The federal government admitted yesterday it has no idea what has happened to more than four tons of the closely guarded uranium and plutonium it has used in the last 30 years to make atomic weapons.

About 60 per cent of the missing nuclear metal is weapons-grade uranium. The rest is plutonium. It takes less than 15 pounds of plutonium to build an atomic explosive.

The 8,437 pounds of uranium and plutonium that have disappeared from an estimated 25 laboratories and factories scattered across the United States is enough to make about 500 Hiroshima-size atomic bombs.

The government denied that any of the missing metals have been stolen diverted into the hands of terrorists or countries seeking their own nuclear weapons or offered for sale on the black market.

There is no evidence of any their attempt or of a black market," said Gen. Edward B. Giller, deputy assistant administrator of the Energy Researdh and Development Administration. "We believe the missing plutonium and uranium can be traced to overestimates, machining [grinding, shaping, or lathing] and scrap losses and unmeasurable amounts bound up in equipment and pipes."

ERDA and the Nuclear Regulatory Commission released their joint estimate lost nuclear metals after being asked to do so by The Washington Post and The New York Times under the Freedom of Information Act almost two years ago. Release had been held up by the National Security Council in the Ford and Carter administrations, pending study of the secrecy issues involved in the release.

The National Security Council finally decided to make public the amount of metals missing from all nuclear facilities in the United States but two the Plutonium weapons factory at Rock Flats, Colo., and a uranium weapons factory (called YF-12) at Oak Ridge, Tenn.

A source close to the National Security Council said the metals missing from both weapons factories were kept classified because their release might suggest changes made over the years in the ways U.S. nuclear weapons were produced.

"Had we released that information," this source said, "the totals of missing metals would have been much higher, three times or more the four tons that were made public."

Of the 8,437 pounds of uranium and plutonium unaccounted for, about 3,400 pounds is plutonium which is used almost entirely for weapons. More than 90 per cent of the missing plutonium was from production plants at Richland, Wash., and Aiken, S.C., which are operated by ERDA.

ERDA's Giller explained that plutonium factories are so intricate that the metal often passed through 50 miles of piping as it is being purified.

"It's inevitable that small quantities of the material will adhere to the walls of the pipes as it passes through," Giller said. "Just a coating of less than one hundred thousandth of an inch on the interior walls of the pipes would account for the entire plutonium difference [303 pounds] at the Savannah River plant in Aiken, S.C."

Five laboratories and plants operated by ERDA cannot account for almost 3,900 pounds of missing weapons-grade uranium, which is uranium enriched with a least 20 per cent of the fissionable isotope of uranium called U-235. The five include the weapons laboratory at Los Alamos, N.M., which has "lost" 233 pounds - a large amount of - weapons-grade uranium although it does not handle the quantities off the metal needed to produce weapons.

In the 20 years that ended in 1967, ERDA (then the Atomic Energy Commission) also lost 2,380 pounds of uranium through contractor factories that produce weapons parts and nuclear submarine fuel from weapons, grade uranium. One plant at Apollo, Pa., could not account for 583 pounds of weapons-grade uranium in that time.

Until 1967 the AEC owned all weapons-grade uranium adn plutonium. Since then the AEC (and now the NRC) has licensed the metals to private contractors. In the nine years since 1967 the licneses lost 72 pounds of plutonium and 1184 pounds of weapons-grade uranium.

About two-thirds of the uranium missing from NRC licensces is metal bound for nuclear submarines, which use eapons-grade uranium to make power.

While insisting there is no sign of theft, ERDA and NRC officials yesterday conceded that employees or visitors on seven occasions had taken "minute" amounts of metal as souvenirs. One souvenir hunter was convicted for the theft, and another was arrested by police in Mexico but not tried.

The figures released by ERDA and NRC show that enough uranium and plutonium has been lost in every year since 1954 for a terrorist or a thief to have fashioned a nuclear bomb.

ERDA cannot account for 120 pounds of plutonium and 200 pounds of weapons-grade uranium in the year that ended July 1, 1976. Three contractors making submarine fuel under license from the Nuclear Regulatory Commission lost more than 200 pounds of weapons-grade uranium in the same 12 months.