The controversial Clinch River breeder reactor, an advanced nuclear power plant authorized by Congress 12 years ago and left for dead by President Carter, won a major fight in its battle for life yesterday when the Nuclear Regulatory Commission approved the start of site preparation work at Oak Ridge, Tenn.

The NRC, voting 3 to 1 with all of President Reagan's appointees in favor, granted the Energy Department an exemption from certain licensing proceedings to let it start clearing and grading the 271-acre site, building access roads and digging the hole for the foundation for the building.

Assistant Energy Secretary Shelby T. Brewer hailed the NRC decision as a "watershed event," and voiced hope that it will permit completion by 1989 of the plutonium-fueled breeder, so-called because it produces more plutonium than it uses.

Carter, concerned about the possibility of increasing the worldwide spread of plutonium that could be used in nuclear weapons, had attempted to kill the Clinch River project and slow development of the breeder technology.

Brewer, however, noted yesterday that Reagan had made "expeditious completion of Clinch River a major policy objective" and called the NRC vote an important step toward helping to restore the U.S. leadership position in nuclear technology development.

A DOE spokesman said a contract for site preparation, which will cost an estimated $35 million to $50 million, will be awarded later this month.

The crucial third vote in support of the exemption was cast by the newest NRC commissioner, James K. Asselstine. Asselstine voted against the proposal hours after he was sworn in three months ago on grounds that to do otherwise might "raise serious questions regarding my own independence and objectivity."

Asselstine said yesterday he was joining Chairman Nunzio J. Palladino and Commissioner Thomas M. Roberts in voting for the exemption because he had been persuaded by administration arguments that a further delay in start of construction work at the site would set back completion of the $3.2 billion project by an additional six to 12 months.

The two environmental groups that took the lead in fighting the administration's effort to start work at Clinch River, the Natural Resources Defense Council and the Sierra Club, vowed to continue in court their effort to block the project.

The environmental groups disputed the Reagan administration's claim that waiting for completion of the normal licensing procedure would create major delays for the project.

They suggested that the administration's eagerness to break ground was primarily related to the tough battle expected in Congress this year over continued federal funding of the breeder project.

Although Congress kept the breeder alive in the late 1970s by appropriating funds over Carter's objections, Clinch River's opponents last year came within two votes of cutting off funding in the Senate.

Senate Majority Leader Howard H. Baker Jr. of Tennessee, the project's leading supporter, said yesterday he hoped the NRC vote had "removed some of the doubts as to whether Clinch River will be a reality."

Commissioner John F. Ahearne, a Carter appointee and the only commissioner to vote against the exemption yesterday, said he felt "little has changed from the previous" two DOE requests to accelerate work on the project, both of which the NRC rejected.

What has changed, however, is that the Reagan administration now has majority control of the regulatory body. The fifth member of the five-member commission, Victor Gilinsky, was out of the country and did not vote yesterday.