In the Westinghouse design of Advanced Pressurized Water Reactor (AP-1000), a special design named In-Vessel Retention (IVR) is adopted to enhance the heat removal capability through outer wall of reactor vessel during a severe core melt accident. In the present study, RELAP5-3D system thermal-hydraulic code is used to simulate the natural convection flow within the water channel of IVR. The results of simulation are substituted into SULTAN, and SBLB Critical Heat Flux correlations developed specifically for this purpose to access the margin of IVR design of AP1000.

In the model, the cylindrical part of flow channel of IVR is represented by three-dimensional cylindrical components (r, Θ, z) of RELAP5-3D code. The semi-spherical shell of the flow channel is modeled by several inter-connected three-dimensional cylindrical components. Each component has different radius.

The results demonstrated that the critical heat flux ratio is lowest at the upper equator 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 between 0.35 and 1.08 hours after shutdown. The results demonstrated that IVR of AP1000 can terminate the progression of severe accidents if the attacks of vessel lower head starts about 70 minutes after the initiation of the accident.

The results also show that the coolant velocity close to the insulated layer becomes negative (flow downward) near the top (φ = 70°) of the heated region. The same kind of flow reversal phenomena was observed in the experiments conducted by Chang and Jeong[1] and Jeong et al.[2] at KAIST.

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