（IVR）of core melt is a key severe accident management strategy adopted by advanced light water reactors such as AP600﹐AP1000. One of the viable means for In-vessel retention is external reactor vessel cooling （ERVC）which involves flooding of the reactor cavity during severe accidents. In this study﹐RELAP5-3D system thermal hydraulic code is used to simulate the behaviors of IVR design of AP1000 Pressurized Water Reactor. Both one-dimensional and multi-dimensional models of the code are adopted in the simulation. The results from 1-D and 3-D simulations are more or less identical. Nevertheless, it has been seen that counter flow pattern is developed in the coolant channel near the vertical part of the heated section. The results of RELAP5-3D simulations are substituted into ULPU,SULTAN and SBLB critical heat flux correlation to quantify the safety margin of the IVR design of AP1000. The results demonstrated that the critical heat flux ratio is lowest at the intersection of spherical and vertical part of the vessel. It has also been predicted that the heat removal capacity of AP1000 IVR is limited between 33 and 48.2 MW, which correspond to0.35 ~ 1.08 hours after shutdown. Key word：AP1000、IVR-ERVC