South Africa's state-owned power company is suspected of having recruited as many as 40 skilled American atomic reactor operators, who may be working in South Africa in violation of U.S. nuclear nonproliferation laws, according to U.S. officials and congressional sources.

The operators, some of whom are thought to have worked for the Tennessee Valley Authority (TVA) and for private U.S. utilities, apparently were hired by South Africa's Electricity Supply Commission (ESCOM) with promises of tax-free salaries as high as $100,000 a year, free housing, free transportation to South Africa and guarantees of Monday-through-Friday day shifts at a new nuclear plant near Capetown, a congressional source said.

U.S. law requires authorization from the secretary of energy before any U.S. citizen "directly or indirectly" helps certain nations, including South Africa, produce plutonium, a reactor byproduct that can be used to fashion nuclear weapons.

Nuclear reactor operators do not produce nuclear weapons, but control various plant systems. Plutonium, which can be used to make nuclear weapons, is a byproduct of the plant's nuclear reaction.

South Africa has refused to sign the Nuclear Nonproliferation Treaty. There is widespread suspicion within the U.S. government and elsewhere that South Africa is developing or has built nuclear bombs.

Energy and State Department officials confirmed that they are investigating whether the U.S. citizens in South Africa broke the law, which carries a maximum penalty of 10 years in prison and a $10,000 fine. Officials said they are not certain whether the Americans were recruited directly by ESCOM, or whether the South Africans violated laws by hiring them.

A spokesman for the South African Embassy here said Ambassador Bernardus G. Fourie was out of town and that the embassy would have no comment.

Meanwhile, a State Department official said, "We don't know exactly what all of the Americans are doing. We don't have a fix on the numbers, but we are in touch with the South African government. We don't have any evidence of a willful violation of law. We haven't made that determination yet."

U.S. authorities said they hope the operators will identify themselves voluntarily. If they don't, "the extradition agreement between us and South Africa would cover this," an Energy Department official said.

"We obviously can't go into their plants and point out the Americans," another State official added. "The word has gone out through ESCOM, and some [operators] have come forward . . . . If they are outside the law, then they must stop work immediately. But it's not a question, as far as we know at this point, of their being in sensitive nuclear areas like nuclear weapons."

In addition to identifying the Americans and their duties, U.S. officials are trying to determine when the operators began working in South Africa. The law requiring official authorization was enacted in February 1983, and anyone working there earlier may be excluded. However, one congressional source said at least 11 of the operators are thought to have gone to South Africa within the last year.

One official said the issue surfaced in November, apparently when officials at the U.S. Embassy in South Africa heard that unauthorized Americans were working for ESCOM's new French-built Koeberg reactor, which opened in 1984. A second reactor is scheduled to begin operations this spring.

Energy Department officials then heard rumors that reactor operators for the Carolina Power & Light Co. and Southern California Edison's San Onofre nuclear plants north of San Diego were contemplating job offers in South Africa. Investigators confirmed those rumors in recent weeks and warned the operators that they would need government permission before accepting the jobs, an Energy Department official said.

Ian McLeod, a spokesman for the federally owned TVA, said, "I understand that some [TVA operators] have left for South Africa, but I have no idea how many or how many followed through . . . . It's a general consensus that our operators are pretty well trained and are generally sought after by other utilities, and I couldn't rule out South Africa."

Of 265 operators licensed to work at TVA's reactors since 1973, 56 have quit, although it is not clear how many are recent departures to South Africa, McLeod said. He noted that starting pay for licensed operators at TVA is $31,000 a year, and pay for the most senior operators is capped at $52,000.

Energy Department officials, led by Carlton E. Thorne, director of the Energy Department's politico-military security affairs division, are planning to brief some members of Congress this week.

On Friday, Rep. Edward J. Markey (D-Mass.) sent a letter to Energy Secretary Donald P. Hodel requesting "complete information," including classified cables, "about your knowledge or that of any other Department of Energy officials" on the subject.

"It is my understanding," Markey wrote, "that officials at the Department of Energy, and possibly the State Department, may have known of the activities of U.S. citizens in South Africa for as long as a year but failed to take action to correct this situation."

That allegation "is absolutely not true," a senior Energy Department official said. Three State Department officials also said they first became aware of the matter late last year.

In September 1983, 13 authorizations were granted to U.S. companies seeking ESCOM contracts for maintenance and safety work. Confidential Energy Department documents show that 10 other requests are "pending" -- some have been in limbo for a year -- while members of Congress watch to see what the administration decides.

Under a 50-year agreement signed in 1957 as part of the "Atoms for Peace" program, the United States helped build South Africa's Safari I research reactor in the early 1960s.

However, further assistance, including supplies of enriched uranium needed for reactor fuel, was suspended in 1975 because of South Africa's refusal to sign the Nuclear Nonproliferation Treaty. The government has built a pilot enrichment plant and is constructing a larger one.

ESCOM's new Koeberg reactor, built to generate electricity for metropolitan Capetown, has been open to inspection by international nuclear watchdogs. But some U.S. public interest groups say they think that the South Africans could secretly divert plutonium from the plant into a bomb-building program. The Nuclear Control Institute estimates that the two Koeberg reactors will produce more than 600 pounds of plutonium a year, enough for 43 atomic bombs.

"I think the really significant thing is the administration obviously has been sitting on this for some time now," said Paul L. Leventhal, president of the institute. "It is yet another example of the administration facilitating the South African program . . . . Everything that the South Africans do contributes to their capability to produce nuclear weapons."

But Warren F. Witzig, a nuclear engineering professor at Pennsylvania State University, said, "Nuclear power for the production of electricity and nuclear weapons have about as much in common as the Marx brothers with Karl Marx. They share the same name but beyond that it gets a little thin."

Despite the widespread perception that the nuclear industry is moribund in the wake of the Three Mile Island accident and plummeting oil prices, there is "a lot of body-snatching" between nuclear utilities looking for experienced technicians and engineers, according to Don Winston of the Atomic Industrial Forum.

Ninety-one nuclear plants now operate in the United States, and about three times that number operate in other countries. Another 36 plants are under construction in the United States, although 100 U.S. plants have been canceled since the early 1970s.

"That puts on a demand for engineers and for help," Winston added.