In-Vessel Retention (IVR) strategy is one of the most important severe accidents management strategies. When severe accident occurs, cooling water will be injected into the reactor pit to conduct the External Reactor Vessel Cooling (ERVC) for the outer wall surface of Reactor Pressure Vessel (RPV).The cooling water in the reactor pit continuously takes away the corium decay heat inside the reactor through the boiling heat transfer mode on the wall surface of the RPV, preventing the lower head of the RPV from being melted through and maintaining the integrity of the RPV. Considering the research and development needs of high-power reactors and the fact that the local Critical Heat Flux (CHF) values obtained from current experiments are gradually approaching the heat flux of the lower head, this paper focuses on exploring new CHF enhancement measures that can be applied to the outer wall surface of RPV lower head. The typical enhancement means "flow disturbance" and "surface treatment" have been taken as the research objects, and “surface grooves” and “twisted belts turbulators” structure has been designed and the improvement effects of different structure on flow boiling CHF value has been studied by experiments. The experimental results have shown that both the “surface grooves” structure and the “twisted belts turbulators” structure can effectively increase the CHF. Compared with the conventional structure, the “surface grooves” structure can increase the CHF value by ~ 50%, and compared with the structure without turbulators, the “twisted belts turbulators” structure can increase the CHF value by ~ 14%.