For the industry, that means that now, as in the past, there will be no direct correlation between the number of warheads decommissioned and the quantity of highly enriched uranium or plutonium, also used in weapons, that the two countries declare surplus.

(This summer, Mr. Obama and President Dmitri A. Medvedev of Russia agreed to a new limit on delivery vehicles of 500 to 1,100 and a limit on deployed warheads as low as 1,500. The United States now has about 2,200 nuclear warheads and the Russians 2,800.)

Mr. Medvedev has reaffirmed Russia’s commitment to a 2000 agreement to dispose of plutonium, and both countries plan to convert that into reactor fuel as well.

An American diplomat and an official with a federal nuclear agency in Washington have confirmed, separately, that the two countries are quietly negotiating another agreement to continue diluting Russia’s highly enriched uranium after the expiration of Megatons to Megawatts, using some or all of the material from warheads likely to be taken out of the arsenals.

The government officials were not authorized to publicly discuss these efforts.

This possible successor deal to Megatons to Megawatts is known in the industry as HEU-2, for a High Enriched Uranium-2, and companies are rooting for it, according to Jeff Combs, president and owner of Ux Consulting, a company tracking uranium fuel pricing.

“You can look at it like a couple of very large uranium mines,” he said of the fissile material that would result from the program.

American reactors would not shut down without a deal; utilities could turn to commercial imports, which would most likely be much more expensive.