On the mighty Parana River above the Iguacu Falls, the huge Itaipu hydroelectric project now under way is expected to provide Brazil with 10 million kilowatts of electricity by the mid-1980s.

Even when hydroelectric plants currently under construction are completed, Brazil will have an untapped potential of more than 100 million kilowatts of power in its rivers -- about one-fifth of the current electric generating capacity of the United States.

With this vast hydroelectric potential, critics ask, why is the Brazilian government rushing into the costly, complex and potentially hazardous business of nuclear power?

Similar questions are asked in Iran, which today is flaring off natural gas that could meet the country's electricity needs for at least the next 100 years.

The questions are heard, too, in South Africa, where abundant coal reserves enabled its thermal power plants until recently to produce some of the world's cheapest electricity.

"In a lot of these developing countries, there is simply no justification for pursuing nuclear power programs," a U.S. energy expert says.

Yet thoughtful government officials in Brazil, Iran and South Africa -- three nations that on the surface would appear to have less than a compelling need for nuclear energy -- make very persuasive cases for their atomic power programs.

"People think we are investing in nuclear energy to compete with hydropower," says Brazilian Minister of Mines and Energy Shigeaki Ueki. "That's not the case at all."

While Brazil's power system has always been based in hydroelectricity, Ueki explains, the government always wanted a certain percentage of the total output to come from thermal plants because "some years we do not have enough rain to supply the water for our hydro plants."

After the oil crisis, which sent Brazil's payments for foreign petroleum skyrocketing, the government allowed the ratio to slide from 70 percent hydro and 30 percent thermal to 85 percent hydro and 15 percent thermal.

"But we consider this 15 percent thermal the minimum," Ueki says. "If we increased the hydro to 90 or 95 percent and we have an abnormally dry year, we would face serious problems with the entire electrical system of the country."

With vast plans under way to increase Brazil's hydroelectric capacity, the question then becomes whether the best way to maintain the 15 percent thermal capacity is by building new oil power plants or nuclear.

"We are competing nuclear with oil," says Ueki. "And we cannot think at this time to increase the installed capacity based in oil because it is much more expensive than nuclear energy. So really, we need nuclear energy."

Iran's arguments on behalf of its nuclear energy program follow a different line. While Brazilian officials frequently mention the desire to gain access to advanced technology as one of the factors motivating their move into atomic energy, this is the driving force propelling Iran's nuclear program.

"For the shah, the nuclear program was his man to the moon," a highranking West German official says. "Iran's motivation for going into nuclear power had nothing to do with energy or with economics. The motivation was industrial."

In South Africa, the argument raised by critics of nuclear power is coal. But most of South Africa's coal deposits are located in the eastern Transvaal, hundreds of miles from the cities where the power is needed in the southern part of the country.

"Our studies showed that by 1980, nuclear power stations along the Cape would be more economical than importing power from the north," an official of the South African electric company says.

South Africa, however, intends to continue building mammoth coal-fired stations near the coal fields.