THE HAGUE — Japan announced on Monday that it would turn over to Washington a large cache of weapons-grade plutonium and highly enriched uranium, a decades-old research stockpile that is large enough to build dozens of nuclear weapons, according to American and Japanese officials.
The move is the biggest single success in President Obama’s five-year push to secure the world’s most dangerous materials, and comes as world leaders gathered here on Monday for a summit meeting on nuclear security. Since Mr. Obama began the series of meetings with world leaders — this will be the third — 13 nations have eliminated their caches of nuclear materials and scores more have hardened security at their storage facilities to prevent theft by potential terrorists.
In a joint statement by the United States and Japan, the two countries said that Japan had agreed to “remove and dispose” of hundreds of kilograms of nuclear material from the Fast Critical Assembly at the Japan Atomic Energy Agency.
The statement said the elimination of the uranium and plutonium would “help prevent unauthorized actors, criminals, or terrorists from acquiring such materials. This material, once securely transported to the United States, will be sent to a secure facility and fully converted into less sensitive forms.”
The statement did not specify the amount of the material to be eliminated. American and Japanese officials said it would include 700 pounds, or 320 kilograms, of weapons-grade plutonium. The amount of highly enriched uranium has not been announced but is estimated at 450 pounds.
The Japanese agreement to transfer the material has both practical and political significance. For years these stores of weapons-grade material were not a secret, but they were lightly guarded at best; a reporter for The New York Times who visited the main storage site at Tokaimura in the early 1990s found unarmed guards and a site less-well protected than many banks. While security has improved, the stores have long been considered vulnerable.
Iran has cited Japan’s large stockpiles of bomb-ready material as evidence of a double standard about which nations can be trusted. And last month China began publicly denouncing Japan’s supply, in an apparent warning that a rightward, nationalistic turn in Japanese politics could result in the country seeking its own weapons.
At various moments right-wing politicians in Japan have referred to the stockpile as a deterrent, suggesting that it was useful to have material so that the world knows Japan, with its advanced technological acumen, could easily fashion it into weapons.
The nuclear fuel being turned over to the United States, which is of American and British origin, is a small fraction of Japan’s overall stockpile. Japan has more than nine tons of plutonium stored in various locations and it is scheduled to open in the fall a new nuclear fuel plant that could produce many tons more every year. American officials have been quietly pressing Japan to abandon the program, arguing that the material is insufficiently protected even though much of it is in a form that would be significantly more difficult to use in a weapon than the supplies being sent to the United States.
The result is that nuclear security — eliminating or locking down nuclear material — may be the biggest element of Mr. Obama’s nuclear legacy. The only other aspect of his agenda that may yet come to fruition centers on Iran, where economic sanctions, covert action and diplomacy have brought Tehran to the table to negotiate over its nuclear program. But even Mr. Obama says his chances of reaching a deal are at best 50-50.
“The Obama team came in thinking a lot of things would be easier than they turned out to be,” said Matthew Bunn, a professor at Harvard’s Belfer Center for Science and International Affairs.
One of Mr. Obama’s major goals has been to stop the production of new supplies of nuclear material; at the last nuclear security summit meeting, in 2012, he said “we simply can’t go on accumulating huge amounts of the very material, like separated plutonium, that we’re trying to keep away from terrorists.” But Pakistan has blocked his effort to negotiate a treaty that would end the production of more material — called the Fissile Material Cutoff Treaty — and it is unclear whether the summit meeting communiqué will contain language urging other countries to disgorge their plutonium stockpiles.
There have been other obstacles to Mr. Obama’s agenda.
He succeeded in negotiating a modest arms control treaty with Russia in 2010, but the rapidly deteriorating relationship with President Vladimir V. Putin of Russia has all but ended hopes for further reductions in the arsenals of the two countries.
Nonetheless, the effort to secure dangerous nuclear materials in Russia and the former Soviet states has been one of the big successes of the post-Cold War era, Just last year Ukraine, then still under the control of the now ousted President Victor F. Yanukovych, sent more than 500 pounds of weapons-grade uranium from a reactor back to Russia. Ukraine gave up its nuclear weapons — left over after the fall of the Soviet Union — two decades ago. Had the weapons and materials remained in Ukraine, the current standoff with Russia might have taken on far more dangerous dimensions.
But Mr. Obama’s agenda has also run into major troubles in the Senate. In 2009 and 2010 the White House promised to reintroduce the Comprehensive Test Ban Treaty, which was defeated in the Senate during the Clinton administration. It has never been put back in front of the Senate, for fear of a second rejection. Even seemingly noncontroversial legislation, including passage of two nuclear terrorism conventions that deal with the physical protection of materials, has been stuck.
Administration officials and advocates of major nuclear reductions argue that Mr. Obama has focused a level of attention on securing stockpiles even if his arms reduction efforts have come up short.
“What President Obama has done is put it more on the front burner and accelerated the process,” said Sam Nunn, a former Democratic senator from Georgia who played a central role in creating the American-backed program to help dismantle nuclear weapons and clean up nuclear material around the world.
“Significant progress has been made — not enough,” said Mr. Nunn, the chairman of the Nuclear Threat Initiative, a research group that presses for deeper cuts.
The summit meetings, which have taken place every two years, have forced national leaders to focus on their stockpiles of materials and their protections, and engaged the United States on their processes for securing them, blending them down so they cannot be used in bombs, or getting rid of them.
“This process has given us the opportunity to build relationships that have opened new doors to cooperation, some of which we can talk about and some of which we can’t,” said Elizabeth Sherwood-Randall, who heads the effort at the National Security Council and has been negotiating with countries participating in the meeting.
Of the agreement with Japan, she said: “This is the biggest commitment to remove fissile materials in the history of the summit process that President Obama launched, and it is a demonstration of Japan’s shared leadership on nonproliferation.”
Ms. Sherwood-Randall said that even Russia “has continued to work on nuclear security at a professional level,” despite the tensions over Ukraine. But she conceded: “It is true that at this moment, we will not begin a new discussion about new arms control. This is not something the Russians are interested in at this time.”
In fact, Russia is now modernizing its nuclear force. So is the United States: To pass the New Start treaty in 2010, the administration told Congress it would spend at least $80 billion on a “life extension” program for its existing nuclear arsenal, and that it would cost far more to upgrade nuclear submarines in years ahead.