The last segment of a crippled Soviet nuclear-powered reconnaissance satellite--its 1,000-pound uranium fuel core--fell out of Earth orbit yesterday and burned up over the South Atlantic 1,100 miles east of Brazil.

The satellite's 110 pounds of high-enriched uranium, shielding, faulty rocket engine and 400 pounds of fuel left orbit at 6:10 a.m. EST, burned up and scattered radioactive dust into the atmosphere along a track 500 miles wide and 1,000 miles long, all of it over the South Atlantic.

There have been no reports of the half-ton hulk having been seen falling, though its largest pieces may have fallen and sunk unobserved.

"We assume it all re-entered the atmosphere and burned up harmlessly," a Pentagon spokesman said. "All I can tell you is that it fell off our radar screens and disappeared."

The segment of Cosmos 1402 that burned up yesterday was the second of two pieces to fall out of orbit and shower radioactive debris into the atmosphere over the world's oceans. The first segment, its four tons of radar and electronics used by the Soviets to follow the movement of U.S. surface ships, burned up and fell into the Indian Ocean Jan. 23.

The Pentagon said that Air Force WC135 weather reconnaissance planes will soon be flown along yesterday's re-entry path to gather samples of air to be tested for radioactivity.

When an identical reactor aboard Cosmos 954 burned up over northwest Canada in 1978, it left enough radioactive fission products in the atmosphere to double the "background" radiation the Earth receives every day from cosmic rays and the sun.

While there were no immediate visual reports of the satellite's burnup, it should have provided a spectacular sight for persons aboard planes or ships in the South Atlantic area at the time. Not only did the hot uranium fuel break up and burn like a falling meteor shower, the 400 pounds of hydrazine and nitrogen tetroxide fuel on board ignited in the sky like a large kerosene bomb.

The rocket engine and its fuel were supposed to fire the reactor to a 600-mile-high orbit to keep it from falling to Earth and posing any hazard. However, the engine failed to fire as ordered Dec. 28, and the satellite fell out of orbit, its heavier segment falling first.