For more than two decades, a compact, powerful warhead called the W-76 has been the centerpiece of the nation's nuclear arsenal, carried aboard the fleet of nuclear submarines that prowl the Atlantic and Pacific oceans. But it has become the subject of a fierce debate among experts inside and outside the government over its reliability and its place in the nuclear arsenal.

The government is readying a plan to spend more than $2 billion on a routine, 10-year overhaul to extend the life of the aging warheads. At the same time, some weapons scientists say the warheads have a fundamental design flaw that could cause them to explode with far less force than intended.

Although the government has denied that assertion, officials have disclosed that Washington nevertheless is considering replacing the W-76 altogether.

"This is the one we worry about the most," said Everet Beckner, who oversees the arsenal as director of defense programs at the National Nuclear Security Administration.

Some arms-control advocates oppose the 10-year overhaul program, saying it could produce not only refurbishments but also deadly new innovations. They like the replacement option even less, saying it could prompt the government to conduct underground detonations that would undo the global ban on nuclear testing and start a new arms race.

Moreover, some argue that nuclear weapons are dinosaurs that have little use in U.S. military strategy and that it makes no difference if the W-76 is ineffective.

"That's why people are so passionate about this," said Daryl Kimball, executive director of the Arms Control Association in Washington.

The W-76, developed in the early 1970s for destroying large targets such as military bases, now sits packed in clusters of up to eight atop hundreds of missiles in a dozen nuclear submarines. While exact figures are secret, federal officials and private weapons experts agree that it is the nation's leading weapon by virtue of sheer numbers. Of 5,000 active warheads in the arsenal, experts say 1,500 are W-76s aboard submarines. Each is meant to be about seven times as powerful as the bomb that destroyed Hiroshima.

Workhorse of a weapon

The importance of the W-76 is rising as the nation's nuclear force relies more on submarines and less on bombers and land-based missiles.

"It's by far the most numerous" warhead, said Hans Kristensen, a weapons expert at the Natural Resources Defense Council, a private group in Washington that monitors nuclear trends. "It's the workhorse in terms of targeting."

Several factors lie behind the current worries and repair plans. The W-76 is one of the arsenal's oldest warheads. As warheads age, the risk of internal rusting, material degradation, corrosion, decay and the embrittling of critical parts increases.

The overhaul to forestall such decay is scheduled to go from 2007 to 2017. In all, it is expected to cost more than $2 billion, say experts who have analyzed federal budget figures.

Questions also surround the weapon's basic design

Four knowledgeable critics — three former and one current scientist at the Los Alamos National Laboratory in New Mexico, which designed the W-76 — have argued recently that the weapon is highly unreliable and, if not a complete dud, likely to explode with a force so reduced as to compromise its effectiveness.

Federal officials, while denying that, disclosed in interviews that the warhead is being considered for a new program that intends to replace old warheads with more reliable ones. Congress and future administrations would have to approve a replacement for the W-76.

Officials would give no estimate for that endeavor's cost or length of time. But they acknowledged that they carefully had weighed the W-76's problems and the alternatives for fixing them.

"I've spent a lot of personal time on this," Beckner of the National Nuclear Security Administration said.

The W-76, and its troubles, were born during the Cold War, when U.S. bomb makers sought to win the arms race with designs that made nuclear arms lightweight, very powerful and in some cases so small that a dozen or more could fit atop a slender missile.

Escalating tensions

Where most nuclear powers had to make do with weapons that were ponderous if dependable, more like Chevys than race cars, the W-76 epitomized the American edge. It was a hydrogen warhead — known as thermonuclear because a small atom bomb at its core worked like a match to ignite the hydrogen fuel. Shorter than a man, it had undergone an extraordinary degree of miniaturization.

"It was the tightest design we had," said one top nuclear scientist who did not want his name used for fear of retaliation for releasing confidential information. "They crammed in everything with a shoehorn."

Tensions ran high, especially for senior designers such as Charles Cremer, leader of thermonuclear design at Los Alamos. In 1974, as W-76 plans took shape, Cremer committed suicide.

Richard Morse, a physicist at the weapons laboratory who directed advanced concepts for bomb design as well as a separate group devoted to laser fusion, said in an interview that much tension centered on the weapon's so-called radiation case. In usual fashion, it was to be made of uranium, which is nearly twice as heavy as lead.

Leaders at Los Alamos wanted the case to be as lightweight as possible, so they envisioned it as extraordinarily thin — in places no thicker than a beer can (albeit with plastic backing for added strength).

Its physical integrity was vital. The case had to hang together for microseconds as the exploding atom bomb generated temperatures hotter than the surface of the sun, forcing it to emit radiation that kindled the thermonuclear fire. If the case deformed significantly or shattered prematurely, the weapon would fail, its thermonuclear fuel unlit.

