News Navigator: How is spent nuclear fuel cooled without water? via Mainichi

With air-cooled dry casks recently receiving attention as a means of storing spent nuclear fuel, the Mainichi answers common questions readers may have about this and other ways in which nuclear waste is stored.

Question: How is spent nuclear fuel from nuclear power plants stored?

Answer: Spent nuclear fuel continuously emits heat, so it has to be cooled. At many nuclear plants, it is kept in pools of water.

With dry storage, however, the fuel is stored in a cask made of a material like metal or concrete. Pools for spent fuel require electricity so a pump can circulate the water, but with dry storage, the natural flow of air is used to cool the containers, and no electricity is needed.

Before dry casks can be used, however, the fuel has to be cooled in pools of water for seven to 10 years. In Japan, resin that blocks radiation is applied to the inside of metal casks, and they are filled with helium gas to prevent fires and corrosion. The surface of the cask holding the fuel will range from 30 to 60 degrees Celsius, depending on the surrounding air temperature. The fuel inside does not melt, and the surface radiation is only 5 to 12 microsieverts per hour — low enough for the containers to be held with bare hands without problem.

Q: Why is dry cask storage receiving attention now?

A: Originally it was seen as a way to free up space in the cooling pools because, as nuclear plants were operated, spent fuel accumulated and the pools were coming close to filling up. Now, it is receiving attention because of the Fukushima No. 1 Nuclear Power Plant disaster triggered by the March 2011 Great East Japan Earthquake and tsunami. At the No. 4 reactor building, all nuclear fuel was in the cooling pool. When the building exploded, that pool was left exposed to the open air. For a time, cooling of the fuel ceased, and if the water had vaporized, the fuel could have melted. On the other hand, an air-cooled cask at the plant was fine despite the tsunami.

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