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information about Fukushima published in English in Japanese media info publiée en anglais dans la presse japonaise

Unused plutonium deteriorating at Monju

December 8, 2015

Monju reactor fuel deteriorating as reactor sits unused



Plutonium fuel at the Monju fast-breeder reactor in Tsuruga, Fukui Prefecture, is deteriorating as the reactor sits unused, it has been learned from the facility's manager, the Japan Atomic Energy Agency (JAEA).

Over the 20 years since a sodium leak on Dec. 8, 1995, the reactor has barely been run, and part of its plutonium has converted to americium-241, which is difficult to break down in nuclear reactions. The agency says that currently the changed fuel does not present an obstacle to running the reactor, but if the reactor remains shut down it may have to prepare new fuel.

The Monju reactor breaks down nuclear fuel like plutonium to make electricity. Neutrons leave the broken-down plutonium, colliding with other plutonium and breaking it down, too. The continuation of this chain reaction keeps the nuclear reactions going.

However, americium absorbs neutrons and puts a brake on nuclear fission. Hideyuki Ban, co-leader of the Citizens' Nuclear Information Center, says, "If you have a growth in the amount of non-nuclear reacting americium in your fuel, then you can call that deteriorated fuel."

When the agency briefly restarted the reactor in 2010, it switched 117 of 198 fuel units for new ones due to parts of them having converted to americium. The agency says that an investigation at the time found that over 1 percent of the total weight of the reactor's fuel had changed into americium.

An agency representative says, "We believe we can currently run the reactor at full capacity." The representative also admits, though, "If the current shutdown of the reactor continues long-term (and the amount of americium grows), we will have to change some fuel for new fuel."

Construction work on the Monju reactor began in 1985, and in 1995 it began producing and sending electricity. However, after a sodium leak it was shut down. It was finally restarted in May 2010, but in August that year a fuel-exchanging device fell in the reactor, and the reactor was shut down again.

Even while offline, it costs around 22.3 billion yen a year to maintain it. The total amount spent on the reactor as of the end of March this year was around 1.17 trillion yen. According to the agency, it costs tens of millions of yen to produce a single unit of fuel. There are 42 backup fuel units for the plant, but if these units had to be changed when restarting the reactor, that could bring additional costs.

Meanwhile, the Nuclear Regulation Authority has recommended that the management of the reactor be changed from the JAEA. Whether or not the reactor can be restarted remains unclear.



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