Japan's Monju Fast Breeder Reactor Sodium Coolant Overseas Transport Planned to Start in Fiscal 2028

Japan's Ministry of Education, Culture, Sports, Science and Technology plans to include 23.7 billion yen in its fiscal 2027 budget request to advance decommissioning work for the “Monju” fast breeder reactor in Tsuruga City, Fukui Prefecture, including preparation costs for overseas transport of sodium coolant. The amount represents an increase of 4.9 billion yen over the previous fiscal year's budget, including supplementary budgets. Under the plan, overseas transport of sodium coolant from the “Monju” reactor will begin in fiscal 2028.

“Monju” differs from conventional light water reactors in that it uses metallic sodium rather than water as a coolant to remove heat from the reactor. Sodium has a boiling point of approximately 880 degrees Celsius and does not easily pressurize at high temperatures, but it can burn upon contact with water or air, making handling and management highly difficult. In 1995, during trial operation, “Monju” experienced a sodium leak from piping that triggered a fire, after which it was shut down for an extended period. The Japanese government decided in 2016 to decommission it.

Currently, approximately 1,665 tons of sodium remain within the “Monju” facility. Of this, approximately 758 tons are in the main system equipment containing radioactive substances, approximately 755 tons are in the auxiliary system free of radioactive substances, and approximately 153 tons are in the fuel storage facility. The Japan Atomic Energy Agency, which is responsible for “Monju” operations, began decommissioning work in fiscal 2018, with the overall decommissioning plan expected to be completed in fiscal 2047. The current work is in the second of four phases, focusing on preparations for dismantling sodium-related equipment.

The transfer and removal of sodium is regarded as a major challenge in the decommissioning work. At present, sodium extracted from the reactor and other equipment is stored in tanks. Subsequent preparations include laying transport piping to the tanks and transferring the sodium to removal tanks. The related operations involve temperature control, requiring sodium to be converted between solid and liquid states to meet storage and transport requirements.

Regarding overseas processing, the Japan Atomic Energy Agency signed a sodium processing contract with the British company Cavendish in 2023. The UK side is advancing decommissioning work for the Prototype Fast Reactor (PFR) and has relevant processing experience. Under the plan, sodium removed from “Monju” will undergo chemical processing in the UK and be used as an industrial neutralizing agent. Construction of the sodium processing facility at Workington Port in Cumbria, western England, began in May of this year.

When officials from Japan's Ministry of Education, Culture, Sports, Science and Technology recently visited Tsuruga City Hall, they stated that the ministry will be responsible for preliminary preparations, regulatory procedures, and coordination with overseas and related organizations. The Tsuruga City side expressed its hope that the related work will proceed steadily with safety as the top priority.

In addition, a construction survey for a “test research reactor” is also being advanced at the “Monju” site. The facility is planned to utilize neutrons for scientific research activities and to serve a function in nuclear energy-related personnel training, with the Japan Atomic Energy Agency responsible for its construction and operation. The related survey will be conducted in cooperation with Kyoto University, Fukui University, and other institutions. However, because materials indicate that active faults may exist within the “Monju” site, the Japan Atomic Energy Agency is currently conducting geological surveys and other investigations at multiple locations on the site.