In-vessel retention (IVR) is a widely adopted severe accident mitigation strategy in the advanced commercial PWRs. External water cooling on the outer surface of the reactor pressure vessel (RPV) is designed to efficiently remove the decay heat from the in-vessel corium, such that the RPV integrity can be kept and the radioactive corium retained inside. The effectiveness of the IVR strategy requires that the maximum heat flux along the RPV lower than the critical heat flux (the switch point from nucleate boiling to film boiling). In existing engineering design, water cooling is realized by flooding the RPV outer surface. Spray cooling on the outer vessel surface is considered to be a potential method to increase the safety margin. In this paper, we investigate the effect of spray cooling in the IVR applications with respect to the influences on the heat transfer and cooling capacity (limit). Molten pool heat transfer analysis is performed to analysis its influence on the thermal behavior of the melt pool and RPV. The cooling capability (limit) is also analyzed between the spray cooling and conventional cooling measurements. Results indicate that the spray cooling does not show significant influence on heat removal as the changes of the heat flux profile and the wall thickness (that are of interest in safety analysis) are insignificant; the main contribution to the enhancement of the safety margin by spray cooling would be from the expected significant improvement of critical heat flux (CHF).

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