Japan has won approval from the United States for operation of a controversial nuclear fuel reprocessing plant in an agreement that represents a partial turnabout by the Carter administration.

Japanese officials regard the agreement, announced here today by U.S. and Japanese negotiators, as a victory for the government of Premier Takeo Fukuda.

Fukuda's government had staked considerable prestige in getting the experimental plant into operation this year dispite the Carter administration's contention that it would endanger U.S. efforts to halt nuclear proliferation.

While the chief U.S. negotiator, Gerard Smith, called the agreement a said both sides had gained points in the few details of the agreement that trickled out indicated that the Japanese have gooten pretty much what they wanted.

The United States had objected to Japan's plan for starting up the pilot project this year and had urged Japan to modify the plant to provide for a system of "co-processing" of uranium and plutonium in a form that could not be tranformed into material for nuclear weapons.

Japan, however, contended that the modifications the United States proposed would more than $2 billion and require more than 10 years to complete.

Under the agreement, the Japanese will be able to start up the plant using the simpler reprocessing method they originally preferred and will be able planned to reprocess as much fuel as they had.

The only restriction appears to be two-year limitation on the use of this reprocessing system but sources on both sides said that the agreement does not clearly define what happens after the two years.

The agreement still must receive the approval of Fukuda and president Carter. The Japanese said they expect final approval later this month when a delegation of experts visits Washington.

Smith defended the agreement today, saying that "the United States has no doubt about Japan's strong determination not to possed nuclear weapons."

The spent fuel to be used at Tokal comes from the United States which, under a 1958 agreement, has the right to insist on safeguards for any of the reprocessed fuel.

If finally adopted, the agreement would remove one of the major controversies between the two countries.

Encouraged by previous U.S. administrations, the Japanese had spent about $300 million devolping the plant at Tokai, 60 miles northeast of Tokyo for reprocessing plutonium. When the Carter administration, as part of its worldwide effort to curb nuclear proliferation, raised objections Japan countered that it could not tolerate delays and a lengthy, costly modification. Japan insisted on a single-extraction process that would produce pure plutonium.

According to official U.S. sources, the agreement provides that Japan may process with its original plan for two years. During that period, Japanese scientists will intensively study ways to shift to a co-processing methods. In the meantime, the sources said, the plutonium produced at Tokai would be stored separately from the uranium and in liquid form.

The U.S. sources insisted that they won some points in the negotiations. They said that the Japanese side offered to defey the planned construction of a fuel-conversion plant that would have transformed the plutonium into a form from which weapons-grade material could be produced. They also said, the Japanses co-processing studies would provide valuable technical information that other nations could use as a guide.

Japanese sources seemed elated by the outcome. An official of the science and technology agency said the U.S. negotiators had made "fairly big concessions" to Japan.

Neither side specified what would happen after the two-year experimental operations are over. There apparently was no understanding that the Japanese would have to cease operations then or would have to begin a co-processing method.

The U.S. official said there was only an agreement to reconsider the problem in the light of Japanese technical discoveries in the period.

A Japanese official was asked what would happen if no new technique is discovered during the two years. "Well, Japan will go on the same way," he said. "Anyway, that will be the point which will be discussed after two years."