Senate opposition mounted yesterday to a plan to speed up the nation's nuclear-waste disposal program, with a key GOP leader -- and nuclear power advocate -- declaring the proposal "unwise."

Sen. Alan K. Simpson (R-Wyo.), assistant minority leader, said he opposes the bill because it would force the Energy Department to pick a site for the country's first nuclear-waste repository before crucial scientific information is available.

Simpson said federal experts concluded more studies are needed at the three finalist sites for the waste dump -- studies that cannot be finished before the legislation's Jan. 1, 1989, deadline for picking a site. The leading sites are Deaf Smith County, Tex.; Yucca Mountain, Nev., and Hanford, Wash.

"We don't have sufficient technical information on the three candidate sites," said Simpson, a supporter of nuclear power. "Why kid ourselves? We don't have it, and we won't have it by Jan. 1, 1989. This {legislation} would result in the selection of a site prior to completion of tests," he said. "I think that's a very unwise course of action."

The legislation, sponsored by Sen. J. Bennett Johnston (D-La.), would accelerate by about five years the schedule for picking a site for the first permanent high-level waste dump, needed to hold thousands of tons of radioactive waste from the nation's 108 commercial nuclear power plants. The facility, to be built in a rock formation deep underground, is supposed to prevent waste from leaking into the environment for thousands of years.

Also voicing opposition to the bill were Sen. Robert T. Stafford (R-Vt.), ranking GOP member of the Senate Environment and Public Works Committee, and Sen. John B. Breaux (D-La.), chairman of a Senate subcommittee on nuclear regulation.