WASHINGTON (AP) _ Cleaning up contamination and radioactive waste at the government's nuclear weapons plants may cost much less than the $60 billion to $100 billion commonly estimated, Energy Secretary James D. Watkins told Congress Wednesday.

''I believe we can do this for a lot less,'' Watkins told a Senate Budget subcommittee in his first appearance on Capitol Hill since his swearing in last week.

Watkins did not make his own estimate but promised to produce later this year a detailed blueprint, including cost projections, for an initial five- year effort to clean up the weapons sites and modernize the aging facilities in 12 states.

The Energy Department, in an estimate published before Watkins took over, said modernization and cleanup of the weapons complex would cost at least $81 billion over the next 21 years. The General Accounting Office has put the cost at more than $100 billion.

In testimony before the Senate panel, Keith O. Fultz, the GAO's director of energy issues, maintained that current cost estimates probably were too low, given that no one yet knows how bad some of the contamination is and the feasibility of cleanup.

Watkins refuted this. He said scenarios for accomplishing the cleanup often contain expensive, unneeded frills.

Watkins also said he was confident his department could safely restart the Savannah River Plant nuclear reactors in South Carolina, which are the nation's only source of tritium, a radioactive gas needed in the manufacture of nuclear warheads. The three reactors there have been closed for nearly a year because of safety concerns.

''I believe we can get those reactors up,'' he said, adding that he planned to meet soon with senior executives of Westinghouse Electric Corp., which on April 1 takes over for E.I. du Pont de Nemours & Co. as contract manager at Savannah River.

The Energy Department and Westinghouse have said they would produce a detailed plan for restarting the Savannah River reactors by the end of this month.

The timing of a restart at Savannah River is considered important because the shutdown has raised questions about the adequacy of U.S. tritium supplies.

Watkins said he already was developing a plan that would get the Savannah River reactors running again and keep them in safe operation for at least a decade - time enough to allow for the construction of a new generation of reactors.

He defended the Bush administration's plan for spending $6.8 billion to build two tritium-producing reactors, one at Savannah River and the other in Idaho. Critics have said only one reactor, at most, will be needed in the foreseeable future.

Robert B. Barker, the Pentagon's assistant secretary for atomic energy, told the panel the two new reactors would be capable of producing 150 percent of the nation's tritium needs. He strongly defended the plan, saying extra production capacity was needed to guard against unforeseen technical or other problems.

Across Capitol Hill, a House subcommittee sent to the full Energy and Commerce Committee legislation to create an independent, 11-member board to oversee the cleanup of the government's nuclear weapons plants.