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In-Vessel Retention (IVR), which arrests relocated molten core materials in the vessel during severe accident, has been singled out as an appealing accident management approach to many reactors. The heat transfer imposed by in-vessel corium is a vital part for IVR success considering the difficulty of significantly altering ex-vessel CHF. For a given decay power, corium pool configuration determines the heat flux profile along the vessel wall, which may produce uncertainties associated with IVR strategy. In this paper, a thermodynamic tool is employed to study the corium pool configurations by analyzing the possible interaction among relocated corium, zircalloy cladding and core internals. The results reveal the immiscibility gap phenomena under high temperature which separates molten materials into oxidic and metal phase in the lower head. The oxidic phase is quite stable and its density is only slightly changed by various accident scenarios. The metal phase is relatively unstable and its density is susceptible to the condition of cladding oxidation degree and crust integrity. The corium pool configurations in the lower head are determined based on the results of thermodynamic analysis and phase density comparison. Both two-layer and three-layer corium pools are likely to be formed under different accident scenarios. CAP1400 has intentionally increased the mass of lower core support plate, which is a beneficial design change to prevent possible focusing effect if material infiltration through crust is assumed to be impossible.

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The In-containment Refueling Water Storage Tank (IRWST) provides low-pressure safety injection flow for passive CAP1400 Nuclear Power Plant (NPP) during Loss-Of-Coolant-Accident (LOCA) and subsequent Long Term Core Cooling (LTCC). The Passive Residual Heat Removal Heat Exchanger (PRHR HX) and the spargers of Automatic Depressurization System (ADS) stage 1∼3 are submerged in the IRWST. During small break LOCA, heat and mass are delivered through PRHR HX and ADS spargers to IRWST, and IRWST is heated up before its safety injection. However, numerical and experimental investigation has shown that IRWST is not perfect mixing, and thermal stratification exists. During ADS-4/IRWST initiation phase, the temperature of IRWST injection flow is of great importance, and is affected greatly by IRWST simulation method when modeling with system code like RELAP5. In this paper, two different IRWST simulation methods where one use multi channels in horizontal direction while the other use only one, are analyzed for CAP1400 SBLOCA with RE-LAP5, and their effects are compared. Finally, the better method which uses only one channel in horizontal direction is recommended.