Japan kept the unreported plutonium as mixed oxide (MOX) fuel, a plutonium-uranium mixture that could be burned in a reactor. It was found in an offline reactor in a nuclear plant in Saga Prefecture in the southern Japanese town of Genkai, RT reported Sunday.

According to the report, the MOX fuel was loaded into the No. 3 reactor of Kyushu Electric Power Co.'s Genkai nuclear plant in March 2011 during its regular checkup, shortly before the Fukushima Nuclear disaster occurred later that month. It was then taken out two years later as the reactor remained offline.

The unreported plutonium was first found by Kakujoho, a nuclear information website.

According to the reports Tokyo submitted, plutonium reserves in the country stood at 1.6 tons, while they were nearly 2.2 tons in 2011. It appears that Japanese government excluded the loaded plutonium.

However, an official of Japan's Atomic Energy Commission argued in an interview with Japanese Kyodo News that the unreported plutonium is still considered being used and hence exempt from reporting to the IAEA.

But experts both in Japan and abroad warn that the Tokyo’s reporting does not reflect the actual state of unused plutonium that could be diverted for nuclear weapons.

"From the safeguards point of view this material is still un-irradiated fresh MOX fuel regardless of its location," former IAEA Deputy Director General Olli Heinonen said. "If it has, indeed, not been irradiated, this should be reflected in the statements."

Keeping the largest amount of plutonium among non-nuclear armed nations, Japan claims to possess over 44 tons of plutonium (9.3 tons within the country and 35 tons in Britain and France), while the actual amount is 45 tons, said Japan's Kyodo News Agency.

Japan’s nuclear reactors remain idle after the 2011 disaster at Fukushima. In the past the country used plutonium for power generation.

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