In the 1950s, both the US and the USSR tried to develop nuclear propulsion systems for piloted aircraft, but the plans were eventually scrapped because it would have cost too much to protect the crew from the on-board nuclear reactor, as well as making the aircraft too heavy. However, the US Air Force Research Laboratory (AFRL) now intends to abandon the conventional fission reactor design and instead focus on a new type of power generator called a quantum nucleonic reactor. In 1999, Carl Collins and colleagues from The Center for Quantum Electronics at the University of Texas at Dallas discovered that shining X-rays onto certain types of hafnium caused 60 times as much energy to be released as was put in. The X-rays encourage particles in the nuclei of radioactive hafnium-178 to jump down several energy levels, thus liberating energy in the form of gamma rays. These gamma rays could produce a jet of heated air, providing the thrust for the UAV.
The quantum nucleonic reactor is considered safer than a fission reactor 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,” says Christopher Hamilton at the Wright Patterson Air Force Base in Ohio, who conducted the latest nuclear UAV study.
The US effort in the 1950s was known as Project Pluto (as in king of the dead underworld) and some neat previously classified film footage about it is available. Seems like Air and Space magazine did an article on it a few years back, too. There is also a French site with some neat photos of the Pluto nuclear engine; be sure to check out the (English) links at the bottom of the French page to find more info on this incredible atomic chapter of aviation history. What goes around comes around.