Senior weapons scientists emphatically denied a report this week that the most important US nuclear warhead has a design fault that could make it unreliable.
The New York Times reported on 3 April that the W76, a thermonuclear warhead launched from submarines, has a flaw in the design of its casing that could cause it to explode with much less force than expected — or not at all.
Leading scientists who designed the warhead say that the accusations are completely unfounded, however, and that the report's sources weren't heavily engaged in the design project. "There is nothing wrong with the W76," says Harold Agnew, who was director of the Los Alamos National Laboratory in New Mexico when the lab designed and tested the warhead 30 years ago.
Exact numbers are classified, but arms-control experts estimate that the W76 makes up almost one-third of the US stockpile of 10,000 warheads, as well as dominating the far smaller British nuclear arsenal. The warheads are compact and lethal: up to eight of them can sit in a single missile, each yielding an explosive force more than five times bigger than that of the bomb dropped on Hiroshima in 1945.
For about a year, some scientists have been expressing concern to government officials about the uranium case surrounding the warhead. The case is thin and light so that it can be carried on smaller, submarine-launched missiles. The critics claim that it might fail when the fission trigger of the bomb detonates, meaning the bomb's powerful fusion fuel would not ignite.
The bombs are "at best unreliable and probably much worse", Richard Morse, a retired plasma physicist from Los Alamos told the The New York Times. Morse did not respond to Nature's requests for an interview.
Other scientists familiar with the weapon dispute Morse's assertions. "I think he's wrong," says Richard Garwin, a prominent former hydrogen-bomb designer. Agnew, who was present at several tests of the W76, says that it never failed to detonate. He adds that six to nine warheads are carefully analysed every year to make sure that ageing will not affect their performance.
"We have looked into this concern extensively, and our best technical judgement is that it is simply wrong," Linton Brooks told the Senate on 4 April. Brooks is the administrator of the National Nuclear Security Administration, which oversees the US nuclear stockpile.
But Brooks added that his agency is now planning a study to create a more robust type of warhead. This might eventually replace weapons such as the W76, which was designed for minimum size and weight. He says that the Reliable Replacement Warhead programme will design weapons that have wider performance margins and can be more easily maintained without testing.
Arms-control experts are sceptical of the project, which will cost $9 million in 2006. "The existing stockpile is safe and reliable, and it is likely to be so for some time," says Daryl Kimball, executive director of the Arms Control Association in Washington, DC. "This programme seems unnecessary."
The replacement warhead programme would itself replace an 'advanced concepts' project, which has been criticized as a step toward development of new nuclear weapons (see Nature 428, 455; 200410.1038/428455a). Opponents of new weapons note that doubts about the W76's reliability could boost political support for the replacement programme.