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Japan nuclear regulator says high level radioactive wastes could explode if safeties fail

24 December 2013

Stockpiles of unprocessed plutonium solution and liquid waste at a nuclear reprocessing facility in Tokai, Ibaraki Prefecture, could boil and spew radioactive substances or cause hydrogen explosions if safety devices were to fail, the Japanese Nuclear Regulation Authority (NRA) says.

According to Asahi Shimbun, this assessment was part of a report issued by the NRA in early December on Japan Atomic Energy Agency’s Tokai Reprocessing Plant, where 3.5 cubic metres of solution containing plutonium and 430 cubic metres of high-level radioactive liquid waste are stored.

The reprocessing plant extracts plutonium from spent nuclear fuel for use in plutonium-uranium mixed-oxide fuel for consumption in the Monju prototype fast breeder reactor and other facilities.

The plutonium solutions are supposed to be processed into mixed-oxide powder before going into storage, whereas the high-level radioactive liquid waste is supposed to be vitrified before going into storage. But the processing equipment for those operations has remained offline since 2007, partly because of anti-seismic retrofitting and partly due to equipment malfunctions.

NRA investigations showed that a loss of cooling functions could allow the liquid waste to boil and spew radioactive substances in 55 hours, while plutonium solutions could boil in 23 hours.
A loss of hydrogen removal functions could cause an explosion of hydrogen generated by the radiolysis of water in the liquid waste in 38 hours and a similar hydrogen explosion in the plutonium solutions in 11 hours, the NRA added.

The JAEA plans to take 18 months to process the plutonium solutions into 640 kilograms of mixed-oxide powder and 20 years to process the high-level radioactive liquid waste into vitrified products.

The processing equipment can be restarted only after clearing the government's new regulatory guidelines, which took effect on December 18.

In view of the risk involved in leaving the liquid waste unprocessed, however, the JAEA has requested a special exemption to allow the agency to operate the equipment even before it passes safety screenings.

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