LIVERMORE, Calif. (AP) – The scientists who crack open the nation’s nuclear weapons for a living are never quite sure what they will find inside.

Many of the warheads were designed and built 40 years ago, and their plutonium and other components are slowly breaking down in ways that researchers do not fully understand. With no new bombs in production, the government spends billions of dollars each year tending to its aging stockpile.

The Bush administration wants to revamp the entire arsenal with a weapon now on the drawing board named the Reliable Replacement Warhead.

The redesigned weapon is needed to ensure “a safe, secure, reliable and effective nuclear deterrent for the indefinite future,” said Linton Brooks, chief of the National Nuclear Security Administration.

The administration ordered up a competition between Lawrence Livermore National Laboratory near San Francisco and Los Alamos National Laboratory in New Mexico. The two laboratories submitted their proposals for the weapon in March. The White House plans to pick a winner by November.

As envisioned, the next-generation nuclear weapon would have the same destructive power as existing ones, but be durable enough to last for decades.

The next bomb is also meant to be so secure that it has jokingly been dubbed the “nuclear doorstop” – useless for any other purpose, should it fall into the wrong hands.

The government and the labs refuse to discuss details of the two designs, citing national security. But they describe both proposals as “conservative” blueprints meant to assure reliability without violating a moratorium on full-scale nuclear testing in place since 1992.

“We’re not going to come up with anything cutting-edge and stick it in the stockpile without testing,” said David Schwoegler, spokesman for Lawrence Livermore’s nuclear weapons program.

The United States has not built a nuclear warhead since 1991. The government spends about $5 billion a year maintaining the weapons, and engineers have patched problems by opening up warheads that were never meant to be opened. The accumulation of tiny engineering changes meant the bombs moved incrementally away from their original designs, with unknown effects.

The White House believes designing a replacement warhead is vital to preserving the nation’s nuclear edge, particularly amid looming questions about North Korea, which reportedly possesses several nuclear weapons, and Iran, which the administration fears wants them.

The redesign project “means making sure that aging phenomena don’t cause us any questions about nuclear reliability,” Brooks said in a telephone interview with The Associated Press. “It means making sure that we incorporate safety and security and use-control in a way we didn’t know how to do when we designed the stockpile.”

Critics, including some former nuclear weapons scientists, question the need to resume nuclear weapons production.

They also question whether a next-generation bomb can improve reliability and safety if it cannot be tested. Congress has financed the research on the condition that the redesigned weapon reduce the need for testing.

Opponents fear the project could send the wrong signal to the world at a time when the United States and its allies are trying to curb the spread of nuclear technology.

Brooks said North Korea and Iran play into the project only “indirectly,” explaining that the administration would press for the program anyway. “We didn’t sit down and say, ‘Look, there’s problems in Iran. Let’s go and invent a new design,”‘ he said.

The project also aims to improve safeguards against accidental detonation or use of the weapons by terrorists, Brooks said. It marks the first time that an American nuclear bomb has been designed with those goals as the top priority.

Proponents say a revamped weapon could help the United States to reduce the number of warheads held in reserve in case other weapons are found to be faulty.

A new weapons production line would be needed to produce the bomb. For instance, the Rocky Flats, Colo., plant that once made plutonium triggers for nuclear warheads was shuttered in 1989. Los Alamos can only build a handful per year; the administration is aiming for 10 next year.

The Livermore and Los Alamos labs set aside bomb-designing more than a decade ago in favor of maintaining the current stockpile.

Each year, the nation’s nuclear arsenal loses about a half-dozen bombs from its reserve of several thousand as the Livermore and Los Alamos teams rip them apart in what is called “destructive analysis.” Others are painstakingly dismantled and refurbished with new parts.

On Thursday, engineers gathered at a high-security plant near Amarillo, Texas, to toast a milestone: the first rebuild of a B-61 nuclear bomb. It’s the oldest warhead in the arsenal, having been designed in the early 1960s and built into the 1970s.

The government is spending $470 million over nine years to refurbish the B-61s. That’s money the Bush administration would rather channel into an overhaul of the entire arsenal and the mostly dormant nuclear-weapons complex.

Brooks sees the bomb-redesign project as making that complex more adaptable.

“Any weapon we have will sooner or later go through some type of modernization or have (some) problem to repair, and right now that takes a very long time,” he said.

AP-ES-06-30-06 1553EDT