U.S. says 'not at all concerned' about Japan's plutonium

VIENNA (Reuters) - The United States is not worried about Japan's plutonium stocks, a senior U.S. diplomat said on Wednesday, following criticism raised by China in a new dispute between the east Asian neighbors.

"We are not at all concerned that the plutonium is either being handled improperly or that there isn't a plan for disposition," Joseph Macmanus, the U.S. ambassador to the U.N. nuclear agency, told reporters in response to a question.

Last month, Beijing said it was "extremely concerned" by a report that Japan has resisted returning to the United States more than 300 kg (660 lb) of mostly weapons-grade plutonium.

Japan's Kyodo news agency said the United States had pressed Japan to give back the nuclear material, which could be used to make up to 50 nuclear bombs. Japan had balked, but finally given in to U.S. demands, Kyodo said.

The material was bought for research purposes during the 1960s and the two governments will likely reach an official agreement on its return at the Nuclear Security Summit in The Hague in March, an official at Japan's Education Ministry said.

Japan also has plutonium contained in spent nuclear fuel at civil reactor and reprocessing sites - totaling 159 metric tons (175 tons) at the end of 2012, according to Japanese data posted on the website of the International Atomic Energy Agency (IAEA).

Macmanus said "plutonium and the disposition of plutonium stocks" was a central element of what he called a very successful diplomatic and energy partnership with Japan.

"We are satisfied that Japan understands what the conditions are for the use and the maintenance of those stocks and we are not concerned," he said.

IAEA Director General Yukiya Amano earlier this week also said there was no reason for concern that plutonium held by Japan could be diverted for nuclear arms purposes.

Tasked with preventing the spread of nuclear weapons, IAEA inspectors regularly check nuclear facilities around the world, including in Japan and other countries with nuclear power plants and other atomic sites.

The U.N. agency's most high-profile case is Iran, where it regularly visits uranium enrichment sites and now also monitors the implementation of a breakthrough agreement late last year between Tehran and six world powers. However, unlike Japan and most other IAEA member states, Iran has not granted the U.N. watchdog wider inspection powers under the so-called Additional Protocol. Iran rejects Western accusations that it is seeking to develop the capability to make bombs.

China is involved in a bitter territorial dispute with Japan. It denies Japanese accusations that it is a threat to peace and in turn has accused Japan of trying to rearm and failing to learn the lessons of its brutal behavior during World War Two, when Japanese forces occupied China.

Unlike China, Japan, the world's only target of atomic bombs, in the final stages of World War Two, does not have nuclear weapons, and it is the government's stance that it will not seek to obtain them. Deteriorating relations between Beijing and Tokyo have been fuelled by a dispute over a chain of uninhabited islands in the East China Sea.

(Reporting by Fredrik Dahl; Editing by Hugh Lawson)