American and Russian officials announced an agreement 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 to be issued later Monday, outline 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.