2 This paper presents a numerical investigation of two-phase natural circulation flows established when 3 external reactor vessel cooling is applied to a severe accident of the APR1400 reactor for the in-vessel 4 retention of the core melt. The coolability limit due to external reactor vessel cooling is associated with 5 the natural circulation flow rate around the lower head of the reactor vessel. For an elaborate prediction 6 of the natural circulation flow rate using a thermal-hydraulic

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... m code, MARS-KS1.5, a three-7 dimensional computational fluid dynamics (CFD) simulation is conducted to estimate the flow rate and 8 pressure distribution of a liquid-state coolant at the brink of significant void generation. The CFD 9 calculation results are used to determine the loss coefficient at major flow junctions, where substantial 10 pressure losses are expected, in the nodalization scheme of the MARS-KS code such that the single-11 phase flow rate is the same as that predicted via CFD simulations. Subsequently, the MARS-KS analysis 12 is performed for the two-phase natural circulation regime, and the transient behavior of the main 13 thermal-hydraulic variables is investigated. 14 15 J o u r n a l P r e -p r o o f 3