The refusal by the United States to supply South Africa with nuclear fuel for its atomic energy program since 1976 has forced this country to slow down its atomic research.

In fact, there is a possibility that a French-made nuclear power plant now under construction here will remain inoperable for as long as a year after its completion as a result of a fuel shortage.

The U.S. embargo on the shipment of any more nuclear fuel to South Africa stems from this country's refusal to sign the Nuclear Nonproliferation Treaty or to submit its entire nuclear program to international inspection and safeguards.

Negotiations between the United States and South Africa, under way for more than three years, have so far failed to break the impasses. France has also been involved in the talks, because a French consortium is building South Africa's two nuclear power plants at Koeberg near Capetown.

The nuclear field is one of the few areas where Washington has been able to exert some concrete influence over South Africa. On other sensitive issues, such as this country's internationally decried racial policies, U.S. leverage has been minimal.

South Africa's official justification for refusing to sign the nonproliferation treaty is that it is developing its own secret process to enrich uranium, which it hopes to exploit commercially by making use of its vast uranium reserves and does not want to be discovered by other countries in the process of international inspection. t

It also resents U.S. efforts to get it to sign a treaty that other nuclear powers, like France, India and China, as well as some non-nuclear ones, such as Israel, Argentina and Brazil, have refused to sign.

[U.S. officials in Washington said that while it was true South Africa was developing its own enrichment process, other countries also doing this, such as Japan and the Netherlands, had allowed international inspection of their facilities. "They are hardly in a unique position in this regard." one official said.]

The U.S. government feels that South Africa's ability to enrich uranium -- and therefore to produce weapons-grade material if it chooses to do so -- makes it particularly important that this country accept international inspection of its facilities. Washington has refused to fulfill fuel supply contracts for the two Koeberg plants until it does.

South Africa already suffers from a high international profile because of its racial policy of apartheid. "More and more countries feel there should not be any nuclear relations with South Africa until there are full-scale safeguards," said one source familiar with the nuclear dispute between Pretoria and Washington.

The American concern about the program here, which the United States helped to build up, has been heightened by suspicions that South Africa already has a nuclear weapons capability.

In August, 1977, a Soviet spy satellite detected what appeared to be a test site in the desert area of South Africa. Then, last Sept. 22, a U.S. satellite registered a "blue flash" in the atomosphere near South Africa, setting off speculation it had tested a nuclear device. American scientists are still divided on whether or not there really was a nuclear explosion.

Pretoria has repeatedly said its nuclear program is for peaceful purposes only, and it labeled reports it had tested a device last year as "nonsense."

U.S. leverage over South Africa in this dispute appears to be only temporary. South Africa responded to the fuel cutoff with a decision to make its own by expanding a pilot uranium enrichment plant operating at Palindaba just west of Pretoria since 1975. It is this facility Washington is most interested in bringing under safeguards, because weapons-grade material as well as fuel for the power plants could be made there.

Officials will not say exactly when that project will be completed. But it is generally thought that the first of the two power plants at Koeberg will be ready in 1982. The 2,000-megawatt plant may have to remain idle at a reported cost of $1.3 million a day until the enrichment plant is completed. The second power plant is due to be finished in 1983.

Nuclear physics reseacher J.P. Sellschop from the University of Witwatersrand estimates that the locally enriched uranium fuel may not be available until the end of 1983.

Even then, only half the problem would be solved because the South Africans must find a country willing to process that fuel -- slightly enriched (3 percent) uranium hexafluoride -- into fuel elements to fit into the power plant reactors.

Although South Africa has the technical knowledge to do this, it would require a massive allocation of resources to build up the sophisticated metallurgical industries to do it, according to Sellschop.

In the parlance of research and development, "it's not so much a matter of 'r' but of 'd,' and it would be a pity because our needs are so small," Sellschop said.

There have been public reports outside South Africa that a French-Belgian company named Eruofuel has already contracted to fabricate the fuel rods either from American-supplied or South African-made fuel.

France, which has a great interest in seeing the nuclear power plants come into operation once they are built, nevertheless has been cooperting with U.S. efforts to get South Africa to agree to some kind of internationally recognized safeguards, according to an informed source.

"The French don't seem to want responsibility for South Africa on their shoulders," the source said.

But if South Africa finds a company that will fabricate the fuel elements from South Africa's fuel, U.S. efforts would be undercut.

Althogh the financial investment in the power plants has made this the most immediate problem for the South Africans, they are also vexed by the threat to their atomic research program.

The United States also refuses to supplying highly enriched (93 percent) weapons-grade fuel for the American-made research reactor at Pelindaba.

Atomic Energy Board president Wynand de Villiers said over a year ago that the small (2') megawatt) Safari reactor, which has run on U.S. supplied fuel since 1965, was only working it one-eighth its capacity. Research there includes work on the use of atomic energy in medicine and geology.

The reactor has only enough fuel left for a few months, authoritative sources say.

De Villiers has also stated that South Africa has the technology to make highly enriched uranium for its Safari reactor. "If the government should so decide, in the future we could produce it," he said.

"Safari may go cold," said Sellschop, who does research there. "But not indefinitely. It would only be for a few years. The U.S. will have thrown away their initiative on this" by refusing to supply South Africa fuel.