The Japan Atomic Energy Agency has announced that defueling has been completed at the Monju prototype spodium-cooled fast breeder reactor (FBR) in Tsuruga City, Fukui Prefecture. The work started on 16 August, and a total of 530 fuel assemblies have now been transferred to a storage pool. These include 160 fuel assemblies being held within a sodium-filled storage tank when decommissioning of the reactor began in 2018. The other 370 assemblies remained in the sodium-filled core. These were first moved into the sodium-filled tank before being moved to the storage pond.
This completed phase 1 of the reactor decommissioning, JAEA said. A detailed four-stage decommissioning plan. submitted to the Nuclear Regulation Authority (NRA) in December 2017, was approved in March 2018. In the second and third stages, the liquid sodium coolant will be extracted from Monju and related equipment will be dismantled. The reactor building is to be demolished and removed by fiscal 2047 in the final stage.
"The used fuel will be transferred to domestic and foreign operators with licenses for reprocessing in Japan or in countries with which Japan has signed agreements for cooperation on the peaceful uses of nuclear energy," JAEA noted. In January, Cavendish Nuclear and Jacobs signed a Memorandum of Understanding with JAEA for the treatment and re-use in the UK of sodium coolant from the Monju reactor.
The decommissioning of Monju will take 30 years and cost more than JPY375 billion ($2.5bn), according to government estimates. This includes JPY225 billion for maintenance, JPY135 billion for dismantling the plant and JPY15 billion for defuelling and preparations for decommissioning.
The 280 MWe Monju, named after a Buddhist deity of wisdom, was initially considered a "dream reactor" because of its ability to produce more nuclear fuel than it consumes, but a series of accidents eventually led to cancellation of the project. Monju was shut down in 1995, just four months after it began operation when about 700kg of liquid sodium leaked from the secondary cooling loop. Although there were no injuries and no radioactivity escaped plant buildings. JAEA tried to conceal the scale of the damage.
Monju restarted in May 2010 but refuelling equipment fell into the reactor vessel during a refuelling outage later that year, and it has not operated since then. Although the equipment was retrieved and replaced, NRA did not allow the reactor to restart. In November 2015, following concerns over lax equipment inspections, NRA determined JAEA was not competent to operate the reactor, and in December 2016, the government decided to decommission the reactor. The JPY1000bn ($9bn) Monju had operated for only 250 days since its start-up in 1994.
Image: The Monju FBR (courtesy of Mainichi)
