Saturated Pool Nucleate Boiling on Heat Transfer Surface with Deposited Sea Salts

Seawater was injected into the reactor cores following the accident at the Fukushima Daiichi nuclear power station. Saturated pool nucleate boiling heat transfer experiments with NaCl solution, natural seawater, and artificial seawater as well as distilled water were performed to examine the effects of salts on boiling heat transfer. The heat transfer surface was made of a printed copper circuit board. The boiling phenomena were recorded with a high-speed video camera. The surface-temperature distribution was measured with an infrared camera. In the experiments, the concentrations of the NaCl solutions and the artificial seawater were varied over a range of 3.5-10.0 wt.%. Boiling curves were well predicted with the Rohsenow correlation although large coalescent bubble formation was inhibited in the NaCl, natural seawater, and artificial seawater experiments. Deposits of calcium sulfate (CaSO4) on the heat transfer surface were observed in the experiments with artificial seawater. This formation of a deposit layer resulted in the initiation of a slow surface-temperature excursion at a heat flux lower than the usual critical heat flux (CHF). A unique relationship was confirmed between the salt concentrations of the artificial seawater in the bulk fluid and the vaporization rate at the surface at which the slow surface-temperature excursion initiated. This relationship suggested that if the bulk concentration of sea salts in the seawater exceeded 11 wt.%, the deposition of calcium sulfate on the heat transfer surface occurred even if the heat flux was zero.