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US backs revamp of nuclear warheads

Weapons stockpile set for 'unnecessary' upgrade.

The US government is pushing ahead with a plan to overhaul its nuclear stockpile, despite a scientific review showing that existing warheads will last at least another half-century.

On 1 December, the National Nuclear Security Administration — the agency that oversees the US nuclear stockpile — announced that it will pursue plans to develop a new Reliable Replacement Warhead (RRW), which it claims will be safer and more robust than current designs. But the announcement came just two days after the release of research showing that the plutonium in existing warheads has a shelf-life of at least a century. Most warheads in the US stockpile are only about 20 years old.

Critics say the decision to proceed is an example of politics trumping science. “It is clear that the present stockpile is going to be reliable beyond our lifetime,” says Jay Coghlan, executive director of Nuclear Watch, a watchdog group in Santa Fe, New Mexico. “It's not the science that rules, it's the special interests.”

But officials maintain that the programme was never solely about replacing old warheads. “I believe the reasons for the RRW are in some ways independent of plutonium ageing,” says Charles McMillan, associate director for weapons physics at Los Alamos National Laboratory in New Mexico. “RRW enables us to bring new technologies to the table.”

Since 2004, the RRW concept has been gathering momentum — both at the nation's nuclear weapons labs and in Congress (see Nature 442, 18–21; 2006). The idea is to build a new generation of robust warheads to replace existing designs. They would be larger than existing warheads, but would have a greater margin of error, be easier to manufacture and include more safeguards.

The proposed designs have mainly focused on making changes to the plutonium triggers, or pits, of the current warheads. Some argue that adding more plutonium or changing the way they are cast would result in a more stable, dependable trigger that could sit on the shelf for years without testing.

Research suggests that the present crop of US nuclear warheads should last for at least 80 years. Credit: P. SHAMBROOM /NUKEPHOTO.COM

Officials have pushed for the programme to proceed quickly because of concerns that the ageing triggers on the current warheads might become unreliable within a few decades. The chief worry was that the constant stream of radiation that comes from the plutonium itself could create cavities in its crystalline structure, causing the weapon to fail.

But a trawl of old nuclear test data and a battery of lab tests have shown this isn't the case. Some of the 1,054 nuclear tests carried out by the United States before 1992 were done with ageing weapons, and the data helped scientists understand how older plutonium triggers behave. Meanwhile, researchers also discovered that artificially aged plutonium heals itself by shifting new atoms back into its crystalline lattice. The current generation of weapons can therefore last for at least 85–100 years.

“The arguments with which replacement warheads won congressional support have fallen apart.”

These findings are likely to have implications elsewhere, especially in Britain, which is thought to have warheads similar to the US design. Other nations such as France use plutonium triggers and have stockpile stewardship programmes.

The findings were “a real surprise”, says Raymond Jeanloz, a geologist at the University of California, Berkeley, and a member of the JASONs — an independent scientific group that advises the US government on security issues and that reviewed the work. “The labs have done an outstanding job.”

The revised lifetimes call into question the need for an RRW programme, say many critics. RRW was sold to congressional supporters on the basis that the ageing stockpile would soon have to be replaced anyway. But as it will be in good condition for the conceivable future, such a plan now seems unnecessary, says Christopher Paine, a nuclear analyst at the Natural Resources Defense Council, an environmental group based in New York. “The arguments with which they won congressional support have fallen apart.”

McMillan counters that the RRW has always been about more than just replacing older warheads. “RRW will bring to the stockpile the most modern technologies for safety and security,” he says. In addition, he says, the process of developing and producing the RRW will help transform the weapons complex into a smaller, more responsive one.

If nothing else, the extended shelf-life of the current warheads should allow more time for debate.

Additional information

See Editorial, page 653.

Related links

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Enough warheads, already

Rich in plutonium

Nuclear weapons: The next nuke

Bush buries US bunker-buster project

Nuclear chiefs scotch story on frailty of ageing warheads

Related external links

The JASON report on Plutonium

CDI beginners guide to RRW

Los Alamos National Laboratory

National Nuclear Security Administration

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