WASHINGTON, June 3 - The United States will reduce its stockpile of nuclear weapons by nearly half over the next eight years, the Energy Department said Thursday.

The Bush administration made the decision last month and informed Congress on Tuesday in a classified report.

Linton F. Brooks, administrator of the National Nuclear Security Administration, which is part of the Energy Department, said in a conference call with reporters that the reductions would leave the nation with "the smallest nuclear-weapons stockpile we've had in several decades." He called the decision historic.

Mr. Brooks would not discuss specific numbers for the cuts. "The numbers I'm prepared to use are 'almost in half' and 'smallest in several decades,' " he said.

The decision by the administration followed an announcement by President Bush in November 2001 that the nation would reduce the number of "operationally deployed" strategic warheads by about two-thirds by 2012, leaving 1,700 to 2,200 warheads.

But that announcement did not commit the United States to reduce the total number of weapons in its inventory, only the number of strategic weapons that were ready to use immediately.

The new decision includes additional categories of weapons, including short-range weapons that are not considered strategic, weapons held in reserve and weapons in places like nuclear submarines that are in overhaul and "logistical spares," which are used to swap with weapons being recalled for overhaul.

When Mr. Bush promised in 2001 to cut the number of actively deployed strategic weapons to no more than 2,200, the United States had 6,100, according to Tom Cochran, an expert at the Natural Resources Defense Council, a group that specializes in nuclear weapons, among other environmental issues. The United States had 10,000 nuclear weapons in all categories, and the announcement made Thursday will cut that to 6,100, Mr. Cochran said, suggesting that the overall reduction would be somewhat less than Mr. Brooks's figure.

Some of the weapons to be removed from the active category will be dismantled, and some will go into the reserve category, meaning that they could be returned to readiness quickly; some of the weapons now in the reserve will be decommissioned, Mr. Cochran said.

In practice, the weapons to be retired will join a long queue at an Energy Department plant in Amarillo, Tex., called Pantex, which is now busy with "life extension" of existing weapons, Mr. Brooks said. He said that President George Bush, who left office in 1993, decided to retire the nation's stock of nuclear artillery shells, "and we just finished dismantling the last one last year."

Mr. Brooks said in a letter to members of Congress that making the stockpile smaller would require more work on the remaining weapons. "We must continue the administration's efforts to restore the nuclear weapons infrastructure," he said in an unclassified cover letter to the memo describing the schedule for reducing arms from now to 2012.

In the conference call, Mr. Brooks said that the decision to reduce the stockpile meant that a new bomb plant that the administration wants to build, the Modern Pit Facility, could be smaller than it might have otherwise been, but that it would still be needed. Pits are the hearts of plutonium weapons, and the Energy Department lost most of its capacity to make pits when it closed the Rocky Flats, Colo., plant, near Denver, in the 1990's, because of environmental and production problems.

The plutonium in the pits in existing weapons is breaking down over time, Mr. Brooks said, and at some point the department will have to melt down and recast the pits. One reason for that the memo was issued Tuesday was to convince members of Congress that a new pit plant is needed, he said.

"We've not yet been able to convince some of our Congressional colleagues that the Modern Pit Facility is unrelated to any notion of future weapons development or future weapons growth," Mr. Brooks said.

In fact, the administration has shown intermittent interest in a new class of small nuclear weapons, an idea bitterly opposed by some members of Congress.

Mr. Brooks said the reduction was the largest in history in percentage terms.

Mr. Cochran, at the Natural Resources Defense Council, agreed that the reduction was significant. But he said: "These cuts are over eight years. That's two presidential administrations. This is not a fast-paced reduction."