WASHINGTON — The Bush administration took a major step Friday toward building a new generation of nuclear warheads, selecting a design that is being touted as safer, more secure and more easily maintained than today's arsenal.

A team of scientists from Lawrence Livermore National Laboratory will proceed with the weapons design with an anticipation that the first warheads may be ready by 2012 as a replacement for Trident missiles on submarines.

The new weapons program, which has received cautious support from Congress, was 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 by pushing a new warhead — although characterized as a replacement for existing ones — at a time the U.S. is trying to curtail nuclear-weapons development in North Korea and Iran. The U.S. has not built a nuclear warhead in more than 20 years.

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 Sen. Dianne Feinstein, D-Calif.

Rep. Ellen Tauscher, D-Calif., chair of the House Armed Services strategic-forces subcommittee, promised "a long evaluation" in Congress to ensure 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," said Thomas D'Agostino, acting head of the Energy Department's National Nuclear Security Administration (NNSA), which oversees 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, the lowest number since the 1950s.

There are thought to be about 6,000 warheads deployed and 4,000 more in reserve.

D'Agostino said the intent is to develop a safer, more secure warhead to ensure increased reliability without the need for underground nuclear tests.

He said 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 costs, the scope of the program and a schedule toward full-scale engineering and production.

The administration is asking for $119 million for the next fiscal year for design work. The officials said they could not say how much the program eventually will cost.

The "reliable replacement warhead" has been the focus of a yearlong, intense design competition between Livermore in California and scientists at the Los Alamos National Laboratory in New Mexico, the government's two premier nuclear-weapons labs.

Congress authorized design work on the new warhead in 2005, but with a stipulation that its goal be to ensure the reliability of the nuclear arsenal without resumption of bomb testing.

Some lawmakers have 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.

AP writer Scott Lindlaw in San Francisco contributed to this story.

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