U.S. and Russian officials announced agreement Monday on how to safely dispose of 34 metric tons of Russian weapons-grade plutonium, overcoming a major hurdle in a joint nuclear nonproliferation effort that at times has been close to falling apart.

The two countries, in a joint statement, outlined a plan where Russia agrees to modify its fast-neutron reactors so that they can burn the plutonium, yet ensure that additional plutonium will not be produced.

In turn, the United States, which also will dispose of 34 tons of excess plutonium from its weapons program, will continue to help Russia pay for construction of a plant in Russia to turn the plutonium into a mixed oxide fuel for the reactors and in research of a more advanced reactor that could speed up the disposal process.

The two countries tentatively agreed to the plutonium disposal program seven years ago when it was hailed as a breakthrough in safeguarding some of Russia's nuclear material. But progress stalled because of a variety of disagreements, most recently over how Russia would destroy the plutonium.

Russia's ambivalence in turn caused Congress to balk at approving money for the U.S. portion of the plutonium disposal effort because of what lawmakers called the apparent inability to get Russia to agree on a disposal plan.

Energy Secretary Samuel Bodman and Sergey Kiriyenko, director of Russia's Atomic Energy Agency, Rosatom, in a joint statement outlined a "mutual understanding" as to how Russia's plutonium would be disposed of and reiterated both countries' commitment to the program.

The agreement "reflects measurable progress towards disposing of a significant amount of weapons grade plutonium in Russia," said Bodman in a separate statement released by his office.

William Tobey, deputy administrator at DOE's National Nuclear Security Administration, said in an interview that the agreement should resolve some of the key concerns in Congress and keep the U.S. program on track.

"We nailed down some important details," said Tobey.

Among them, he said, was assurance from the Russians that the reactors used to dispose of the plutonium will be modified to burn more than they produce, that the plutonium they produce will not be weapons grade, and that the U.S. contribution will be capped at $400 million.

While viewed as major nonproliferation effort, the plutonium disposition is expected to take several decades and cover only a fraction of the weapons-grade plutonium both countries possess. The United States is believed to have about 100 metric tons of weapons-grade plutonium and Russia about 140 metric tons.

Nevertheless, former Sen. Sam Nunn, co-chairman of the Nuclear Threat Initiative, a nonproliferation advocacy group, called the agreement a "major advance toward achieving the elimination of enough plutonium to make more than 8,000 nuclear bombs."

The agreement calls for 34 metric tons of Russia's excess plutonium to be turned into a mixed oxide, or MOX, fuel and then burned in its existing BN600 fast-neutron reactor and in a larger version, the BN800, once it is built. It also calls for continued U.S. help for Russian development of a more advanced gas-turbine reactor that could speed up the disposition.

In recent years the nonproliferation effort has been stymied by Russia's insistence that the plutonium program mesh with the country's expanded civilian nuclear energy programs.

Rosatom plans to implement the plutonium disposition "within the framework of the strategy for developing Russian nuclear energy," says the joint statement.

Disposal would begin in the BN600 reactor in 2012, three years before the United States is scheduled to begin processing plutonium into MOX for use in a commercial reactor. Construction of the $4 billion MOX processing facility began last summer year at the Savannah River nuclear facility in South Carolina.

The announcement Monday anticipates 1.5 metric tons of Russian plutonium being burned a year once both of Russia's reactors are operating, meaning it will take more than 22 years to destroy the 34 metric tons, once both reactors are operating.

Ed Lyman, nuclear weapons expert at the Union of Concerned Scientists, said use of the relatively small BN600 reactor "will put Russian plutonium disposition on the slow track" because the reactor can burn only about three-tenths of a ton of plutonium a year, and the larger reactor has yet to be built.

Lyman said "this is a total retreat from the original concept" which would have disposed of the plutonium in larger light-water reactors, an option the Russian rejected.