Japan did not inform U.N. regulators for two years of a reactor fuel supply containing enough plutonium for 80 nuclear bombs, Kyodo News reports.
A Japanese government insider said Tokyo was right to exclude roughly 1,410 pounds of mixed-oxide fuel from its 2012 and 2013 declarations to the International Atomic Energy Agency, the news agency reported on Saturday. A nonoperational reactor was storing the material, making its disclosure unnecessary, according to the Japan Atomic Energy Commission source.
A one-time commission official, though, suggested the panel had omitted the material by mistake. The Japanese regulatory body "should make efforts to improve" its oversight, former commission vice chairman Tatsujiro Suzuki argued.
The nuclear specialist website Kakujoho initially publicized the existence of the MOX fuel cache, which reportedly entered a reactor at Japan's Genkai nuclear power plant for testing in March 2011. That month's crisis at Japan's Fukushima Daiichi atomic site prompted Tokyo to suspend atomic operations across the country, and the fuel sat idle in the Genkai plant reactor until its transfer to a holding pond in March 2013.
Japan's statements to the agency suggested its domestic stockpile had dropped in that period, from 2.2 tons in 2011 to 1.6 tons the following year. Tokyo declared the same, lower quantity in 2013.
Former IAEA safeguards chief Olli Heinonen said Japan should have declared any MOX fuel that had not yet been "irradiated" in a power reactor.
"From the safeguards point of view, this material is still unirradiated fresh MOX fuel regardless of its location," he said.
China pressed Japan on Monday to justify the apparent exclusion, the Xinhua News Agency reported.
"We expect Japan to respond to the concerns of the international community, take practical action at an early date and address the imbalance between its demand and supply of sensitive nuclear materials," Chinese foreign ministry spokeswoman Hua Chunying told reporters.