The impact of thermal chemical phenomena on the heat fluxes into the RPV during in-vessel melt retention

In a severe accident, one possible strategy to prevent basemat melt-through is to flood the outside of the RPV (Reactor Pressurized Vessel) and to stabilize the molten core inside the vessel. In-Vessel Retention (IVR) is effective if the heat fluxes on the water-cooled outside of the vessel permanently stay below related CHF-limits. The prediction of the underlying heat flux profiles is typically based on the assumption of a stratified molten pool with a metal layer on top and natural convection-driven heat transfer inside and between the layers as well as to the vessel walls and the free surface. This paper summarizes the results AREVA NP assessment of thermo-chemical phenomena with respect to their impact on IVR. Among the analyzed phenomena, the formation and relocation of a dense metallic phase, the growth of a refractory surface crust, and the increase in the fission product inventory of the metal phase were identified as the most relevant ones. Among these the potential risk related with a dense metal layer was found to be highest, because of, first the reduction of the light metal layer thickness and the related aggravation of the focusing effect and secondly the high superheat of the dense metal (inside the oxidic pool) and the possibility of its transient thermal contact with the vessel wall in cases of: i) a local failure of the oxidic crust that protects the vessel wall, or ii) a 'late' rise of the superheated metal back to the surface. The potential impact of the second issue, transient thermal loads on the vessel, is considered to be potentially even higher but its relevance can not be evaluated because appropriate experimental data are missing.