The Senate was winding up its work on the nuclear waste disposal bill, after two years of work and two days of debate, when Sen. James A. McClure (R-Idaho) slipped in an amendment that he said would merely clarify the intent of the bill.

The "clarifying" language went in without objection, and it wasn't until after the bill had passed, 69 to 9, that startled staff aides to Democratic senators realized that McClure had bulldozed away a major roadblock to nuclear power plant construction in seven states, including Maryland.

Laws in those states restrict the construction of new nuclear plants until the problem of how to dispose of nuclear waste is solved.

McClure's amendment declares the problem solved.

"The amendment was obviously designed to vacate a number of state laws and even court decisions on waste disposal," said David Berick of the Environmental Policy Center. "The provision basically says 'when we pass this legislation, there is a solution because this legislation determines there is a solution. It's true because we say it is.' "

Leonard Bickwit Jr., general counsel of the Nuclear Regulatory Commission, agrees that the McClure amendment "appears to satisfy a number of state requirements that safe disposal methods must exist as a condition to the licensing of nuclear plants."

And Emanuel Gordon, a spokesman for the Atomic Industrial Forum, a nuclear power trade association, said McClure's provision "seems like it would wipe or eviscerate requirements" that nuclear plant construction or expansion be tied to a disposal solution. "Obviously we think it's good," Gordon said. "It looks like it will be a great help."

California, Maryland, Oregon, Connecticut, Maine, Montana and Wisconsin have laws restricting the construction of new nuclear plants until a demonstrable means of disposing of high-level nuclear waste is developed, according to the NRC's state programs office.

The California law has been challenged by industry, which won in the district court but lost in appeals court. The U.S. Supreme Court has been asked to take the case.

The McClure provision could still be knocked out in the House, where final action is several months away.

But the Justice Department has already made use of it in an amicus curiae brief before the Supreme Court, pointing to it as evidence that something is going on in Congress regarding this issue--"an attempt to give the court a hook to take the case," one observer suggested.

Justice attorneys argued that if the McClure provision or something similar should be enacted, "it would be appropriate for this court to consider vacating the court of appeals decisions and remanding for reconsideration in light of that legislation."

McClure offered the disposal solution item with a group of housekeeping amendments needed to clear up technical problems on the bill, which would establish a national policy for the permanent disposal of high-level radioactive waste, before a final vote was taken April 29.

Only a handful of senators remained on the floor, most waiting for a vote on the Falkland Islands resolution.

McClure described the amendment as "clarifying legally the intent of this bill . . . to establish the definitive federal policy" for disposal of radioactive waste from nuclear power plants.

The amendment said that "enactment of this act shall be construed in any federal, state and local administrative or judicial proceeding to satisfy any legal or statutory requirement . . . for the existence of and the approval of the United States of a demonstrated technology or means for the disposal of spent fuel and high-level nuclear waste and . . . for assurance of the safe disposal of spent fuel and high-level nuclear waste and the scheduled availability of storage and disposal facilities for high-level nuclear waste."

The language was not distributed to other senators beforehand, and was "completely unexpected," according to one Senate aide involved with the bill.

Sen. J. Bennett Johnston Jr. (D-La.), the minority member monitoring the action, raised no objections after McClure's explanation. Aides to other Democratic senators, who didn't read the amendment until after the bill had passed, complained that McClure had sneaked it past them and that Johnston had been caught unaware.

But Johnston aide Ben Cooper said the senator "accepted the amendment because he didn't have any problems with it." Senators opposed to the language "have nobody to blame but themselves for not being on the floor to watch for it," he said.

Mike Faden of the Union of Concerned Scientists said, "It's not entirely clear, but our first reaction is that it could have a severe effect, and I think that was the intent. At least there seems a strong possibility of it knocking out laws like California's."

The California law, enacted in 1976, was challenged in 1978 by two utilities and a pro-business public interest law firm, which argued that the power to regulate nuclear energy was reserved for the federal government in the U.S. Atomic Energy Act of 1954.

The state lost in district court in 1980, but last fall won in the 9th Circuit Court of Appeals, where about 25 states joined California.

The disposal issue was also one of the major ingredients in last month's decision by the District of Columbia appeals court overturning the NRC's environmental standards.

Judge David Bazelon, writing for the majority, said the NRC's rules are "invalid because they fail to allow for proper consideration of the uncertainties" surrounding the disposal of high-level nuclear waste. Some attorneys say they believe the McClure provision would help the NRC in that case.

NRC counsel Bickwit said the amendment would have no impact on that case, but he said it did appear designed to end the so-called "waste confidence" proceedings at NRC, which stem from yet another court decision ordering the NRC to prove it has "reasonable confidence" that permanent waste diposal facilities will be available when needed.

Those proceedings stem from Minnesota's challenge of the NRC's right to increase temporary disposal capacity on plant sites when there is no permanent disposal solution.

Minnesota argued that the on-site facilities would become de facto permanent waste repositories. In 1979, a federal court ruled that the NRC had the right to make those decisions but not before it built a record on which to base its action. The commission has received written and oral testimony, and is expected to decide within months.