The Bush administration is taking a major step toward building a new generation of nuclear warheads by selecting a design that is being promoted as safer, more secure and more easily maintained than today's arsenal.
A team of scientists from Lawrence Livermore National Laboratory will continue with designing the weapons design in anticipation of having the first warheads ready by 2012 as a replacement for Trident missiles on submarines.
The new weapons program, which has received cautious support from Congress, was immediately criticized by some nuclear nonproliferation groups as evidence the government wants to expand nuclear weapons production, not move toward eliminating the stockpile.
Critics also maintain that it sends the wrong signal around the world by pushing a new warhead, although characterized as a replacement for existing ones, at a time the United States is trying to curtail nuclear weapons development in North Korea and Iran.
Some lawmakers agreed.
"The minute you begin to put more sophisticated warheads on the existing fleet, you are essentially creating a new nuclear weapon. And it's just a matter of time before other nations do the same," said Democratic Sen. Dianne Feinstein. "This could serve to encourage the very proliferation we are trying to prevent."
Another Democrat, Rep. Ellen Tauscher, chairwoman of the Armed Services strategic forces subcommittee in the House of Representatives, expressed cautious support but promised "a long evaluation process" in Congress to assure the warhead will do what is promised without future underground testing.
Nuclear underground tests have not been done since a ban in 1992.
"This is not about starting a new nuclear arms race," countered Thomas P. D'Agostino, acting head of the Energy Department's National Nuclear Security Administration, which oversees the nuclear weapons programs.
Steve Henry, deputy assistant to the secretary of defense for nuclear matters, said the new design is hoped to lead to fewer warheads being needed. He said it has not changed administration determination to reduce the number of deployed warheads to fewer than 2,000, which would be the lowest number since the 1950s.
The United States is believed to have about 6,000 warheads deployed and another 4,000 in reserve.
D'Agostino, briefing reporters on the design decision, said the intent is to develop a safer, more secure warhead to assure increased reliability without the need for underground nuclear tests.
He cautioned that the program remains in the early stages and that in coming months the Livermore team will expand on its design work to give a better estimate on overall costs, the scope of the program and a schedule toward full-scale engineering and production.
The administration is asking for $119 million (â‚¬90.4 million) for the next fiscal year for design work. The officials said they could not say how much the program eventually will cost.
The so-called "reliable replacement warhead" has been the focus of a yearlong, intense design competition between Livermore in California and nuclear scientists at the Los Alamos National Laboratory in New Mexico - the government's two premier nuclear weapons labs.
Both the labs developed proposals, and at one point there was discussion to combine the designs into a single program. But that was rejected and D'Agostino made clear Friday the program would be Livermore's to develop.
The Livermore design was based on an existing warhead that reportedly had been exploded in an underground test in the 1980s but never was put into the stockpile. The Los Alamos design was based on a fresh approach but without a history of actual testing.
It was this "very robust test pedigree," as D'Agostino put it, that gave Livermore the upper hand.
"It ... gave us the confidence ... to certify and go forward without underground testing," he said, adding that without that assurance "we were not going to go forward."
Congress authorized design work on the new warhead in 2005, but with a stipulation that its primary goal be to assure the reliability of the nuclear arsenal without resumption of bomb testing, and that it would help in the consolidation of the Energy Department's nuclear weapons complex.
Some lawmakers have also questioned whether the new warhead is needed, especially in light of a recent finding that the plutonium in the current warheads will last nearly 100 years, twice as long as previously thought.
Some nuclear weapons critics warned the warhead could lead to an increased likelihood of future testing and called it a ploy to rebuild, not dismantle, the nuclear weapons infrastructure.
"This is a first installment on a plan to develop and produce warheads on an ongoing cyclical basis ... similar to what we had during the Cold War," said Lisbeth Gronlund, a scientist at the Union of Concerned Scientists, a nuclear nonproliferation advocacy group.
John Isaacs, executive director of the Center for Arms Control and Nonproliferation, said there's no need for a new warhead when "the US nuclear stockpile, based on 50 years of research and over 1,000 underground nuclear tests, has been confirmed safe and reliable for at least another half-century."
"The bottom line is we're returning to what we used to do in the Cold War years. That's the message to the world," said Hans Kristensen, director of the nuclear information project of the Federation of American Scientists.