Japan failed to include 640 kg of unused plutonium in its annual reports to the International Atomic Energy Agency in 2012 and 2013, in what experts are terming an "inappropriate omission."

The stock is part of mixed plutonium-uranium oxide (MOX) fuel stored in a reactor that was offline during this period, and was thus deemed exempt from IAEA reporting requirements, said an official at the Japan Atomic Energy Commission.

Experts warn that Japan's reporting does not reflect the actual state of unused plutonium that could be diverted for nuclear weapons. The unreported amount is enough to make about 80 nuclear bombs.