The nation's plutonium stockpile had shrunk as of the end of 2018 by about 1.6 tons from a year earlier, to 45.7 tons, reflecting increased plutonium thermal power generation, the Cabinet Office said Tuesday.
Concerns have grown internationally as Japan possesses a large amount of plutonium, which can be converted into nuclear weapons.
For thermal power generation, spent fuel from nuclear reactors is reprocessed to extract uranium and plutonium, which is then recycled into fuel called mixed oxide (MOX).
MOX fuel is used at the Nos. 3 and 4 reactors at the Takahama nuclear power plant in Fukui Prefecture, operated by Kansai Electric Power Co., and by the No. 3 reactor at Kyushu Electric Power Co.'s Genkai nuclear plant in Saga Prefecture.
Of the total, some 9 tons are held in Japan and 36.6 tons are in the U.K. and France for reprocessing.
Most of Japan's nuclear power plants remain offline as they are required to pass newly established safety regulations following the 2011 Fukushima nuclear disaster.