In contrast to the United States, where President Carter has moved to restrict the use of plutonium, the Soviet Union is developing it as a nuclear fuel to generate electricity.

The Soviets are in the midst of a construction program that in two years will find them generating 720,000 kilowatts of electricity from the two largest plutonium breeders in the world.

If no unforseen difficulties are encountered in these two plants, the Soviets plan to start construction in 1983 of a third breeder that will more than double their breeder-produced electric capacity.

The United States does not operate a breeder electric plant and has no firm plans to build one. Carter ordered a halt to the proposed Clinch River.Tenn. breeder reactor, which would have produced 500,000 kilowatts of power, to find an alternate nuclear process that does not breed plutonium.

Plutonium breeder reactors produce their own source of fuel and do not rely on what many view as a diminishing supply of uranium. Critics of the process complain that the reactors also produce plutonium, which can be used to make nuclear weapons.

Carter opposes development of the breeder plants for that reason and has urged other nations to follow suit. Carter has also deferred indefinitely U.S. plans to produce plutonium for civilian use.

Indications here are that Soviet civilian nuclear energy technicians receive only small amounts of plutonium for experimental use from the Soviet military establishment. The overwhelming bulk of the Soviet plutonium is controlled by the military, Western experts think.

"We don't see any difficulties moving to plutonium," said Mikhail Troyanov. deputy director of the Institute of Physics and Power Engineering. The institute, 80 miles southwest of Moscow, is where the Soviet nuclear power program was born 24 years ago.

"After 1990, breeders should be built in the Soviet Union in large numbers," Troyanov added.

Troyanov was speaking to American science writers on a trip arranged in cooperation with the Soviets by the Atomic Industrial Forum, a Washington based trade association. It is made up of major nuclear suppliers including Westinghouse and General Electric. The forum vigorously opposed Carter's efforts to stop the Clinch River breeder reactor.

Soviet Nuclear leaders, while not criticizing Carter's Clinch River cancellation, question the wisdom of the move. The Soviets say they see no alternative to plutonium as a replacement for what they say is an inevitable dwindling of uranium supply.

There is a worldwide debate on how much uranium can be dug out of the earth's crust, in part because mining interests are only beginning to explore agressively for it.

The Carter adminstration takes the optimistic view that there is as much as 4 million tons of extractable uranium in the United States. The National Academy of Sciences believes the United States can hope to mine one third that much.

The Soviet Union does not advertise the size of its uranium reserves, except to point out that they are not limitless. The Soviets already, import raw uranium ore from East Germany, Poland and Czechoslovakia.

While clearly ahead of the rest of the world in engineering electric plants to burn plutonium, the Soviets have had their share of trouble getting there.

The Soviets placed their first breeder near Shevchenko on the eastern shore of the Caspian Sea, a dessert where oil, natural gas, managanese and phosphates are produced. The plant was built to produce electricity and also steam to desalt Caspian seawater for industry in the surrounding desert.

In 1974, one year after the Shevchenko breeder went into operation, one of the six loops of tubes that carry liquid sodium metal to transport heat from the nuclear reactor to the steam generators swelled and broke. The accident that followed mixed 125 gallons of water with one ton of sodium, more than enough to start a flash fire.

Parts of the Western press described the fire as an explosion saying that U.S. spy satellites had witnessed the accident. The West had no details of what really happened until Soviet nuclear scientists began a year ago to tell their colleagues in the West what took place.

Sodium is a combustible metal that will ignite on contact with water in a violent enough way to release explosive hydrogen from water. One American nuclear engineer said that the Soviets described the incident at Shevchenko as "something like a grain elevator explosion" but that nobody was killed or injured. The Soviets insist the accident was not serious enough to shut down the plant.

"This failure was so insignificant," Troyanov said, "that not only was it impossible for sputniks to see it but the local populace didn't even notice it."

Whoever noticed it, the accident has proved to be costly and time consuming for the Soviet Union. THE RUPTURED COOLANT TUBE STILL HAS NOT BEEN REPAIRED, FORCING THE Shevchenko breeder to operate ever since at 65 per cent of its ratee power - where it is to stay until next year, when a new steam generator is shipped from Czechoslovakia and a new collant loop installed.

The incident at Shevchenko may have had something to do with the delays the Soviets have encountered with their second big breeder, a 600,000-kilowatt plant being built near Belyoyarsk in the Ural Mountains.

The Belyoyarsk breeder has been under construction since 1968 and was to be finished in 1973. Officials now say it will be ready next year or the year after. Troyanov insists the Shevchenko accident did not cause the Belyoyarsk delays. He blamed normal machinery and construction delays."

Belyoyarsk will use an entirely different kind of sodium coolant.

While far advanced in designing and building the machinery for plutonium breeders, the Soviets curiously are five to ten years away from operating their breeders on plutonium. They use uranium as the fuel in Shevchenko and will start up the Belyoyarsk breeder with uranium in the core of the reactor.

This is significant because breeders are more efficient if they use plutonium as their start up fuel. Uranium will breed plutonium but not nearly at the rate that plutonium itself breeds fresh plutonium. A plutonium breeder will double its fuel supply in 10 years or less. A uranium breeder might take more than 20 years to breed as much fuel as it burns.