From 1978 to 1987, about 3,400 W-76s rolled off the production line, Kristensen of the defense council said. The design was considered so good that Britain made a variant of the W-76 for its submarines.

Ban on nuclear tests

Even with their seeming success, arms designers continued to do underground tests to determine how cases would behave in the first milliseconds after the atomic blast. But in 1992, after the Cold War, the United States joined a global moratorium on nuclear tests. It no longer was possible to detonate weapons to check their reliability.

In secret, experts and officials say, debate on the W-76 began almost immediately after the test ban; suggestions included an alternative design that would thicken the radiation case and give the new warhead a much longer life. The work had become formalized by 1995 in a joint effort between the Navy and the nation's nuclear-weapons complex.

As the test ban persisted, U.S. nuclear officials singled out the W-76 as the first warhead to undergo precautionary scrutiny. The program employed teams from Los Alamos and Lawrence Livermore National Laboratory, its archrival. The meetings usually were cordial.

But a vocal dissenter emerged. It was Morse, who had left Los Alamos in 1976 for the University of Arizona but returned in 1996 and aided the W-76 assessment.

Morse specialized in scientific explanations for the complex flows that curl through the extraordinarily hot gases known as plasmas, which lie at the heart of an exploding nuclear weapon. His main goal was to help scientists develop a giant laser that, in lieu of an atomic match, would fire on a tiny radiation case surrounding an even tinier pellet of hydrogen fuel, releasing a burst of nuclear energy. Heat from such miniature hydrogen bombs was envisioned as a future source of electricity.

But Morse found that nature had erected tricky barriers to that goal. In particular, he documented how a form of turbulence known as Rayleigh-Taylor instability (named after the physicists Lord Rayleigh and Geoffrey Taylor) could perturb the expanding plasma of the very hot radiation case, forming waves, ripples and whorls that blocked ignition of the thermonuclear fuel.

He also found that extremely small variations in the case were responsible for the onset of turbulence, making it difficult to eliminate. It was a typical finding of the new science of chaos — much like understanding how the beat of a mythical butterfly wing could spawn a hurricane.

Festering concerns

Morse in 1996 brought similar analyses to bear on the W-76's thin case, arguing that it probably would fail. He said officials for decades had swept the issue under the rug and that Cremer, the designer, had struggled with the problem.

In an interview, Morse said he soon was "disinvited" from the evaluation and left Los Alamos for Sandia National Laboratories in Albuquerque. But he added that concerns about the W-76 only festered and grew.

Beckner disagreed. He said the joint review found that the W-76 "looks like a pretty good weapon."

Even so, the government began preparing for an extensive refurbishment of the warhead in a bid to extend its life by 30 years. The planning started around 2000 and foresaw the installation of new fuses, electronics, batteries, cables, valves and the conventional high explosives that light the atomic match. It also sought to increase the warhead's accuracy and flexibility in targeting.

In 2003, amid preparations for the refurbishment, Morse again sought to stir debate. He says he felt compelled to do so because of the W-76's rising importance to the nation's nuclear forces.

At a secret meeting in March 2004 at Los Alamos, Morse led four critics who laid out concerns to lab and federal officials, including Beckner. Morse characterized the discussion as acrimonious.

"It was a verbal mud-wrestling match," he recalled. The officials "would not be candid with us."

"We told them things they didn't know," Morse continued. "It was very, very disappointing."

In contrast, Beckner said the meeting and subsequent analyses left him with "high confidence that this nuclear weapon is a good design, was built properly and will function if required."

In early July, press reports in New Mexico began to describe the dispute, and the director of Los Alamos days later scheduled a secret lab symposium to review the "technical challenges" to understanding how radiation cases act in the first microseconds of a nuclear blast, according to a synopsis of the planned meeting.

As the number of press reports grew, lab and federal officials denied that there was any problem with the W-76. They cited a history of detonations of the weapon at the Nevada Test Site.

Dependability questioned

In late November, the dependability issue emerged nationally as Congress approved a small budget item that began a new weapons design effort known as the Reliable Replacement Warhead program. Its goal is to have weapons scientists design a new generation of nuclear arms that are more reliable and longer lived, reversing the Cold War trend of making small, lightweight, powerful weapons. If possible, it is to do so without nuclear testing.

Beckner of the nuclear administration said the W-76 was a candidate for redesign. The current work to extend the warhead's life, he said, could expand to include more fundamental design changes. "That is not the plan at present, but that could happen," he said, adding that he could not discuss the issue of thickening the radiation case.

Thomas Cochran, a senior scientist at the Natural Resources Defense Council, said a thicker, heavier case for the W-76 might force compensating cuts in the weight of its hydrogen capsule. And that, he added, would reduce the weapon's overall force.

Morse applauded the new federal interest. "What's out there in those boats," he said, "is at best unreliable and probably much worse."

New York Times reporters Sandra Blakeslee and Kenneth Chang contributed to this report.