Vapor condensation from humid air on zinc-silicate epoxy-coated mild steel plate used in containment liners of nuclear power plants

Steam condensation plays a critical role as an accidental safety measure in the passive containment cooling system of a nuclear power plant. Zinc-silicate epoxy-coated mild steel plate is widely used for steel containment or as liner material for steel lined concrete containment of nuclear reactor. The purpose of the coating is mainly to increase the corrosion resistance of mild steel liner. However, high condensation rate is of utmost importance to prevent large pressure and temperature buildups within the nuclear reactor in case of a loss of coolant accident (LOCA) and maintain necessary safety margin. In this work, the effect of the epoxy coating on mild steel liner during vapor condensation in humid air environment has been investigated. Both as-purchased, uncoated (bare) and epoxy-coated mild steel plates have been used as condenser surfaces. A wide range of heat fluxes are realized by carrying out condensation experiments both under natural and forced convection scenarios. It is found that the epoxy-coated condenser plates show lower contact-angle hysteresis and hence promote better droplet mobility and condensate drainage from the surface. However, measurements reveal that the overall condensation rate on the epoxy-coated surfaces is less than that on uncoated mild steel plate. Our study shows that the lower nucleation density coupled with the additional thermal resistance due to the coating thickness causes lower condensation rates on the epoxy-coated mild steel surface. Results of the study are relevant for benchmarking reactor thermal hydraulics codes for realistic simulations of containment pressure under accident scenarios.