To win a crucial conference committee vote for his natural gas bill, President Carter has agreed to spend $1.55 billion over the next three years on breeder reactor research he previously said he didn't think was desirable.

Sen. James A. McClure (R-Idaho), who provided a key vote that will send the controversial natural gas pricing bill to the floors of the House and Senate, yesterday released details of his compromise with the administration on breeder research.

The compromise - which provoked immediate opposition from environmental and consumer activists - commits the administration to support major new investments in the development of a liquid metal fast breeder reactor (LMFBR), but does not include immediate construction of a prototype breeder reactor on Clinch River, Tenn. Carter has opposed this.

McClure said in an interview yesterday, "This is a fundamental dedication to a liquid metal fast breeder reactor program." That is just what the Carter administration had previously opposed.

The liquid metal fast breeder reactor is meant to produce electricity in a process that creates more atomic fuel than it uses.This fuel is in the form of plutonium, a substance used to make atomic weapons, and is highly toxic.

In his presidential campaign and subsequently, Carter has argued that, by multiplying the use and production of plutonium, the breeder reactor adds to the danger that more and more countries will develop their own nuclear weapons.

Carter has also traditionally argued for more emphasis on innovative, non-nuclear sources of energy. For example, during the presidential campaign of 1976, he said the amount of money being spent by the government on the fast breeder reactor "should be drastically reduced."

In office, the Carter administration has sought to block the construction of the country's first LFMBR at Clinch River, a project begun earlier that has enjoyed strong congressional support. Last year Carter vetoed a bill authorizing construction money for the reactor. But congressional votes have kept the project alive, and more floor fights have been expected on Clinch River this year.

To the extent that the compromise with McClure avoids a commitment to build Clinch River, the administration can claim a partial victory. But the compromise would keep Clinch River alive as a possible future option at the end of a three-year research program costing more than $1.5 billion.

The compromise allows the president (Carter or a successor) to make a final decision on whether to build a LMFBR in 1981, basing that decision on the results of the new research progam the administration is now committed to support.

Whether the McClure-Carter compromise will survive debate in House and Senate is uncertain. Administration officials that it is likely to be approved in the Senate, but that the House prospects are cloudier.

The compromise angered many interested parties, including Sen. Howard H. Baker Jr. (R-Tenn.), the Senate minority leader, whose home state stands to lose millions if Clinch River is not built. Baker said yesterday he was withdrawing his support for the president's natural gas bill, apparently in protest of the McClure compromise, and would try to prevent cloture in an expected Senate filibuster on the gas bill.

According to McClure, Sen. Mark O. Hatfield (R-Ore.) is also upset at the compromise, and might withhold his support for the gas bill in protest. Hatfield could not be reached last night.

If Baker actively supported a filibuster on the gas bill, administration lobbyists conceded yesterday, its prospects would be considerably dimmed. But the same administration officials said they thought Baker's statement yesterday was intended for home consumption in Tennensee, and that he might still support the gas bill, which has become the symbol of Carter's energy program.

McClure said he would understand Baker's concern, given the potential benefit Clinch River could bring to Tennessee. Administration officials noted that an expanded breeder research program of the kind the compromise envisages would be a boon to McClure's state of Idaho, which is home to some of these research efforts.

Industry spokesmen yesterday withheld comment on the McClure compromise and asked time to study it. Several said that the atomic energy industry in general is deeply suspicious of the Carter administration intentions.

McClure said, in effect, that the suspicion was justified, since the administration's plans had been to spend a steadily declining amount on fast breeder research over the next three years, leaving the program in a much-reduced state by 1981.

Jim Cubie of the Union of Concerned Scientists, who has led a lobbying effort of environmentalists, consumer groups and skeptics against the LMFBR, said yesterday the new compromise is "a clear retreat" from previous administration policy and represented "a waste of money."

Carter "looks like a fool on this," Cubie said, arguing that Carter now supports spending money on something he has previously called wasteful.