Six months after South Korea's first atomic power station went into commercial operation, the reactor is shut down for repairs.

While one group of engineers works to correct a problem with the power plant's water pumps, other workers are performing the foul-smelling task of removing thousands of mussels that were starting to clog the coolingwater intake from the sea.

"The mussel growth was very serious," said B.H. Chung, acting director of the Kori division. "The intake was becoming smaller and smaller. We are now removing all the mussels, and are rigging a device to inject cholorine gas at the very beginning of the intake to keep mussels out."

Both of these problems that contributed to the temporary shutdown of the South Korean plant are not unlike difficulties that seem to keep atomic power stations in developing countries out of service more of the time than nuclear reactors in the industrial nations.

No conclusion can really be drawn yet about the operating record of nuclear power stations in Korea, since the first Kori station only went into commercial service April 29, or about Taiwan, which put its first reactor into operation a year ago.

But of the three developing countries that have had atomic power stations since the early 1970s, only Argentina -- which in September ran its Atucha reactor at 107.5 percent of its rated capacity -- has come close to matching the operating record of the industrial countries.

Pakistan's lone small reactor outside Karachi, which has never operated at capacity since it went into commercial operation in 1972, is limping along in sick condition.

The two Indian reactors at Tarapur have never exceeded 65 percent of capacity for a calendar year in their nine years in service. The third operational Indian ationa atomic power station, at Rajasthan, has an average generation of 33.7 percent since it went into operation in 1973 -- poorest record in the world for its type of reactor.

In fairness, it should be noted that one of the major problems of all developing countries -- obtaining spare parts from the manufacturer -- has been a particular nightmare for India since it staged its nuclear test in May 1974.

Canada, which had largely built the Rajasthan plant, not only cut off all cooperation with India, but would not even stand behind defective equipment that was under warranty.

But over and above this special case, equipment breakdowns -- even where they do not occur much more frequently than in industrial nations -- clearly keep atomic power stations shut down for longer periods of time in developing countries.

"In the U.S., if you need a spare part, you pick up a phone to get it in one day," said Sergio Guimaraes, chief of operations for the first Brazilian atomic plant. "Here, it takes three months."

One consequence is that power plant officials often assemble large staffs in the hope of getting back into service quickly through makeshift repairs.

India's Tarapur plant was designed by General Electric to be operated by 156 people, an Indian nuclear official said. It employs 700 today.

Because the out-of-service time tends to be greater, there is a natural reluctance in developing countries to shut down nuclear reactors for repair -- particularly since shutdowns often tend to mean blacking out major cities.

Nevertheless, officials at Atucha appear to have shown little hesitation in ordering shutdowns, despite the fact that this has meant lights out for sections of Buenos Aires. And Korea's Kori plant is not expected to be put back into operation until January even though this is the country's traditional peak period of electricity demand.

These pressures become more severe, however, when an atomic power station in a developing country represents more than 10 percent of an interconnected electricity grid.

"It is extremely difficult to shut a plant down when it represents more than this," an International Atomic Energy Agency economist said. "If you have to close it down for a month or so, the system is plunged into chaos."

When India put its two Tarapur power stations -- each rated at 210,000 kilowatts -- into operation in 1969, the entire grid in the Bombay area had only the relatively small capacity of 1.8 million kilowatts.

The result, IAEA sources say, was that India kept these power stations running "while maintenance was performed on them, under circumstances that would not have occurred in the United States."

"Officials at the Tarapur plant concede that they have allowed themselves to be persuaded to keep the reactor running at times when they would have liked to shut down.

"Ours were the biggest units in the electric grid," an official said. "We had to take the convenience of the grid into account."

The result, an IAEA safety inspector says, is that "India, in maintenance of these plants, has dosed a lot of people."

Another worry in the case of most developing countries is the absence of a strong regulatory agency, empowered and willing to order immediate operating changes in light of new safety information.

At the Tarapur power station, both nuclear reactors -- which are housed in the same building -- are operated from the same control room. All the auxiliary systems of both units are fed from a common control panel.

The Browns Ferry atomic power stations at Decatur, Ala. -- like Tarapur built by General Electric -- also had some systems in common until a serious fire hit the facility in March 1975.

"Since the fire there, there have been a good many changes," an Indian official noted. No similar changes, however, are envisioned at Tarapur.