A second segment of the falling Soviet nuclear-powered surveillance satellite, a one-ton package that contains the satellite's uranium core, is expected to burn up in the atmosphere and scatter its radioactive debris over 20,000 square miles of Earth as early as Feb. 5.

The Pentagon said yesterday that the nuclear-powered reactor of Cosmos 1402 was in an orbit 127 miles from Earth at its lowest point and 132 miles at its highest.

It has fallen about 30 miles in the last 30 days, and is now falling about three miles every day, which is a little more than half the five-mile-a-day descent of the four-ton radar surveillance segment of the satellite before it dropped into the Indian Ocean on Sunday.

The nuclear reactor segment travels around the earth in precisely the same orbit that the radar segment did before it fell out of orbit, moving in a northeasterly path that covers most of the inhabited Earth and all of the world's oceans below the Arctic icecaps and above the Antarctic icecaps.

The only difference in their orbits is that the reactor segment has descended more slowly because it is smaller and lighter than the radar segment.

Designed to burn up in the uppermost reaches of the atmosphere so that its radioactive debris scatters around the world in infinitesimal particles, the reactor segment of Cosmos 1402 contains 110 pounds of high enriched uranium (U-235) that is packaged in a stainless steel vessel to protect its electronics from most of the radiation produced by the uranium as it fissions.

If the reactor plunges into the atmosphere, there will be enough destructive heat produced by friction with the atmosphere to burn up all of its fuel elements at an altitude of 250,000 feet.

When Cosmos 954 burned up over Canada five years ago, its uranium fuel did not separate from its container, but suffered so much destructive heating that almost all of it scattered through the upper atmosphere. The only fuel particles found in the Canadian tundra were no larger than flakes of snow.