Picture Global Hawk Unmanned Aerial Vehicles (UAVs) that can fly continuously for months.
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.
A tutorial on quantum nucleonics provides some details on the science involved.
A friend who knows a fair amount of physics thinks that pumping the hafnium nuclei up to a high energy state to make them into a suitable fuel source would be a fairly inefficient process because most of the x-ray energy would not hit the nuclei to cause the needed energy jump. Many times more energy would be needed to pump the nuclei up into a higher energy state than would be given back as the jump down to a lower state and release their energy. Therefore quantum nucleonics is not an approach suitable for large scale energy storage use.
The advantage over batteries or even hydrogen fuel is that quantum nucleonics can be used to produce a material with a much higher energy density. In specialty applications such as military UAVs the energy costs may be worth it because the high energy density would allow continuous operation of a UAV for a long period of time.
The technology seems like it would be more attractive for UAV use over oceans than over land. A UAV that crashed over an ocean would likely sink to the bottom taking its radioactive material with it far beyond the reach of humans. There are plenty of naval military applications for UAVs such tracking enemy fleets, surveilling ships to look for possible terrorist ships carrying WMD, as well as search and rescue. There are also civilian scientific applications such as continuous data collection on weather and environmental monitoring. Plus, search and rescue is a civilian need as well. One can even imagine long duration UAVs being used as lower altitude equivalents of communications satellites.
Update: Back in August 2001 some Lawrence Livermore National Laboratory scientists published a result that argues against the possibility of using hafnium-178 as an energy storage material.
LIVERMORE, Calif.—Physicists from the Lawrence Livermore National Laboratory, in collaboration with scientists at Los Alamos and Argonne national laboratories, have new results that strongly contradict recent reports claiming an accelerated emission of gamma rays from the nuclear isomer 31-yr. hafnium-178, and the opportunity for a controlled release of energy. The triggering source in the original experiment was a dental X-ray machine.
Using the Advanced Photon Source at Argonne, which has more than 100,000 times higher X-ray intensity than the dental X-ray machine used in the original experiment, and a sample of isomeric Hf-178 fabricated at Los Alamos, the team of physicists expected to see an enormous signal indicating a controlled release of energy stored in the long lived nuclear excited state. However, the scientists observed no such signal and established an upper limit consistent with nuclear science and orders of magnitude below previous reports.
|Share |||Randall Parker, 2003 February 20 11:34 AM Airplanes and Spacecraft|