According to the report, 'The USAF hopes that such a vehicle will be able to "loiter" in the air for months without refuelling, striking at will when a target comes into its sights."

However, the idea is expected to raise serious issues with the wisdom of flying radioactive material in a combat aircraft. If shot down, for instance, would an anti-aircraft gunner in effect be detonating a dirty bomb? It raises political questions, too.

To have Unmanned Aerial Vehicles (UAVs) constantly flying over a region would amount to a new form of military intimidation, especially if they were armed, says Ian Bellamy, a Lancaster University arms control expert.

But right now, there seems no stopping the proliferation of UAVs, fuelled by their runaway success in the Kosovo and Afghanistan conflicts. The obvious attraction of UAVs is that they don't put pilots' lives at risk, and they are now the norm for many reconnaissance and even attack missions.

The endurance of a future nuclear-powered UAV would offer military planners an option they might find hard to turn down. Last week, the Pentagon allocated $1 billion of its 2004 budget for further development of both armed and unarmed UAVs.

The US Air Force Research Laboratory (AFRL) has funded at least two feasibility studies on nuclear-powered versions of the Northrop-Grumman Global Hawk UAV (pictured above).

The latest study as reported by New Scientist and revealed earlier this month at an aerospace technology conference in Albuquerque, New Mexico, concluded that a nuclear engine could extend the UAV's flight time from hours to months.

But nuclear-powered planes are not a new idea. In the 1950s, both the US and the USSR tried to develop nuclear propulsion systems for piloted aircraft. The plans were eventually scrapped because it would have cost too muchto protect the crew from the on-board nuclear reactor, as well as making the aircraft too heavy.

The AFRL now has other ideas, though. Instead of a conventional fission reactor, it is focusing on a type of power generator called a quantum nucleonic reactor.

This obtains energy by using X-rays to encourage particles in the nuclei of radioactive hafnium-178 to jump down several energy levels, liberating energy in the form of gamma rays. A nuclear UAV would generate thrust by using the energy of these gamma rays to produce a jet of heated air.

Military interest led from research published in 1999 by Carl Collins and colleagues at the University of Texas at Dallas. They found that by shining X-rays onto certain types of hafnium they could get it to release 60 times as much energy as they put in.

The reaction works because a proportion of the hafnium nuclei are "isomers" in which some neutrons and protons sit in higher energy levels than normal. X-ray bombardment makes them release this energy and drop down to a more stable energy level.

So the AFRL has since been looking at ways in which quantum nucleonics could be used for propulsion. "Our directorate is being cautious about it. Right now they want to understand the physics," says Christopher Hamilton at the Wright Patterson Air Force Base in Ohio, who conducted the latest nuclear UAV study.

The AFRL says the quantum nucleonic reactor is considered safer than a fission one because the reaction is very tightly controlled. "It's radioactive, but as soon as you take away the X-ray power source its gamma ray production is reduced dramatically, so it's not as dangerous [as when it's active]," says Hamilton.

Paul Stares, an analyst with the US Institute of Peace in Washington DC, wonders what would happen if a nuclear UAV crashed. But Hamilton insists that although hafnium has a half-life of 31 years, which according to Britain's National Radiological Protection Board is equivalent to the highly radioactive caesium-137, the structural composition of hafnium hinders the release of this radiation. "It's probably something you would want to stay away from but it's not going to kill you," claims Hamilton.

This article is based upon a New Scientist press release

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