The US Air Force is examining the feasibility of a nuclear-powered version of an unmanned aircraft. 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. Instead of a conventional fission reactor, the US Air Force Research Laboratory (AFRL) 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.