Despite asking for slightly more money for nuclear-weapons work than last year and substantially more than the Cold War average, the Bush administration is easing its push for a new nuclear arsenal, as well as new factories and experimental facilities to maintain it.

Energy and Defense department officials still are gunning for a nuclear "bunker buster" to be designed at Lawrence Livermore weapons lab and its neighboring weapons-engineering lab, Sandia-California.

As part of his $2.57 trillion request, the president is seeking $8.5 million, split between the Energy and Defense budgets, to slam a hardened mockup of the nation's highest-yield nuclear weapon through concrete walls and possibly drop it from a jet bomber.

That money is less than half of the $25 million the administration sought for its Robust Nuclear Earth Penetrator last year — cut entirely by congressional appropriators — and the administration has slowed or compromised on other controversial weapons projects as well.

Energy Secretary Samuel Bodman called the budget request "a lot of tough decisions and a lot of tradeoffs."

Federal weapons officials called off a decision on building a new factory for plutoniumPush for nukes slowing, budget shows weapons cores, known as pits. They didn't fully replace some congressional cuts in the National Ignition Facility, a giant laser at Lawrence Livermore that is the nation's largest science project and an administration priority.

They also dropped the "Advanced Concepts" program to re-establish Cold War-style teams at the nation's three nuclear-weapons labs to brainstorm new kinds of warheads capable of low-explosive yields and tailored radiation effects, such as electromagnetic pulses.

Instead, taking a blunt suggestion from congressional appropriators, the administration would spend the money on making older warheads less susceptible to aging.

In what Congress titled the "Reliable Replacement Warhead," weapons scientists would reverse the decades-old trend toward lighter, more compact warheads and design bulkier, sturdier explosives that could sit in a storage bunker for decades, without testing.

The goal in essence would be "bombproof" bombs, the nuclear equivalent of a pickup truck instead of today's arsenal full of souped-up race cars. In the lingo of weapons designers, they are "highly optimized" warheads with tight "performance margins" that in theory could break down with age and deterioration.

"We're really pushing the limits on some of the performance margins on some of these systems," said one weapons official.

But redesigning the old warheads for resistance to aging, rather than performance, isn't going to be a cake walk. Both the Energy Department and the Pentagon are likely to demand extraordinary assurances that the weapons will work without exploding them.

"Ideally with these bigger margins there would be a little more room for error. But I think they're going to be held to a high standard of proof without resort to nuclear testing," the weapons official said.

So far, scientists say today's nuclear arsenal of mostly 1970s- and 1980s-vintage weapons is standing up surprisingly well to the effects of aging, with the most sensitive nuclear components expected to last at least 60 years.

For now, designers are far from offering any hardier designs for development into a prototype, a step that would have to be approved by Congress.

The nuclear earth penetrator could come up for such a vote in 2007, when the administration expects to finish studying its feasibility with a last installment of $14 million.

Democrats and at least one powerful Republican chairman have signaled their intent to kill the study this year.

"That's a waste of money on a weapon (that) commanders in the field have not asked for, is of highly questionable utility and may trigger a new global nuclear arms race," said Rep. Ellen Tauscher, D-Alamo, a member of the House Armed Services Committee who has led opposition votes in 2003 and 2004.

The Pentagon and the Energy Department plan to draw up a formal military requirement for the penetrator, according to administration budget documents. If so, the Pentagon's statement of need for the weapon will go head-to-head in congressional debate against the most definitive, non-governmental study of the bunker buster to date.

A panel of weapons experts assembled by the National Academy of Sciences has concluded that the weapon has some benefit to national security, but is not of much practical use against underground bunkers of chemical or biological weapons, high on its target list.

Like previous studies, including one by former Livermore director Michael May, sources said the NAS report concludes that a nuclear weapon in theory could incinerate or neutralize chem-bio weapons, but only if the weapon detonates close to the center of the underground room in which they might be stored. That would require knowledge of the detailed layout of secret underground WMD facilities of the sort that administration officials confidently predicted would be found in Iraq.

Researchers have called that kind of elusive targeting knowledge "exquisite intelligence," and without it, even the 1.2-megaton blast of the bunker buster could heave chemical or biological agents to the surface in a column of radioactive fallout.

Contact Ian Hoffman at ihoffman@angnewspapers.com.