The Bush administration said Tuesday that it would make a 15 percent reduction in the nation’s stockpile of nuclear weapons, taking the overall inventory down to less than a quarter of its size at the end of the Cold War in 1991.

A major effort to retire older weapons was accomplished five years ahead of schedule, allowing the new round of cutbacks, said Thomas P. D’Agostino, chief of the National Nuclear Security Administration.

The overall size of the nation’s nuclear inventory is classified. But under the terms of a 2002 treaty, the U.S. and Russia are committed to reducing deployed weapons on missiles and aircraft to between 1,700 and 2,200 by 2012. It is believed the U.S. holds about 4,000 additional weapons in reserve status, although those numbers are classified.

Separately, the Energy Department unveiled a long-term plan to modernize and reduce the size of its weapons research and production complex, saying it would cut about 20 percent of its current 37,000 workers and close about one-third of its physical facilities.

The plan would eliminate the jobs of about 300 employees in nuclear weapons work at Lawrence Livermore National Laboratory in the San Francisco Bay Area, a bomb design center that has an 8,000-member work force involved in a wide range of research. About half of those cuts already have occurred, said Bruce Goodwin, chief of the lab’s weapons program.

“We have seen this coming for some time,” Goodwin said.

The cutbacks at Livermore also would reduce by 90 percent the acreage devoted to nuclear weapons work, mainly because weapons research activities would stop at Site 300, about 17 miles east of the 1-square-mile main laboratory site.

Similar cuts are being made at the National Nuclear Security Administration’s sites in Amarillo, Texas, Albuquerque, N.M., Los Alamos, N.M., Oak Ridge, Tenn., and Aiken, S.C.