With its energy needs scaring. South Korea has tentatively decided to double the size of its already ambitious nuclear programs and spend more than $70 billion to put 43 atomic power stations into operation by the year 2000.

This staggering program would give South Korea a country the size of Indiana a nuclear power generating capacity almost equal to that of the atomic plants currently in operation throughout the United State.

Plants of this magnitude - for a small country that now has only one nuclear power plant in operation - would ordinarily strike observers as highly unrealistic.

By contract, Iran and Brazil - two developing countries that decided to leap into atomic power programs in a big way - are now finding it necessary to scale down their nuclear dreams. But in South Korea where electricity demand since August has been a starting 20 percent higher than a year ago, an expanded nuclear power plan seems not only plausibel, but mandatory.

"We don't have any alternative," Atomic Energy Commissioner Ler Byoung Whie said in an interview. "Our electricty demand for the past few years has been much more than we expected."

Since South Korea has no oil resources, none of the coal best suited for thermal power plants, and has already developed most of its hydro-electric potential, the government intends to rely on nuclear power to meet more than half of the rapidly industrializing country's electricity requirements by the end of the century.

"If we were to find any alternative source much cheaper than nuclear power, then we could shift to that," Lee said. "But at the moment, we don't have any option."

The revised program - replacing an earlier plan to build 21 nuclear plants by the year 2000 - will not be finalized by President Park Chung Hee's government until early next year.

"But we suspect there aren't going to the major changes," said Lee Bong Suh, who is in charge of planning for the Ministry of Energy and Resources.

The Korea Electric Co. moreover, is already making short range plans in accordance with the new program, and now expects to have eight atomic plants - instead of the previously scheduled five - in operation by 1986.

This accelerated timetable is welcomed news for the nuclear construction industry, which has been picking up few new contracts in the developing world recently. It has to be particularly good news for Westing-touse, which won the contracts for four of the first five South Korean atomic power stations.

A $2 billion contract for South Korea's sixth and seventh nuclear power reactors is now expected to be awarded in about a month, and sources say Westinghouse and France's Framatome Consortium have the inside track.

While power industry officials here would like to stick with Westinghouse, informed sources say France has put forward the most attractive financing proposal - offering both a lower interest rate and longer repayment terms. The president of the Korea electric Co. is scheduled to visit Washington late this month to see if the U.S. Export-import Bnak will match the French offer.

Paris reportedly is also bringing strong political pressure to bear on behalf of Framatome, suggesting that if South Korea does not finally place an order for power stations with France, future nuclear cooperation may be in jeopardy.

Officials here are not unmindful of the fact that France was willing in 1975 to sell South Korea a pilot plutonium reprocessing plant - technology that Seoul felt and still feels is important in guaranteeing future fuel supplies for its power stations.

The U.S. government, concerned that the plutonium produced by reprocessing plants is also suitable for the fabrication of nuclear weapons, pressured South Korea into cancelling that deal. But wit their rapidly expanding power program, South Korean officials suggest privately that they may have to review the reprocessing option at some point in the future.

Regardless of who wins the contracts for the sixth and seventh power plants, the contract for two more nuclear reactors will be up for bids in 1979.

South Korean officials show little concern, moreover, with the problem of financing their costly nuclear program. "If we require that much electricity, that itself means economic growth is at a very high rate,' said Lee Bong Suh.

After growing at an average annual rate of more than 17 percent - even though the worldwide recession of the early 1970s - South Korea's electricity demand has continued to rise at more than 20 percent. This phenomenal growth has occurred despite the fact that the tradition peak demand period is just ahead.

"Not only industrial demand is up. This year, the residential growth rate is much higher," said Sung Nack Chung, executive vice president of the Korea Electric Co.

The sharp increase in residential usage reflects a parallel rise in per capita income, which has brought a growing demand by South Koreans for such consumer goods as refrigerators, television sets and air conditioners.

"The rising living standard is a big factor," said Sung. "Many households are purchasing air conditioners. In the summer, we had a very sharp increase in residential [electricity] usage."

The nightmare here, for people who remember the bleak days following the Korean War when not enough electricity was being generated, is once again seeing demand outstrip supply.

"We had a very serious problem last year," Sung said. "We think we're past that this year."

Since it generally takes at least seven years from the time a contract is awarded to the startup of an atomic power station, officials here believe the current pace of their program is more than necessary.

"We have carefully investigated the forecast up to 1985," Sung said. "For the construction of nuclear power plant, we need to start right now."