Characteristics of droplet evaporation on high-temperature porous surfaces for estimating cooling time of fuel debris

At the Fukushima Daiichi nuclear power station, fuel debris is cooled under immersion. However, under an unexpected significant drop in water level, the water comes into contact with fuel debris that possesses a porous structure. In this scenario, rapid cooling of the fuel debris is essential. However, the cooling time has not been estimated, because the thermal behavior of high-temperature debris, including capillary phenomena, is not well understood. In the present study, the droplet evaporation characteristics on metallic porous media featuring pores smaller than 1 mm were investigated. To derive lifetime curves for droplets, the authors conducted experiments using 316L stainless steel and bronze porous materials with pore diameters of 1, 40, and 100 μm to establish droplet lifetime curves. The experimental findings indicate that the Leidenfrost phenomenon is mitigated on porous surfaces because the vapor escapes through the pores of the porous material. Further, as the temperature increases, an oxide film having a fine structure activates capillary action in bronze porous media. By contrast, a stainless-steel porous medium prevents capillary phenomena owing to its low wettability, inhibiting droplet absorption and dispersion into the pores. Consequently, if the fuel debris has similar characteristics to steel porous media, rapid cooling via the capillary action is unexpected. Finally, the cooling heat flux and cooling time of fuel debris are estimated according to experimental results. The estimation indicates that even in the case of stainless steel, the fuel debris can be cooled from 200 to 100 °C within 60 min under a low-decay heat condition. However, when the decay heat exceeds a certain value, the fuel debris cannot be cooled in either the stainless steel or bronze cases. Therefore, for risk management, the cooling system should be established for situations where the decay heat is substantial and simple spray cooling is not sufficient.