WASHINGTON — The plutonium that is the key ingredient in thousands of nuclear weapons sidelined in the new arms control treaty between the United States and Russia is likely to be around for decades at least, according to experts. They say the process for destroying plutonium has not yet started to whittle down the surplus already created by previous agreements.

Plutonium can be consumed in nuclear power reactors, creating the possibility of a swords-to-plowshares conversion that would have the added benefit of making redeployment of the weapons impossible. But converting the weapons plutonium for civilian reactor use has proved much slower than expected.

Since the late 1990s, the United States has been trying to build a factory at the Savannah River Site, near Aiken, S.C., that would convert the plutonium to reactor fuel. Government officials once hoped that such fuel could be loaded into reactors in 2002. But construction did not begin until 2007 and even if all goes well, the plant will not be finished until 2016. The cost of the plant, once estimated at $2.3 billion, is now $4.8 billion. The plant is the largest nuclear construction project in the country.

The plan is to use the amount already declared surplus, 34 tons, over about 15 years, so if the new arms agreement results in more plutonium being declared surplus, it would not start to be converted to fuel until the 2030s, at the earliest, people involved in the project say.