Three years after Brazil concluded an unprecedented agreement to obtain sensitive nuclear technology from West Germany, the government's ambitious atomic power program is lagging for behind schedule. As a result, Brazil now seems unlikely to have a nuclear weapons potential until at least the 1990s.

Some well-informed Brazilian scientists feel that the aspects of the West German deal that have most concerned the Carter administration- Bonn's agreements to help Brazil construct uranium enrichment and fuel reprocessing plants that could produce materials suitable for diversion to a nuclear weapons program-are becoming very distant worries.

"I think these parts of the deal will sort of fade into the horizon," Jose Goldemberg, Prresident of the Brazilian Society of Physicists, said in an interview.

The unique uranium enrichment process that West Germany agreed to share with Brazil still has never been tested on more than a laboratory scale. Plans now call for it to be tested in a larger model in 1981, but even that one will not be able to actually produce enriched uranium.

Even if the technique appears at that time to be feasible on an industrial scale-a big "if," in the opinion of non-Brazilian scientists who have closely followed the West German program-construction of even the small enrichment plan currently envisaged by Brazil would take at least into the late 1980s.

At best, Brazil may be able to produce uranium of low enrichment suitable for use in a nuclear power plant by 1989. Production of weapons grade uranium would not be possible before the 1990s.

The reprocessing plant that West Germany promised to help Brazil build, moreover, appears a far less immediate proliferation worry than was originally feared.

Construction is not expected to actually start until 1965. Officials also say the Brazilian plant will be of laboratory size, designed not to produce any significant amounts of plutonium, which can be used in nuclear weapons, but rather to let Brazilian scientists begin learning reprocessing technology.

What's more, the key to West German aid in helping Brazil build both enrichment and reprocessing plants appears to lie in Brazilian willingness to forge ahead with purchase of eight nuclear power reactors that were scheduled under the original agreement to be completed by 1990.For Bonn, the advanced technology was only the sweetener to land a huge reactor sale.

But three years after announcement of the West German-Brazilian agreement, only two of the eight reactors are actually under contract Construction work on those is already two years behind schedule, and no date has yet been set for signing any additional contracts.

Since the price of the reactors is calculated in deutschmarks, cost of the total package has almost doubled since 1975 to an estimated $12 billion.

"The program is becoming very expensive for us," one high-ranking Brazilzian official remarked privately.

The atmosphere has also been soured by recent charges both here and in Germany of corruption in the Brazilian nuclear program, and shoddy construction work in sinking the concrete pilings that will support the first nuclear reactor.

The allegations, which have begun appearing almost daily in the leading Brazilian newspaper, finally proded a Brazilian congressional committee into opening a public inquiry this past week.

Top officials within the power industry here and also jittery about how Gen. Joao Baptista Figuereido, the ruling military/s designee to be the next president of Brazil, feels about nuclear energy. Many informed observers here believe that he may well decide to cut back the country's nuclear program.

"I feel the new president will move to slow this down," one ranking Brazilian official said.

West German officials have hinted that such a development might cause Bonn in turn to drag its heels in hepling Brazil acquire the coveted enrichment and reprocessing technology.

Brazil's minister of mines and energy, Shigeaki Ueki, insisted in an interview that Brazil intends to press ahead with the purchase of all eight reactors.

"Really, we need nuclear energy," Ueike said. "And we must have all the technology of the fuel cycle, from the concentration of uranium up through reprocessing."

Ueki conceded that the West German deal "many be slowed-extended, for two or three years" to 1982 or 1983. But, he declared, "we're not changing our program. Our nuclear program is very realistic."

As informed foreign observer who has closely followed the Brazilian program, however, did not share Ueki's optimism.

"The nuclear program that they have embarked on is relaly beyond them," he said. "They do not have the industirial infrastructure which is necessary to accommodate this vast development in an orderly, logical way."

If Brazil has four of its West German power plants operating by 1990, he added, it will be a "miracle."