Japan’s Monju fast reactor prepares for decommissioning

3 September 2018

The removal of nuclear fuel from Japan’s Monju fast-breeder reactor in Tsuruga, Fukui Prefecture began on 30 August as the first step in a 30-year-long  decommissioning process. 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 the 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, the operator, the Japan Atomic Energy Agency (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, the Nuclear Regulation Authority (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 startup in 1994.

At the start of the work to remove the fuel, JAEA President Toshio Kodama, warned of the need to make every effort “to prevent trouble by paying very close attention”. He said: "The real work is ahead of us. Safe and steady progress will win the trust of the local community." Removal of the 530 fuel rods from the core and the storage pool will take until at least fiscal year 2022. Maintaining the safety for workers is seen as one of the main challenges of the decommissioning process.

JAEA said work had begun to initially remove the 160 fuel assemblies stored within the sodium-filled storage tank, following completion of simulated training between 19 and 28 August. The team of workers began to remove the fuel one rod at a time using the specially designed remote-controlled equipment. Each rod, 4.2 meters long and 10 centimetres thick, will be removed from a fuel pool filled with liquid sodium, washed and packed in a canister, before being placed inside another fuel pool filled with water.  The aim is to remove one rod each day, and a total of 100 rods will be taken from the sodium pool by the end of this year.  Work to remove the 370 fuel assemblies from the sodium-filled core is scheduled to begin next year. These will be placed in the storage tank prior to being transferred to the storage pool. JAEA also plans to start extracting some 760 tonnes of sodium from the reactor's secondary cooling system before the end of 2018. The liquid sodium will be placed in the storage tank for later disposal.

NRA will conduct regular throughout these procedures. Monju's decommissioning will be carried out in four stages. The NRA has so far only approved the removal of fuel that is scheduled to be completed by fiscal 2022. Plans are yet to be finalised for the second and later stages, which will include removal of some 960 tonnes of liquid sodium coolant from the reactor and the primary circuit. The JAEA will apply for the NRA's approval for decommissioning work for the later stages.

Shuttering Monju in preparation for decommissioning is estimated to cost a total of JPY375bn - JPY225bn for maintenance, JPY135bn for dismantling the plant and JPY15bn for defuelling.

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