UK-based Cavendish Nuclear, a wholly-owned subsidiary of Babcock, together with its delivery partner US-based Amentum, has been awarded the next phase of work by the Japan Atomic Energy Agency (JAEA) to support the decommissioning of the Monju Prototype Fast Reactor, in Fukui Prefecture, Japan.
The 280 MWe Monju 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, four months after it began operation when 700 kg 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. 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 it. The JPY1000bn ($9bn) Monju had operated for only 250 days since its start-up in 1994.
Under the new contract, Alkali Metal Processing Limited, a joint venture between Cavendish Nuclear and Amentum, will construct, commission, and secure regulatory permissions for a new facility in the UK which will treat the sodium coolant removed from Monju. The material, which presents no radiological hazards, will be safely converted into sodium hydroxide for general industrial reuse.
This award builds on a May 2023 contract, which focused on the design and engineering phase of the UK-based project and represents a key milestone in the programme, reflecting the strong collaborative relationship with JAEA.
The joint venture will now move forward with construction, installation of process equipment and preparations for operation, working closely with locally based supply chain partners. The approach draws on both companies’ experience gained from decommissioning the UK’s Dounreay Prototype Fast Reactor, which shares design similarities with the Monju reactor.
JAEA President Masanori Koguchi said: “It is my great pleasure to have successfully concluded the contract for the next phase of this important project. I sincerely hope that, through the implementation of this project, the collaborative partnership between JAEA and Cavendish Nuclear/Amentum will continue to grow and further deepen.”
Cavendish Nuclear Managing Director Mick Gornall, of, said the award “is a testament to the trust and confidence JAEA has placed in us”. Loren Jones, Senior Vice President of Amentum, said the contract is example of Amentum “using our global decommissioning knowledge and experience to provide real benefit to our customers internationally”.
The decommissioning of Monju is expected take 30 years and cost more than JPY375bn ($2.5bn), according to government estimates. This includes JPY225bn for maintenance, JPY135bn for dismantling the plant and JPY15bn for defuelling and preparations for decommissioning.
In 2017, JAEA submitted to the NRA a detailed four-stage plan to decommission Monju, in line with the government’s basic policy. NRA approved the plan in March 2018. In the initial stage, JAEA would transfer all fuel to an on-site storage pool by fiscal 2022. In October 2022, JAEA announced that all the fuel assemblies at Monju had been transferred to a water-filled storage facility. The second and third stages would see liquid sodium coolant extracted from Monju and related equipment dismantled. Finally, the reactor building will be demolished and removed by fiscal 2047.
