THE HAGUE — Japan will announce  Monday that it will turn over to Washington more than 700 pounds of  weapons-grade plutonium and a large quantity of 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 announcement is the biggest  single success in President Obama’s five-year-long push to secure the world’s  most dangerous materials, and will come as world leaders gather here on Monday  for a nuclear security summit meeting. Since Mr. Obama began the 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.

Japan’s agreement to transfer the  material — the amount of highly enriched uranium has not been announced but is  estimated at 450 pounds — has both practical and political significance. For  years these stores of weapons-grade material were not a secret, but 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 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 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.

Mr. Obama’s initiative to lock  down plutonium and uranium around the world was supposed to have been just the  first step in an ambitious agenda to seek “the peace and security of a world  without nuclear weapons,” as he said in Prague in 2009. Now, the downturn in  relations with Russia has dashed hopes of mutual reductions in the world’s two  largest arsenals. At the same time, North Korea has resumed its program,  Pakistan and India are modernizing their weapons, and the Senate has not taken  up any of the treaties Mr. Obama once described as vital.

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  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 Russian President Vladimir V. Putin 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 ousted president Victor 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.

Both 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 upward of $80 billion on a  “life extension” program for its existing nuclear arsenal, and it will cost far  more to upgrade nuclear submarines in years ahead.