The announcement marks an important symbolic and operational milestone in rebuilding the nation's nuclear weapons complex, which began a long retrenchment in the late 1980s as the Cold War ended and the toll of environmental damage from bomb production became known.
"Since 1989 until today, we were the only nuclear power in the world that could not make a pit," said Linton Brooks, administrator of the National Nuclear Security Administration, the Energy Department agency that runs the weapons production complex.
A pit is a hollow sphere made of plutonium, surrounded by conventional explosives that detonate and start fission as the sphere implodes.
Under a Bush administration plan, the Energy Department is beginning limited production of plutonium parts for the stockpile of nuclear weapons and will begin laying plans for a new factory that could produce components for hundreds of weapons each year.
The last time the United States made a plutonium pit was at the Energy Department's Rocky Flats site in Colorado, which was shut down after serious environmental laws were broken and the FBI raided the plant.
Weapons scientists at Los Alamos National Laboratory in New Mexico said Tuesday that they had built a plutonium pit for a W-88 warhead for a Trident nuclear missile.
The production took eight years and ultimately will cost $1.5 billion when the pit is fully certified by the Energy Department in 2007, Los Alamos officials said.
"It is a sign that after a long period of decline, the weapons complex is back and growing," said Jon Wolfsthal, deputy director of the Carnegie Endowment for International Peace and a former Energy Department weapons expert. "To the average U.S. citizen, it would be accurate to say we have restarted the production of nuclear weapons."
Energy Department officials vehemently denied that they are actually producing nuclear weapons and said they need the capability of producing plutonium parts to ensure the reliability of the stockpile of U.S. weapons, which is aging and may need new components.
Although the United States built its first nuclear bomb in a matter of a few years during World War II, the effort to restart component production is going to take about two decades.
The Energy Department expects the future pit factory to begin production in 2018, after lengthy design and environmental reviews.
But critics question whether the Bush administration is going overboard in its investments in the nuclear weapons complex. Thomas Cochran, a scientist at the Natural Resources Defense Council, said the government is now spending about $6 billion annually on the nuclear weapons complex, 50% more than it did during the Cold War.