The Nuclear Regulatory Commission yesterday denied a request by the Reagan administration to hasten construction of the often-delayed $3.2 billion Clinch River Breeder Reactor at Oak Ridge, Tenn.

By a 3-to-2 vote, the NRC refused to grant to the Department of Energy an exemption from its regulations that would have allowed the DOE, the Project Management Corp. and the TVA to begin preparing the Clinch River site next month before hearings are held on their request for a construction permit.

The exemption would have allowed the applicants to dig a quarry, build a wharf to handle river barges and clear and grade 271 acres of land for roads, a railroad spur and a sewage treatment plant on the site.

The deciding vote was cast by NRC Commissioner John F. Ahearne, who sided with Victor Gilinsky and Peter Bradford. Ahearne said he did not think an exemption was "in the public interest" and that he doubted the Energy Department's estimates of how much a delay would cost taxpayers.

"I think the applicant did a very poor job on his application, coming to us with several different explanations as to what a delay would cost," Ahearne said in an interview.

"The way the Energy Department handled its application raised real doubts in my mind that DOE understands the regulation business," he said.

The Energy Department told the NRC that it would cost $20 million a month for each month it was delayed in preparing the Clinch River site because of inflation and the need to keep its construction and design staff on the payroll waiting for the building permit to be granted. The DOE estimated that a delay in site preparation would cost it a total of $120 million to $240 million.

Arguing against the exemption, the Natural Resources Defense Council and the Sierra Club contended that inflation alone would not add that much to the Clinch River costs, that "there are savings associated with delay because the government need not raise funds to pay for construction." They also argued that engineers and designers had completed most of their site preparation work and that the payroll estimates "implicitly assume the staff will not or cannot perform activities beneficial to the project during any delay."

The NRC staff largely agreed, telling the five NRC commissioners: "The staff concludes that the actual cost of delay is significantly less than the $120 million to $240 million cited by the applicants."

"I find that the cost of delay was grossly overestimated," Ahearne said before he voted against the exemption. "I think the DOE submission basically says the cost of delay is really quite small in actual dollars."

First authorized by Congress in 1973, the Clinch River Breeder Reactor was stopped by President Carter in 1977, reinstated at a slowed schedule by Congress the following year and approved by President Reagan and the Congress last year.

DOE said it could complete the project by September, 1988, with the exemption, but it would take until February, 1990, without it. The demonstration project is to see if a fast breeder reactor cooled by liquid sodium metal can produce or "breed" more plutonium than it burns in generating electricity. Plutonium is used in making nuclear weapons.