Abstract: Diluting the core melt with oxide sacrificial material (OSM) during in-vessel retention (IVR) is a newly proposed severe-accident-management strategy of advanced LWRs. When severe accident occurs, the relocated core melt is diluted by OSM, resulting in the inverse stratification of melt pool configuration. Therefore, the researches on the calculations of heat transfer of melt pool are essential to optimize the component of OSM and evaluate the effectiveness of the strategy. The heat flux on the vessel outer surface after diluting the core melt with Fe₃O₄, TiO₂ and Al₂O₃ respectively was calculated. The results show that the structure of upper plenum will melt due to the intense heat radiation after OSM is used. The maximum heat flux on the vessel outer surface reduces with the increase of OSM. When the volume of the OSM ceramics reaches 15 m3, the reactor vessel length should be increased by approximately 2 m. In this case, the heat flux reduces about 45% compared to the reactors without OSM. Moreover, the reduction of heat flux is most pronounced as Fe₃O₄ is used. In addition, the melting point of UO₂-ZrO₂-OSM affects the appearance of a crust on the surface of the oxide mixture, which in turn influences the maximum heat flux on the vessel outer surface.