A 66-nation nuclear conference today rebuffed President Carter's appeal to curb international use of fast-breeder nuclear reactors. The conference instead adopted a technical report strongly supporting rapid development of breeder technology.

The report adopted today would appear to vindicate foreign countries that have forged ahead with breeder programs. With several Republican presidential candidates clearly on record as favoring development of an American breeder, the new report could make the advanced reactor an election-year issue.

The study, carried out as part of the International Nuclear Fuel Cycle Evaluation, which opened its final session here today, concludes that fast-breeder reactors would release less radioactivity into the air than existing atomic plants and would also expose the operators of nuclear power plants to smaller radiation doses.

The report cited a number of other ecological and environmental advantages for fast-breeder reactors as well. In the climate of concern over nuclear safety that has existed since the Three Mile Island accident, it seems likely to reopen the debate over the fast-breeder reactor program in the United States.

President Carter, in organizing the international body's study in 1977, sought to kill construction of a U.S. demonstration fast-breeder reactor at Clinch River, Tenn., and tried to persuade other countries to slow up their breeder programs as well.

The president argued against the spread of fast-breeders -- so called because they breed more plutonium fuel than they use -- out of concern that a sharp increase in the worldwide availabilty of plutonium, which can be used to fabricate atomic bombs, would add to the problem of curbing nuclear proliferation.

But France, Britain, the Soviet Union, Japan and other countries have pressed on with their fast-breeder programs, and Carter has been unable to kill the Clinch River project. Backers of the breeder have each year pushed funding to keep Clinch River alive through Congress, voting an additional $172 million for the project in September by tacking a rider on a federal payroll bill.

Among the advantages cited for the fast breeder in the report:

Radioactive releases. "During the normal operation of a power reactor, there are minor releases of radioactivity into the environment in the form of both gaseous and liquid effluents, there is some evidence to suggest that there may be a difference between the two systems, with the fast reactor appearing to demonstrate a clear advantage."

Radiation exposure. "Occupational exposure should be lower for fast-breeder reactors than for [existing] reactors. Some European utilities are relying on the future fast breeder to decrease the annual does per operator to well below the figure for the [existing] stations."

Thermal pollution. "The thermal efficiency of the fast-breeder reactor is higher than that of the [existing] reactor because the fast breeder operates at higher temperatures; the problems of thermal pollution are therefore reduced."

Nuclear waste disposal. "More waste would be expected to arise from the [existing] power cycle than from the fast-breeder cycle . . . This decrease would require fewer waste handling requirements and, consequently, less permanent storage capacity."

Even members of the U.S. delegation taking part in the fuel cycle talks here concede that the breeder reactor probably would reduce the number of occupational deaths associated with the nuclear industry, since it largely operates on plutonium and thus would reduce the need for the uranium that fuels existing atomic power plants.

A U.S. official said uranium mining is the primary cause of fatalities in the nuclear energy industry.

U.S. officials decided not to dissent formally from the study's general endorsement of breeders for countries with large electricity requirements. They made it clear, however, that they feel the report is too one-sided.

"The need and prospects for breeders understandably are given considerable attention in the light of their promise and the interest that some nations have in this technology," U.S. Ambassador-at-Large Gerard Smith said in addressing the session today.

"But the breeder is not without its costs, risks, and uncertainties," Smith added. "That report focuses much more on the positive features of the technology."

The United States did express satisfaction -- from a nonproliferation viewpoint -- that the report states that "breeder development and deployment is not likely to be economically attractive to countries with a small number of nuclear power plants, or just embarking on such a program."

In an earlier development, the conference delegates, in a last-minute compromise, elected Y. Yatabe, Japan's chief delegate to the talks, to be president of the group's final session.

Sir Hermann Bondi of Britain had been tapped informally to be president, but the group of 77 -- representing 117 Third World nations -- over the weekend objected to selection of a delegate from a nuclear weapons country to chair the gathering.