Reference Material: Japan’s Separated Plutonium Inventory (as of Dec. 31, 2024)
By Matsukubo Hajime
The Japan Atomic Energy Commission (JAEC) drastically changed the way they publicize the plutonium management data from the release at the end of 2022. The JAEC used to sort the inventory by giving details for each facility and process. At present, the data combines the plutonium holdings of Japan Nuclear Fuel Limited and the Japan Atomic Energy Agency, while the plutonium holdings in the individual processes are specified only according to the classification of plutonium. Although there has recently been little change in the inventory, the data is expected to become even more obscure if the Rokkasho Reprocessing Plant is completed and the use of mixed oxide fuel (MOX fuel) increases at nuclear power plants.
In 2024, plutonium separation and MOX fuel fabrication from Japan-origin spent fuel were not performed in France. No MOX fuel was sent back to Japan, and no MOX fuel was newly loaded in Japan. As a result, Japan’s plutonium holdings decreased by only 45 kg (of which 6 kg was a decrease in Japan). The major reasons for this decrease were “inventory withdrawal” and “nuclear losses.” The inventory withdrawal refers to writing off the plutonium that has been retained in a manner where retrieval is deemed impossible in the foreseeable future, such as plutonium in high-level liquid waste. Nuclear losses are reductions due to spontaneous decay. Some plutonium isotopes have a long half-life, such as plutonium-239, whose half-life is 24,110 years, while others have a relatively short half-life, such as plutonium-241, whose half-life is 14.35 years. Of the reactor-grade plutonium isotopes, 12% is plutonium-241, so the losses are relatively large during the time immediately after separation.
Concerning the spent MOX fuel reprocessing verification research project publicized in 2023, the Federation of Electric Power Companies of Japan announced on February 12, 2025 that about 20 tons of spent MOX fuel and about 380 tons of spent nuclear fuel would be transferred to France for reprocessing in the latter half of the 2020s. The two amounts are twice those specified in the initial announcement. The transferred spent fuel is expected to be from Kansai Electric Power. The major purpose of this project is to relocate spent fuel from the almost full spent-fuel pools under the guise of verification research.
