In-vessel retention (IVR) through external reactor vessel cooling (ERVC) is an important severe accident mitigation strategy widely adopted in some advanced light water reactors, e.g. AP1000 and HPR1000. The reactor pressure vessel is supposed to be flooded externally by water, such that the decay heat could be efficiently removed and the radioactive corium materials retained inside the vessel. Lumped parameter models are used in engineering design to calculate the melt pool heat transfer and thus assess the effectiveness of IVR. In this methodology, a simple constant temperature is normally assumed for the ex-vessel surface accounting for the external water cooling. Since the RPV and cooling water are coupled in thermal behavior, modelling of the related heat transfer between them may be of interest. Under the PISAA framework, we developed the IVR module for the IVR analysis. In this paper, we further extend the code capability by considering the ex-vessel heat transfer. Possible conditions of the external cooling water e.g. pure liquid phase convection and boiling as well as the corresponding heat transfer correlations would be considered. The newly implemented model is then verified with acceptable results achieved. With this new feature, the coupled heat transfer analysis of the RPV and external cooling water is investigated. Compared with the original treatment with constant temperature boundary conditions, the new treatment with convective heat transfer affects the predictions of outer vessel temperature and residual wall thickness while the influence on the inner vessel temperature is negligible.

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