A four-ton, highly radioactive section of a Soviet nuclear-powered surveillance satellite fell out of orbit yesterday, reentered the Earth's atmosphere and apparently disintegrated and burned over the Indian Ocean.

It was the second time in five years the same type of Soviet reconnaissance satellite has plummeted from space.

The Defense Department said the portion of Cosmos 1402 that fell is the larger of two sections and is believed to have contained a radar dish measuring 30 feet across, used to track U.S. Navy ships from orbit.

The Pentagon said the satellite chunk reentered the atmosphere at 5:21 p.m. EST and broke into numerous pieces. Most of the fragments, Pentagon officials said, burned up over an area of the Indian Ocean 400 miles long and 100 miles wide.

It was not known whether any pieces of the satellite actually reached Earth. The Pentagon estimated that if fragments had survived reentry, they would have fallen in remote reaches of the Indian Ocean just after 5:30 p.m. EST.

The area is approximately 1,200 miles southeast of the nearest land, the British-owned island of Diego Garcia, which the United States uses for refueling ships and storing equipment for its Rapid Deployment Force.

"If you were looking for one portion of the Earth to get rid of a radioactive satellite," said a Pentagon source, "you couldn't have picked a better place to drop it."

U.S. observers on Diego Garcia witnessed a "40-second burn" in the sky shortly before the satellite section hit the denser parts of the atmosphere, the Pentagon said.

"In all probability, most of the satellite broke up on reentry, but it is impossible at this time to know whether any of it reached the surface," a Pentagon spokesman said. "We may never know."

The Federal Emergency Management Agency canceled an alert last evening and told emergency-response teams, which had been on two-hour alert since Friday, to disband.

A Pentagon statement said U.S. ships and planes had been instructed to monitor radiation levels in the atmosphere.

The Pentagon said one reason it could not pinpoint the area where fragments from Cosmos 1402 might have fallen was that the satellite's huge radar dish gave it an irregular shape, making it aerodynamically unstable when it fell out of orbit into the atmosphere.

The satellite was also tumbling rapidly when it reentered the atmosphere, which also drastically altered its downward flight path.

"It may have plunged abruptly, it may have skipped in or it may have veered to one side by as much as a few hundred miles," a Pentagon official said. "There was no way to predict its behavior once it struck the denser portions of the atmosphere."

The satellite section that fell yesterday was highly radioactive, even though it did not contain the nuclear power source for the satellite's radar.

The reactor, which contains 110 pounds of highly enriched uranium (U-235) and large quantities of poisonous fission products such as iodine-131, strontium-90 and cesium-137, was still in orbit yesterday and is not expected to fall to Earth until the second week of February.

In Moscow last night, the Soviet news agency Tass announced the reentry of "the main part of the satellite's structure" and said "the other fragment of the satellite, the fuel core of the power plant's reactor, is forecast to enter the dense layers of the atmosphere on February 3-8, 1983, and to burn up."

The numerous structural parts of the satellite that came down were made radioactive by four months' exposure to neutrons and gamma rays given off by the reactor section, which was attached to the radar section until Dec. 28.

Then, the Soviets separated the two by remote control and attempted to rocket the more hazardous reactor portion to an orbit higher than its original 150-mile-high position to keep it from falling to Earth.

The maneuver failed, and the Soviets immediately sent a command to turn off the reactor so it would stop producing dangerous fission products.

The two sections soon began to fall out of orbit, and the Soviets could do nothing to halt their descent.

The section that fell yesterday is about four times the size of the reactor section still in orbit, which is why it came down first.

The heavier an object, the more rapidly it will fall out of space once it loses its orbit, because it undergoes more drag to pull it down when it encounters the atmosphere.

Potentially, the reactor section still in orbit is far more dangerous to human health and safety than the radar section now in the Indian Ocean.

The part that fell yesterday was believed to be radioactive enough to give severe radiation burns to anyone who touched fragments from it.

Nuclear physicists around the world hope the 110-pound reactor core will burn up in the atmosphere, so that its radioactive debris will be scattered through thousands of square miles of the atmosphere.

This is what happened five years ago today, when the reactor in Cosmos 954, identical to Cosmos 1402, fell out of orbit and burned up over Canada's Northwest Territories, while the radar portion fell in about 20 pieces. A few of them were as heavy as 30 pounds.

"About all that happened after the Cosmos 954 accident was that the background radiation the Earth receivesfrom the sun and from cosmic rays was doubled, which is not dangerous," one source said.

"That accident certainly implied thatthe Soviets know how to design their space reactors so they deliberately burn up high enough in the atmosphere to make the debris essentially harmless," the source said.

While warships of the British, U.S. and Soviet navies are in the Indian Ocean, sources said it is unlikely that attempts will be made to salvage parts of the satellite wreckage from the water.

The United States has already examined debris from the surveillance satellite that fell in Canada on Jan. 24, 1978. And, sources said, the Soviet satellite is considered technologically inferior to similar U.S. satellites.