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Yuki, Kohei*; Horiguchi, Naoki; Yoshida, Hiroyuki; Yuki, Kazuhisa*
Mechanical Engineering Journal (Internet), 12(4), p.24-00451_1 - 24-00451_8, 2025/08
Fuel debris at the Fukushima Daiichi nuclear power station is typically cooled under immersion. However, an unexpected significant drop in water level results in coolant contact with high-temperature fuel debris having porous structure. In such scenarios, rapid cooling is essential, yet the thermal behavior at the liquid-solid interface, including capillary phenomena, is not well understood. This paper presents basic research evaluating the evaporation characteristics of droplets upon contact with metallic porous media featuring small pores under 1 mm. We conducted experiments using bronze or stainless steel porous media with pore diameters of 1, 40, or 100 
m to derive lifetime curves for droplets. Our findings indicate that Leidenfrost effect is mitigated on porous surfaces as the vapor can escape through the pores. Moreover, in bronze porous media, as the temperature increases, oxide film with a fine structure facilitates capillary action. In contrast, the low wettability of stainless steel porous media prevents capillary action, inhibiting droplet absorption and dispersion into the pores. Consequently, rapid cooling via the capillary action is unlikely if the fuel debris shares similar characteristics with steel porous media. Therefore, for risk management, the cooling system should be established assuming that capillary force does not act in the fuel debris.
Yuki, Kohei*; Horiguchi, Naoki; Yoshida, Hiroyuki; Yuki, Kazuhisa*
Proceedings of 31st International Conference on Nuclear Engineering (ICONE31) (Internet), 4 Pages, 2024/11
Fuel debris in the Fukushima Nuclear Power Station is cooled under immersion condition. However, in the event of an unexpected decrease in water level, coolant contacts high-temperature fuel debris having porous structure. In this event, although fuel debris needs to be cooled rapidly, thermal behavior at liquid-solid contact, such as capillary phenomenon, remains unclear. In this paper, as basic research, we evaluate droplet evaporation characteristics after contact with metal porous media with small pores less than 1 mm. In experiment, to obtain life time curve of a droplet, bronze or stainless steel porous media having 1, 40, or 100 m pore diameter are utilized. Experimental results show that Leidenfrost phenomenon is suppressed on the porous surfaces because generated vapor can be discharged from the pores. Further, for bronze porous media, capillary phenomenon is observed as the temperature of the porous media increase because of generation of oxide film having fine structure. On the other hand, due to low wettability of stainless steel porous media, capillary phenomenon does not occur, and the droplet was not sucked and spread into pore. This indicates that rapid cooling by the capillary phenomenon can not be expected if fuel debris has the same characteristics as the stainless steel porous media.
