Should Brazil persist in its plans to build a nuclear reprocessing plant, Secretary of State Cyrus Vance warned during a visit here two weeks ago, it runs the grave risk of triggering a nuclear arms race with neighboring Argentina, which already has stockpiled enough uranium waste to produce several hundred pounds of bomb-building plutonium once it develops the reprocessing technology.

The message he brought from Beunos Aires, Vance saiud, was that if Brazil would renounce reprocessing, Argentina would likely do the same.

"How childish," a senior official in the Brazilian Foreign Ministry said after Vance had gone. "He probably, said the same thing to Argentina." Brazil, the official said, can handle its relations with Argentina without U.S. help, thank you."

In any case, Vance added during the discussions, if Brazil defers developing its own reprocessing technology the United States could then offer all manner of substitutes - perhaps including the secrets of thorium, a uranium-like fuel found in natural abundance in Brazil.

Nothing but vague promises, the Brazilian, who was present during the discussions, said later. "How do you say it." his aide asked. "A bird in the hand is worth two in the sky?"

Besides, the official said, West Germany, a country that has beem much more generous with its nuclear technology, has its own thorium research program. "The Americans think we don't know anything," he added.

The end of round three in Brazil's nuclear "disagreement" with the United States has left Brazil, by its own account, feeling quite self-satisfield. While the United States has not given up its struggle to keep reprocessing technology from Brazil, it has, a Foreign Ministry spokesman said, "learned the error" of what Brazil considers U.S. heavy-handedness.

Brazil now seems almost taken aback at what it regards as the overwhelming success of its audacity. The Brazilians feel they have proven, for the first time, that a developing country, highly dependent on U.S. trade and support, can strongly disagree and live to tell about it.

Round one was a visit to West Germany early this year by Vice Presietn Mondale to persuade the Germans to cancel the reprocessing part of $10 billion, eight-reactor sale to Brazil on the ground that the Carter administration considers reprocessing technology too dangerous to spread around the world. Brazil, which was not consulted during the talks was furious, West Germany refused to cancel.

Round two, a visit here last March by Deputy Secretary of State Warren Christopher presented a pre-written joint comminuque outlining a curtailed Brazilian nuclear energy program. Brazil refused to sign. Christopher abruptly got on a plane back to Washington, and bilateral relations went into a deep freeze.

Then, came Vance, calm and businesslike. The idea, U.S. negotiators said, was to convince Brazil, on a purely technical level, that reprocessing - the procedure whereby plutonium is separated from the uranium wastes of nuclear reactors - is inefficient and unnecessary. More importantly, the U.S. argument continues, highly volatile plutonium is so dangerous to have around that even the United States has banned construction of reprocessing facilities.

In response, Brazil tried a new tactic. The Brazilians smiled, nodded sagely, said they would take it all into consideration and cordially sent Vance on his way with a brass band, an airport red carpet and a promise to talk again.

"Their objective was to get him back on the plane as soon as possible without opening their mouths," a Latin American diplomat here saud. "They were very satisfied that Vance didn't get anything out of them."

Discussions will doubtless continue this spring, when Carter picks up the southern hemisphere leg of the 11-country tour he postponed last month. "We hope we can convince him," the Foreign Ministry official said.

Carter does not want Brazil, or any other country, to have reprocessing technology. While reprocessed plutonium can be used as a reactor fuel, thus theoretically providing a virtually limitless fuel supply, it can also be converted into nuclear weapons fuel much faster than ordinary enriched uranium. Carter believes it is virtually impossible to safeguard large plutonium stockpiles.

Current international nuclear safeguards amount of regular international inventory checks of nuclear fuels and wastes. Plutonium could quickly be diverted or stole, Carter and others believe, and made into a bomb before anyone would discover it was missing.

U.S. officials say privately that they are less worried about countries like West Germany and Japan developing their own reprocessing technology than they are about a Third Workd century, and they say they're not worried about Japan," the ministry official marveled.

Besides, he said, "We are curretnly negotiating with the Dutch" on a plan that will prove plutonium can be safeguarded. "We are going to show the world . . . The U.S. believes very much in the information it has. Because they are very developed, they are soemtimes inclined to think they have discovered the truth."

He pointed to the fact that while the United States originally objected to both reprocessing and enrichment technology being transferred to Brazil, it has since decided that enrichment is safe after after all.

While reprocessing is only a minor part of Brazil's energy scheme for the next 10 years, the resulting conflicts have made it the symbol of what Brazil considers a life-and-death struggle to continue a monumental development program begun more than a decade ago.

That program, which brought phenomenal yearly growth rates of 10 pr cent, took a sudden dive with the 1974 quadrupling of oil prices.

Brazil now imports more than 80 per cent of the oil it uses, at a cost of more than $300 million a month, To lessen oil dependecy, Brazilian experts have developed an energy plan they predict will reduce oil's share of energy consumption from nearly 44 per cent in 1974 to little more than a third by 1986.

The slack will be taken upo by droelectric capability, through a monumental series of dams now operating, under construction, or planned.

A large-scale switchover to nuclear power is to come much later. Brazil's first power reactor, called Angra I, a 600 megawatt facility now under construction outside a small village on the South Atlantic, will not go into operation until 1978. Three more are scheduled to be on line by 1985, when nuclear energy will still only provide 4 per cent of Brazil's power requirements.

While Brazil is to start enriching its own uranium fuel for those reactors in 1979, thanks to German blueprints for an enrichment facility, explorations for domestic uranium resources have proved nearly as disappointing as the search for oil. To reach the goal of 85 per cent nuclear fuel self-sufficiency within 10 years, the Brazilians say they need reprocessing.