Abstract: In-vessel retention (IVR) of molten corium through external reactor vessel cooling (ERVC) is one of the most important severe accidents management strategies. In this paper, a two dimensional full scale facility FIRM was built to investigate critical heat flux (CHF) behaviors under severe accident conditions. CHF limits of RPV lower head in the process of ERVC were investigated with deionized water as working fluid. The influence of real surface material SA508 Gr3. Cl.1 steel and its mechanism were investigated. The aging effect of SA508 Gr3. Cl.1 steel in deionized water was discussed. The test result will provide a profound understanding of CHF behaviors with real RPV material under IVR-ERVC condition and will be of significance to improve the safety of reactor vessel.