The Soviet Union faces an incredibly expensive cleanup operation in the wake of the accident at its nuclear station in Chernobyl and may be forced to declare some areas off-limits indefinitely, according to U.S. experts in decontaminating radioactive sites.

While the Soviets have released little information on the extent or severity of radioactive fallout from the reactor explosion and apparent meltdown, a U.S. interagency task force has estimated that lethal levels of radioactivity extended as much as three miles from the plant immediately after the accident and that extremely dangerous levels extended as far as seven miles.

"I don't think anybody has ever handled anything this size," said Wayne Bliss, director of the Environmental Protection Agency's radiation division in Las Vegas, which mainly assists in cleanups of military sites. "This is the worst acute contaminating event anybody has ever had."

The U.S. task force is seeking additional information on Chernobyl, partly in an effort to reassure Americans that they are in no danger from a similar disaster but also because of the wealth of data that the accident is expected to generate.

Nuclear Regulatory Commission official Harold Denton, a member of the task force who directed the cleanup at the Three Mile Island commercial reactor near Harrisburg, Pa., after its partial meltdown in March 1979, said the Chernobyl accident could provide the best lessons so far in how to cope with a serious nuclear accident. "This is not the absolute worst case," he said, "but it ranks right up there as to what you could expect."

No environmental cleanup was required in the area surrounding TMI, where a steel-and-concrete containment vessel held in most radioactive emissions. Nonetheless, it was three years before operators could lower a television camera into the contaminated reactor and another year before the first technicians, equipped with special tools, entered the crippled reactor to begin decontamination.

"You can go in TMI now," Denton said. "Workmen are standing above the core, which is underwater, using special robot tools to scoop up the radioactive material for disposal . . . . This one Chernobyl has the core open to the air, with the same level of destruction. This is a major, major cleanup."

The United States has some experience in similar cleanups, with mixed success. A major effort on the Bikini Atoll in the Marshall Islands, contaminated by fallout from an atomic bomb test in 1954, has not reduced radioactivity enough to permit the return of residents.

Islanders returned briefly in 1969 but were evacuated again when it was discovered that food crops on the island were taking up radioactivity at dangerous levels. Two years ago, a scientific committee estimated it would take another $40 million to remove 11 inches of topsoil and revegetate the island.

Bikini Atoll is less than two square miles in size. The area thought to have sustained significant environmental damage around Chernobyl, by contrast, is a minimum of 40 square miles and probably far larger.

The largest areas treated in the United States for radioactive contamination are the nuclear test sites in Nevada, which are neither populated nor suitable for agriculture. Bliss estimated that about 80 acres have been coated with ordinary road oil to prevent radioactive particles from being wafted into the atmosphere.

The Soviets' task will be complicated by the fact that Chernobyl is situated in the Ukraine, the nation's breadbasket, which together with four smaller republics just north of the nuclear station produces more than one-third of the nation's winter wheat and dairy products, as well as substantial amounts of sugar beets, vegetables and fruits.

The plant also sits near the Dnieper River, which supplies agricultural irrigation water as well as drinking water for Kiev, a city of 2.4 million, and other Soviet cities on its way south to the Black Sea.

"We've never had anything contaminated so widely and with such high levels of radioactivity," Denton said.

At the worst, heavily contaminated soils might have to be dredged up and disposed of elsewhere, stripping the land of its most productive topsoil, or abandoned for crop and grazing purposes for decades while radioactive particles decay to safe levels.

Less drastic measures may be possible in less severely damaged areas, such as the use of chelating agents to bind some isotopes and prevent plants from taking them up, or deep plowing to mix and dilute radioactive materials in the soil.

"But there are points where you don't want to use pastures or fields for anything," Bliss said.

The Soviets reportedly faced a similar situation 30 years ago when an explosion contaminated a wide area near Kyshtym in the eastern Ural Mountains. According to published reports from CIA documents and emigre Soviet scientists, 30 villages were abandoned after that accident, more than 100 square miles were declared a "dead zone" and a river was rerouted to avoid contamination.

A far less severe accident happened in 1966, when an American B52 collided with its refueling plane near the coast of Spain and spilled four hydrogen bombs. Two of the bombs ruptured on impact, spreading plutonium and uranium over two areas several hundred feet in diameter. U.S. officials dredged up more than 1,000 tons of topsoil and vegetation and took it back to the United States for disposal.

Just how far dangerous levels of radioactive particles from Chernobyl might have spread is a matter of speculation.

According to the U.S. task force, areas within seven miles of the plant clearly will require long-term decontamination, but without data on local weather conditions and terrain, it is not possible to know how much additional land has been damaged.

The only experience at all comparable to Chernobyl in the United States was a deliberate test by the Atomic Energy Commission in 1965. The commission hauled a small reactor on a flatbed railroad car to a site near Jackass Flats, Nev., and dropped the fuel rods out in an effort to determine the effects of a runaway nuclear reactor.

The reactor exploded, releasing lethal levels of radioactiviy in the immediate vicinity. Cesium and strontium 90 isotopes were detected in samples of milk from cows grazing near Bakersfield, Calif., 175 miles away, but not at levels considered dangerous.

The Chernobyl accident would have released much higher doses, because of the size of the reactor, but just how much more would depend on the amount of fuel in the reactor, how long the fuel had been in use and the power level at the time of the accident.

In a television interview Friday from West Germany, a Soviet official said water reservoirs near the plant were contaminated, an occurrence that U.S. officials considered unavoidable. The plant sits near the Pripyat River, a tributary of the Dnieper.

But water contamination may pose less difficulty than soil contamination, according to Bliss, who said initial doses of radiation have probably been flushed downriver and diluted, and standard city water treatment facilities remove much radioactivity through carbon filters.

Some isotopes may settle in river or reservoir sediment or be taken up quickly by algae and other vegetation, where they can enter the food chain through fish and other animals, he said. "These compounds very likely will dissolve easily, like table salt, and they will move quite a ways," he said.

Rooftop cisterns, used in many countries to collect rainwater for household use, could be heavily contaminated, but well water "should not be contaminated if people have reasonable wellheads," he said.

As for food products, Bliss said that leafy vegetables and grasses will probably have to be destroyed or disposed of in some manner.

"Root vegetables are probably all right if they are washed and checked," he said. "Uptake won't occur rapidly and there are safe levels of radioactivity in consumer products without subjecting the public to great risks."

For future crops, however, the soil might have to be treated with agents to prevent plants from taking up excessive quantities of radioactive materials. Potassium fertilizers, for example, can restrict the uptake of radioactive cesium by plants in much the same way potassium iodide protects humans from radioactive iodine.

While it is impossible to estimate the overall cost of any cleanup operation at Chernobyl, Bliss noted that the United States spent $15 million in just a few days seeking the radioactive remains of a Soviet spy satellite that crashed in Canada in 1978.

"This is going to make that look a trip to the candy store," he said.