The federal government has found it much more difficult than expected to decontaminate sites of nuclear weapons explosions and to make them safe for habitation under U.S. radiation standards.

A three-year, $80 million cleanup program is under way on Enewetak Atoll in the Pacific but it will not make safe for thousands of years the radiation-contaminated northern islands. The islands 20 years ago were the site of U.S. nuclear weapons tests.

The Pacific islands have a special problem: there are only six or seven inches of topsoil over the coral reefs, so deep earth removal or radioactive soil is not feasible.

But, according to some Department of Energy officials, contamination cleanup after a nuclear attack in the continental U.S. "may be more of a problem."

In the Pacific, the islands are porous and water washed the radioactive elements straight into the soil and down to the water table below.

On the U.S. mainland, rain would carry radioactive material left by a nuclear explosion down streams and into rivers, spreading it rapidly over a broad areas.

"I'm not sure we could do surgical removal" in the United States, an Energy Department official said yesterday.

The Enewetak cleanup, directed by the Defense Nuclear Agency with DOE assistance, is "technologically the most complex ever undertaken on earth," Vice Adm. Robert Monroe, DNA's director, said last week.

Energy Department officials said last week they were "surprised at the amount of plutonium fallout" still in the soil, particularly on Enjebi, once the residential island of the northern Enewetak people.

Enjebi, less than a mile square, had been the site of the nuclear test explosions.

Before the cleanup attempt began last May, scientists had expected radioactive strontium and cesium contamination in Enjebi's soil would make it unsafe for living for 25 to 50 years longer.

But they had hoped to be able to locate and remove the longer-lived (hald-decay in 24,000 years) plutonium and americium, heavy radioactive elements created by the nuclear explosions.

These heavy elements emit dangerous alpha rays and if inhaled or swallowed can, over time, cause cancer of the lungs or bones.

An Energy Department official said yesterday he did not believe that Enjebi soil could be cleanup through soil removal or deep plowing to reach the point of being considered safe by new Environmental Protection Agency standards.

He thought a program of soil removal and plowing could cut down the danger of inhalation of radioactive plutonium. But the elements would remain in the soil and be taken up by plants to be later eaten by the natives if they were living on the island.

Natives who returned to Bikini Island, once the site of fallout from U.S. nuclear weapons tests, are going to be removed, from that island shortly because they are absorbing strontium from food grown in the contaminated soil.

Bikini was declared safe for habitation in 1969 by the Atomic Energy Commission after a $5 million cleanup that consisted primarily of plowing the island, removing all vegetation and planting new coconut and other trees and bushes.

Those new trees and bushes produced crops last yer that turned out to be contaminated with strotium and cesium.

DNA's Monroe said the government had "learned from Bikini."

Enewetak, however, presented a different situation. The atoll's southern islands were untouched by nuclear shots and got only minor fallout. Test devices were exploded on northern islands or on barges anchored nearby.

The three former residential islands in the south were safely occupied during the tests and for the 20 years since testing stopped.

Thus, it is on the two-dozen northern Enewetak Island that the radiation cleanup had been focused.

Two of those islands, Enjebi and Runit, may provide the basic information on how large-scale nuclear cleanups can be done.

In 1958, and 18-kiloton device exploded on Runit but failed to chain react, spreading plutonium all over the island.

Officials decided the island could never be decontaminated and instead should be used as the burying ground for radioactive material and soil removed from other islands in the atoll.

Mixed with concrete, the radioactive waste is being buried in Runit's crater.Eventually it will be covered over with an 18-inch thick concrete cap, fenced off and put off-limits for 24,000 years.

Scientists had wanted to make Enjebi habitable once the strontium and cesium in the soil had decayed in 25 years or more.