The United States has granted Brazil a temporary exemption in advance from a $20 million fine the Brazilians could have incurred by refusing full-scope safeguards of nuclear facilities here.

The move was announced yesterday by visiting Vice President George Bush, who said today the exemption could not be blocked by Congress. It is seen as the Reagan administration's first concrete step to relax Carter-era controls on nuclear exports to the Third World.

In 1972, the United States and Brazil signed a 30-year contract guaranteeing U.S. supplies of enriched uranium to Brazil's first nuclear power station, Angra I, built by Westinghouse and due to go on stream next month. The initial fuel load was delivered by the United States several years ago.

But in 1978 Congress passed the Nuclear Nonproliferation Act, setting more strict conditions for export of nuclear fuel than initially were established for Brazil.

Brazil, which is undertaking a $25 billion nuclear power program, has refused to sign the Nuclear Nonproliferation Treaty, despite U.S. pressures. While Brazil disclaims any intention to build nuclear weapons, it contends that the treaty gives unfair advantages to established nuclear powers.

The 1978 U.S. law does not require a purchaser of enriched uranium to sign the treaty, but it does call for sweeping inspections of all current facilities by the International Atomic Energy Agency.

The Brazilians allow international inspections of their West German- and U.S.-designed facilities but have refused IAEA access to a national reactor in Sao Paulo.

If the Brazilians ducked the new U.S. conditions and bought their supplies elsewhere, they faced a $20 million fine for breach of the exclusive supply contract dating to 1972.

Nearing a 1982 deadline for delivery of 20 tons of enriched uranium to recharge Angra I, the Brazilians recently arranged to obtain the needed supplies from a consortium of West German, Dutch and British uranium-enrichment companies. The U.S. action announced by Bush will remove the threat of the $20 million fine for that Brazilian purchase.

The conflict over uranium supplies has strained U.S.-Brazilian relations, but during Bush's three-day visit he has said repeatedly that the United States now wants nuclear cooperation.

"Both countries will work actively during the next year to resolve differences and establish a reliable supply relationship," he said.

The terms of the 1978 U.S. law reflected Jimmy Carter's efforts to prevent spread of nuclear weapon-making capacities. In Latin America, the principal targets were Brazil and Argentina, both with advanced nuclear programs and a reluctance to conform to the most sweeping international controls.

However, the law also complicated U.S. nuclear sales to Mexico, which not only signed the nonproliferation treaty but initiated the Treaty of Tlatelolco, establishing a zone free of nuclear weapons in Latin America. The United States has yet to ratify protocols linking it to that treaty.

The suspension of the fine against Brazil follows press reports from Washington that top administration policy-makers on nuclear control plan to meet Monday to map out a strategy for easing laws restricting nuclear exports to Third World countries.

One proposal reportedly is for weakening laws requiring countries without atomic weapons to allow international inspection of all facilities to ensure continued U.S. nuclear supplies.