Japan had about 47.1 tons of plutonium in and outside the country at the end of 2013, about 2.9 tons more than the year before, the Cabinet Office said on Tuesday.
Newly added were 2.3 tons generated through spent fuel reprocessing outsourced to Britain and 640 kg not reported to the global watchdog in 2012 and 2013. The 640 kg is part of mixed plutonium-uranium oxide (MOX) fuel stored in a reactor that was offline during that time.
Revelations of the unreported 640 kg stoked controversy in June, though the Japan Atomic Energy Commission had said it was exempt from International Atomic Energy Agency reporting requirements, insisting at that time that fuel inside reactors is considered "being used."
Under Japan's nuclear fuel recycling policy, plutonium extracted by reprocessing conventional uranium fuel is consumed by existing reactors in the form of MOX fuel. But this policy is jeopardized by public concerns about nuclear power amid the Fukushima crisis.
A further increase in plutonium could raise concerns in the international community about its possible diversion to nuclear weapons.
The earlier unreported 640 kg of plutonium was contained in MOX fuel loaded in March 2011 into reactor 3 of Kyushu Electric Power Co.'s Genkai nuclear plant in Saga Prefecture during its regular checkup, but has been left there unused as the reactor could not restart in light of the disaster at Tokyo Electric Power Co.'s Fukushima No. 1 complex.