When a representative of the U.S. Nuclear Regulatory Commission visited the Korean Nuclear Fuel Development Institute recently, scientists there say he gave them some blunt advice.

"He told us: 'Don't do research. Don't even read in nuclear science. It scares us," recalled Jac Hyun Yang, president of the institute.

Reluctantly, South Korean officials are for the moment going along with the Carter administration's wish that they not try to develop a plutonium reprocessing plant-a facility that produces material suitable for use in nuclear weapons.

But the size and sophistication of some of the research efforts in Asia, Africa and Latin America make it clear that whatever the Carter administration's desire, there is no long term way of halting the spread "sensitive" technologies to the more advanced developing nations.

"What's known is known," a top West German official remarks philosophically. Even technologies that are not known, moreover, seem within the reach of countries willing to commit money and manpower.

At least half a dozen countries in Latin America, Africa and Asia have long since made that choice.

The Bhabha Atomic Research Center outside Bombay-which produced the device used in India's 1974 nuclear test-has a staff today of more than 12,000, making it one of the largest research complexes of its kind anywhere.

Scientists at the Bhabha center have four atomic research reactors for their experiments, and engineers are currently building a fifth-which will be most powerful in the developing world.

Argentina's Constituyentes and Ezeiza atomic centers, both located on the outskirts of Buenos Aires, have been the focus of major nuclear research efforts for more than two decades.

Nor can it be assumed that research efforts in less industrialized nations will progress move slowly than in major Western countries.

South Africa nuclear scientists have not only managed to develop a new uranium enrichment process, but mastered it faster and more successfuly than scientists working on a related enrichment process in West Germany.

"The whole of the Karlsruhe center-which is four or five times the size of my little center here-was behind the (German) Becker process in its development, and they have all of the German industries to help them in building components. And it spite of that, we're got a plant in operation and they're still struggling." Dr. A. J. A. Roux, president of South Africa's Atomic Energy Board, declared proudly.

Thus far, most developing countries with major nuclear research programs have geared their efforts to making their atomic power stations relatively self-sufficient.

"We came to the conclusion that on all sensitive things, we had to be totally independent," say Homi Sethna, head of India's Department of Atomic Energy.

India's scientists and engineers thus put major emphasis on developing the capacity to produce the fuel for India's natural uranium power reactors.

"I must say we are one of the few countries today that does it from one end to another," Sethna said. "So we are now totally independent as far as that is concerned."

Argentina also decided early to base its nuclear power program on nautral-uranium reactors, and to strive for self-sufficiency. By the end of next year, Argentina expects to be fabricating most of the fuel elements for its Atucha atomic power station.

"If you depend on foreign suppliers, all your decisions are questioned from abroad," says Adm. Raul Castro Madero, head of Argentina's Atomic Energy Commission. "They say 'No, you cannont do this,' or 'You cannot do that.'"

"We think we are going to control the fuel cycle for natural uranium," Castro Madero said. "Then you are in a better position to make your own decisions. You are not weaklings with foreign countries."

South Africa has similarly pursued the route of independence for its nuclear power program-a more complicated undertaking since its plants are fueled with slightly enriched uranium.

"For a small country, it's a big thing to say you want to make your country self-sufficient over the whole of the fuel cycle," Roux said. "But my organizaion has worked toward that end.

"We've got the uranium. We've got enrichment. And in 10 to 15 years time-provided we have a smooth run of development and South Africa's head is not knocked off by sanctions-I see no reason why we can't build reactors," Roux said.

Not all developing countries have the sophisticated nuclear research programs of India, Argentina or South Africa.

While small atomic research reactors are fairly widespread through Asia, Africa and Latin America, most developing countries have modest programs that are far from the frontiers of nuclear technology.

But a growing number of countries like Iran and Brazil, which have had relatively small research programs, are determined to close the gap that separates them from the major Western powers.

In Tehran, scientists at Iran's nuclear research institute have bought a Tokomak for fusion experiments, and have built another fusion device-a linear theta pinch-second in size only to the one at Los Alamos.

"Nobody expects us to have a breakthrough in fusion," says Piran Sioshansi, the center's director. "But should the first fusion reactor be built in the year 2000, we don't want to have to go around looking for some idea of what fusion is all about.

"I'm sure that whatever they can do at Berkeley in a week, it will take us a year to do," Sioshansi adds. "But in certain specific areas, we can already do first class sophisticated research. And like many developing countries, I think we are getting better by the day."

Next: The Nuclear Future CAPTION: Pictures 1 and 2, Scientists at Iran's nuclear research institute perform fusion experiments on a Tokomak (left) and a linear theta pinch: 'In certain areas, we can already do first class sophisticated research . . . and we are getting better by the day.' Iranian Atomic Energy Commission