West Germany has decided to allow the sale of a heavy water nuclear power plant to Argentina despite American objections that Argentina's program lacks safeguards against the development of nuclear weapons.

An American expert said that the agreement reached between Bonn and Argentina after several months of negotiations "does not prevent Argentina from developing an independent nuclear fuel cycle to make a bomb."

West German officials here pointed out that the Argentines pledged to use nuclear energy for peaceful purposes and "indicated" in a separate document that they would ratify the Treaty of Tlateloco banning nuclear weapons in Latin America. The latter action was expected pending the completion of current talks between Argentina and the Vienna-based International Atomic Energy Agency.

The 1967 Tlateloco treaty has been rendered ineffective because of an Argentine-Brazilian nuclear rivalry. Argentina formally agreed to ratify the treaty during a 1977 visit by Secretary of State Cyrus Vance to Buenos Aires. No action was taken because Brazil has maintained that its nuclear plans are "nonnegotiable."

As an added complication, Brazil has refused to put the treaty into effect until every other eligible country has. That includes Argentina and, more importantly, Cuba -- which refuses to sign.

While Bonn officials stressed that new assurances that followed the decision this week to grant an export license for the reactor were significant, it is clear that they fall short of the more encompassing controls urged by the United States --- controls that would cover any and all nuclear operations Argentina might build itself.

The West German sale was criticized by Brigitte Erler, a member of the West German parliament and of the ruling Social Democratic Party, who said the deal will contribute to the ability of Argentina's military regime to produce a nuclear bomb.

Argentina already has the most advanced nuclear power program in Latin America.

Under the terms of the agreement, Argentina assured Bonn that it will use nuclear energy only for peaceful purposes. It promised to place under international controls not only the new plant but any future nuclear facilities it might build by copying the German model. And it provided fresh assurances that all current nuclear material in Argentina corresponds to international controls.

The new project involves construction in Argentina by West Germany's Kraftwerk Union, a Siemens subsidiary, of a 740-megawatt heavy water nuclear reactor. In conjunction, the Swiss firm Sulzer Brothers Ltd. has contracted to build a large heavy water production plant. Together, those projects were bid at a price of $1.6 billion.

Argentina has had a 320-megawatt heavy water reactor in operation since 1974 (also built by a West German firm) and is building a 600-megawatt reactor with a Canadian company. These reactors are especially attractive to Argentina because they run on unenriched uranium -- of which Argentina has plenty -- in contrast with enriched uranium, which would have to be imported.

This plays nicely into Argentina's announced aim of nuclear independence. Argentine officials plan to have six such reactors in operation by the year 2000.

Actually, the heavy water production plant being built by the Swiss will be a more significant addition to Argentina's nuclear program than the West German reactor. Much of the political heat was taken off West Germany by Switzerland's announced determination earlier to proceed with its part of the project without a blanket safeguards agreement from Argentina. i

There was considerable domestic economic pressure on the Bonn government, coming from West Germany's stagnating nuclear industry, to approve the project. Moreover, Argentine officials reportedly showed stiff resistance to the so-called "full-scope" agreement promoted by the United States, Canada, Britain and Australia. It would have subjected all past, present and future nuclear facilities in Argentina to controls and periodic inspections by the International Atomic Energy Agency.

German officials did insist on more from Argentina than was strictly required of them -- and more than the Swiss did. Under the code of business conduct that has been accepted by the 15 major nuclear exporters -- the so-called London suppliers' group guidelines -- West Germany simply was obliged to get a promise from Argentina not to use nuclear material supplied by West Germany in weapons production.

Even so, Argentina's formal pledge to use nuclear energy only for peaceful purposes does not prevent it from setting off a nuclear explosion and calling it -- as India did in 1974 -- a "peaceful explosion."

U.S. officials see the German-Argentine deal as a setback for their efforts to win a tougher set of nuclear supplier guidelines. One American official said, "This deal is a pothole on the road to full-scope safeguards."