Monju reactor decommissioning begins

Following the Japanese government’s decision in December 2016 to scrap the infamous Monju prototype fast breeder reactor (FBR) in Tsuruga City, Fukui prefecture, work has started to remove fuel assemblies from a sodium-filled storage tank, the Japan Atomic Energy Agency (JAEA) has announced. Once all these assemblies have been placed in a water-filled pool, removal of assemblies from the reactor itself will begin.

The JAEA had submitted a detailed plan to decommission Monju, in line with the government’s basic policy, to the Nuclear Regulation Authority (NRA) in December 2017. The plan comprises four stages. In the first stage, the agency will move all fuel to an on-site storage pool by 2022. In the second and third stages, the liquid sodium coolant will be extracted from the reactor and related equipment will be dismantled. The reactor building will be demolished and removed by the 2047 fiscal in the final stage. The authority approved the plan in March 2018.

A key part of Japan’s nuclear energy programme, the 280 MWe Monju FBR began operations in 1994. However, it was shut down only four months later after about 700 kilograms of liquid sodium leaked from the secondary cooling loop. Although there were no injuries and no radioactivity escaped plant buildings, the operator attempted to cover up the scale of the damage.

The reactor was eventually restarted in May 2010 but has not been operated since refuelling equipment fell into the reactor vessel during a refuelling outage later that year.

The equipment was subsequently retrieved and replaced, but the NRA did not permit the reactor to restart.

In November 2015, following concerns over equipment inspections, the NRA determined that the JAEA was not competent to operate the reactor, following which the decision to scrap it was taken by the government.

The JAEA said the sodium would be washed away from the removed assemblies before they were placed in the new storage pool. The plan is to remove one assembly a day, and the agency expects to move 100 assemblies from the storage tank to the pool by the end of this year.

It also plans to start extracting some 760 tonnes of sodium from the reactor’s secondary cooling system by the year end. This will be placed in the storage tank for later disposal. Some 960 tonnes of sodium from the reactor and the primary circuit will be removed at a later stage. Work to remove the 370 fuel assemblies from Monju’s sodium-filled core is scheduled to begin next year.

The decommissioning of Monju will take 30 years and cost more than $3.7 billion, according to government estimates.