Brazil has taken a significant step toward developing a nuclear weapons potential by producing its own plutonium in an American-supplied research reactor and reprocessing it in a facility not open to international inspection, according to U.S. intelligence sources.

The plutonium was produced in the 5-megawatt Babcock and Wilcox reactor at Brazil's leading atomic research center, the government-controlled Instituto de Pesquisas Energeticas e Nucleares (IPEN) on the outskirts of Sao Paulo, according to sources.

Under Brazil's agreement of nuclear cooperation with the United States, the reactor is subject to international inspection. But the plutonium produced in it was put through a laboratory-scale reprocessing plant at IPEN that Brazil refuses to place under international safeguards, the sources said. The fact that Brazil was able to breed plutonium by placing its natural uranium into an American-supplied research reactor--and irradiating it with American-supplied highly enriched uranium fuel--is certain to arouse new congressional concern over the adequacy of U.S. nuclear export arrangements.

The amount of plutonium reprocessed at IPEN to date was said by sources to be "no more than a few grams," and the sources said Brazil at this point does not have the facilities either to produce or to reprocess the five or six kilograms of plutonium needed for a nuclear weapon.

"Brazil is not building a bomb at IPEN," a State Department official said.

Even if the U.S. reactor at IPEN was in constant operation, which it is not, because its American-supplied fuel is becoming burned up and needs to be replaced, Brazil could produce a maximum of 500 to 750 grams of plutonium in the reactor in a year, intelligence sources said.

At the same time, by beginning to gain experience in separating and handling plutonium, Brazil continues to follow the nuclear lead of its regional rival, Argentina.

Argentina, which long has had the most advanced nuclear facilities in Latin America, has reprocessed laboratory quantities of plutonium and is completing construction of a large unsafeguarded reprocessing plant that some analysts believe could produce enough material for a nuclear weapons program.

Brazil, as part of a comprehensive nuclear pact signed with West Germany in 1975, also is developing plans for a larger reprocessing plant to be completed in the late 1980s. But the agreement requires that the plant and all other imported nuclear equipment and facilities be open to international inspectors.

Brazil, however, refuses to permit international inspection of the small reprocessing facility at IPEN, or of IPEN's uranium conversion and fuel fabrication facilities where it produced the materials irradiated in the American reactor, on the grounds that it built these facilities without assistance.

Neither Brazil nor Argentina is a party to the nuclear Non-Proliferation Treaty.

Brazil's use of its American research reactor to produce plutonium for reprocessing in an unsafeguarded facility is certain to be viewed by congressional non-proliferation leaders as a violation of at least the spirit of that country's nuclear agreements with the United States.

State Department sources, however, said Brazil technically has not violated its agreements with the United States. If it had gained access to plutonium by reprocessing the fuel the United States has supplied over the years for the research reactor without seeking U.S. consent, that would have been a violation.

But the bilateral agreement, sources said, apparently did not specifically foresee the possibility of Brazil producing its own plutonium in the reactor and reprocessing it in its unsafeguarded facility.

The United States is barred by the 1978 Nuclear Non-Proliferation Act from exporting new supplies of enriched uranium fuel to Brazil for its three research reactors and its first nuclear power station because of Brazil's refusal to open all its atomic facilities to international inspection.

The method Brazil used to produce the plutonium is, on a very small scale, similar to the route Israel said it feared Iraq was preparing to follow with the 70-megawatt French research reactor that was being built on the outskirts of Baghdad. Israel destroyed the reactor in a strike in June, 1981.

The Brazilian Embassy in Washington did not respond to a request for information about its nuclear program.