Effect of geometric protrusions and depressions on critical heat flux density of the heating surface

In the “Melted Reactor Retention and External Pressure Vessel Cooling (IVR-ERVC)” strategy for nuclear power plants, the Critical Heat Flux (CHF) serves as the critical limit for ensuring pressure vessel integrity. Traditional safety assessment procedures rely on an ideal flat surface assumption, neglecting macroscopic irregularities caused by corrosion and manufacturing defects during long-term operation. To address this issue, this study systematically investigated the impact mechanism of macroscopic irregularities on CHF using a self-developed saturated pool boiling experimental system and high-speed imaging technology. Results demonstrate that as the system approaches boiling crisis, macroscopic convex and concave defects reduce average CHF by approximately 27.4% and 16.7%, respectively. Mechanism analysis demonstrates that concave defects form a “steam trap, ”the vapor retention is attributed primarily to triple-phase contact line (TPL) pinning at the sharp machined edges, leading to bubble retention and localized drying; whereas convex defects impede bubble detachment through “mechanical obstruction,” resulting in premature CHF. Once validated under conditions more representative of IVR-ERVC, a “surface degradation penalty coefficient” may be considered in future nuclear safety analyses as a possible conservative option.