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Spontaneous combustion is not high on most people鈥檚 list of worries, but when it happens to materials at one of the world鈥檚 oldest and largest storage centres for weapons-grade uranium, it is a different matter.

On 22 September, the plastic wrapping around some uranium at the Y-12 National Security Complex in Oak Ridge, Tennessee, burst into flames as a technician was removing it inside a glovebox. Exposed to air, the uranium had heated up and ignited the plastic.

The fire took place in a large wooden warehouse built in 1944 to help the Manhattan Project, set up to develop nuclear weapons. The warehouse is one of the facility鈥檚 main stores for its 400 tonnes of highly enriched uranium, and is now officially rated as a fire hazard, according to an assessment in 1996 by the US Department of Energy (DoE).

In this case the incident was contained, but a major fire would have catastrophic consequences. The DoE says a fire could result in uranium containers breaking open and releasing their contents in a plume of toxic, radioactive smoke. About 700,000 people live within a 160-kilometre radius of Y-12, including 174,000 in Knoxville 25 km away and 28,000 in Oak Ridge itself. In the worst case, the DoE estimates that the local population could receive radiation doses of up to 900 millisieverts, enough to cause nausea, hair loss and in some cases death.

鈥淚n the worst case, local people could receive enough radiation to cause nausea, hair loss and death鈥�

The dangers at Y-12, revealed in a study this week, are not unique. Worldwide more than 1750 tonnes of highly enriched uranium have been produced over the last 62 years to supply bombs, submarines and research reactors.

鈥淪ignificant amounts are stored at dozens of sites in Russia and other countries, often inadequately accounted for, protected and controlled,鈥� says Morten Bremer M忙rli, a nuclear expert from the Norwegian Institute of International Affairs in Oslo. 鈥淢any of the stores are old, some pose environmental threats and some may be at risk from terrorists. International standards are currently too weak to ensure safety and security.鈥�

But what is surprising about Y-12 is that the world鈥檚 richest nation has allowed it to deteriorate to such a poor state. It has been 鈥渇estering for decades鈥�, says Robert Alvarez, a senior environmental adviser to the Clinton administration now with think tank the Institute for Policy Studies in Washington DC.

鈥淲hile a considerable amount of attention has been drawn to dubious storage conditions in Russia and former Soviet states, long standing nuclear weapons material storage problems in this country pose unacceptable risks to workers and the public,鈥� he says.

Alvarez is the author of the detailed study of safety at Y-12 due to appear in a forthcoming issue of the journal Science and Global Security, published by Princeton University. He reveals that the incident on 22 September is just the latest of 22 fires and explosions that have beset the Y-12 complex since 1997, a rate of about two a year (see 鈥淐atalogue of chaos鈥�).

Some were caused by electrical faults, some by ignition of reactive materials such as lithium, calcium and sodium, and some by the tendency of small particles of uranium to ignite spontaneously when exposed to air. Workers have been injured and contaminated and, in one instance, an explosion blew out all the windows in a building.

The Y-12 complex, almost 1 km wide and 5 km long, fills Bear Creek valley in east Tennessee. It comprises about 500 buildings, most of which were built in the 1940s and 1950s. It was this facility that produced the highly enriched uranium for the bomb dropped on the Japanese city of Hiroshima on 6 August 1945. It is now run by a contractor for the DoE鈥檚 National Nuclear Security Administration.

Though the complex still services US nuclear warheads and submarine reactors, its main task these days is to look after 400 tonnes of waste uranium. It is probably the world鈥檚 largest store of weapons-grade materials.

Managing this kind of material has proven hard. Alvarez says the site has a huge backlog of materials to process, such as unstable uranium oxides, solutions and residues in some 200 different types of containers. The exact contents of thousands are unknown because of poor records, and many have remained unopened since they were shipped across the US from other sites.

As well as the risk of explosions and fires, there is also the danger that enough highly enriched uranium might come together to start a chain reaction, a process called criticality. This can produce a burst of potentially lethal radiation. In May this year, there was a major safety violation when uranium was found to be accumulating in a filter in exactly the kind of way that could lead to a criticality event.

Nor is it just the nuclear materials at Y-12 that pose a safety hazard, Alvarez says. He says that a major release of anhydrous compounds of fluorine, used to process uranium, could result in 鈥渁 significant number of public casualties鈥�, with some people receiving 30 times the lethal dose.

Attempts by the DoE over the last 12 years to address Y-12鈥檚 manifold problems have failed because of 鈥渓arge cost overruns, delays and project failures鈥�, Alvarez alleges. The estimated cost of a planned new uranium store has risen from $120 million to nearly $500 million in the last five years because of design changes and 鈥渋nadequate concrete pouring鈥�. The new store is now almost a decade behind schedule.

What鈥檚 more, potential security problems at Y-12 are highlighted by another report published last week by the Project On Government Oversight (POGO), an independent watchdog in Washington DC. It concludes that the site will not be able to meet the US government鈥檚 post-9/11 recommended security standards until at least 2013.

The POGO report also argues that improvising a nuclear bomb using Y-12鈥檚 highly enriched uranium would be a relatively easy task for terrorists on site. Simply dropping one 45-kilogram lump onto another from about 1.8 metres could create a 10-kiloton explosion, it says, killing or injuring up to 60,000 people. The prospect, however, is dismissed as 鈥渇anciful鈥� by the DoE鈥檚 National Nuclear Security Administration. 鈥淭here are better odds that an asteroid would hit Oak Ridge,鈥� says an NNSA spokesman. 鈥淭he facility is well prepared to defeat a terrorist attack.鈥�

The NNSA also attacks Alvarez鈥檚 report as 鈥渕isleading鈥� and 鈥渨oefully out of date鈥�. It accepts that Y-12 was allowed to deteriorate in the 1990s, but points out that since 2001 the Bush administration has invested more than $300 million on top of normal budgets to demolish or upgrade old facilities. 鈥淲e are currently building one facility and planning another to consolidate all uranium operations in modern, secure structures,鈥� says the NNSA spokesman.

Alvarez rejects the NNSA鈥檚 criticisms, arguing that without firm targets and schedules, the money invested could disappear 鈥渄own a black hole鈥�. His study and that by POGO are also backed by Frank von Hippel from Princeton University鈥檚 Program on Science and Global Security, who describes the security situation at Y-12 as 鈥渄eplorable鈥�. The complex houses enough weapons-grade uranium to improvise 鈥渕any thousands鈥� of nuclear devices, von Hippel points out, each of which would be capable of making an explosion similar to that which destroyed Hiroshima.