The Times said Timothy Frazier, head of radioisotope power systems at the US Energy Department, vigorously denied in a recent interview any of the classified missions would involve nuclear arms, satellites or weapons in space.

"The real reason we're starting production is for national security," Frazier was quoted as saying.

Officials at the Energy Department could not be reached for comment.

The program, which the newspaper said had raised concerns among environmentalists, would produce 330 pounds (150 kg) over 30 years at the Idaho National Laboratory. The program could cost $1.5 billion and generate over 50,000 drums of hazardous and radioactive waste, federal officials told the Times.

Plutonium 238 is hundreds of times more radioactive than plutonium 239, which is used in nuclear arms, according to the newspaper. Medical experts say inhaling even a speck posed a serious risk of lung cancer, the Times said.

The newspaper said plutonium 238 had no central role in nuclear arms, but was valued for its steady heat that could be turned into electricity. Nuclear batteries made from it power spacecraft to go where sunlight is too dim to energize solar cells.

Federal and private experts not connected to the project were quoted as saying the new plutonium would likely power devices for espionage under the sea and on land.