|Fig. 1: On September 17, 1955, the Convair NB-36H was the first aircraft to fly with a nuclear reactor operating on board.  (Source: Wikimedia Commons)|
It is widely accepted that burning fossil fuels for energy is neither sustainable nor environmentally friendly. Oil and coal reserves will eventually run out, and the greenhouse gases emitted from their combustion threaten the climate in the form of global warming. As far as the transportation sector goes, the emission of greenhouse gases from air travel is particularly bad because the greenhouse gases are released at high altitudes directly into the atmosphere. For this reason, it is estimated that the gases and particulates emitted from jet aircrafts are two to four times as harmful as those emitted from automobiles. 
Unfortunately, the transition away from fossil fuels has been more difficult for aviation than for ground transport. While batteries are far from achieving the energy density of carbon-based fuels - gasoline contains nearly 100 times more energy per unit volume than the best lithium-ion batteries can currently offer - they are already replacing internal combustion engines in cars.  This is possible partly because, unlike airplanes, automobiles can afford the additional weight brought on by the heavy batteries.
In addition, the power required to operate a car is far less than that required to lift an airplane off of the ground. While a typical car engine provides around 100-300 horsepower (74-225 kW), a single Boeing 777 jet engine delivers 110,000 horsepower (820 MW), several orders of magnitude greater than the highest performing automobiles. Even when normalized by the mass of the transportation vessel, the jet engine is astonishingly powerful: a car operates using 50-150 W/kg while the Boeing 777 runs on approximately 4,700 W/kg. So far, no battery-powered engine has been designed that is capable of meeting this power density. 
Therefore, nuclear energy has the potential to do what neither fossils fuels nor electricity can do: power a commercial-scale flight with little to no atmospheric emissions. Nuclear boasts unparalleled energy density - a kilogram of uranium converted via nuclear processes contains three million times the energy of a kilogram of coal - and its waste is contained as either a liquid or a solid.  But how would a nuclear airplane work?
A standard jet engine functions as follows. First, air from the atmosphere is compressed to increase the concentration of O2 for combustion. Second, the air is combusted with heavy organics - i.e. jet fuel - to generate heat and pressure. And third, the high-energy gas mixture expands out the back of the engine, generating thrust per Newton's third law. A nuclear-powered engine would work the same way except the air would not be heated by combustion but via heat exchange with a nuclear fission reactor. Two designs for this engine were proposed during the United States' Aircraft Nuclear Propulsion (ANP) program in the 1950s, which was created to develop bombers that could fly to the Soviet Union and back without refueling. For the first design (Direct Air Cycle), the atmospheric air would pass directly through the reactor core for heating. For the second design (Indirect Air Cycle), which was pursued to minimize radioactive pollution, the reactor core would first heat up a liquid metal or pressurized liquid. This fluid would, in turn, heat the air, thereby creating a degree of separation between the nuclear reaction and the atmosphere. In both cases, a molten salt reactor was to be used because of its high operating temperature and unique safety features. 
Despite having a process powerful enough to keep a large aircraft airborne for months at a time as early as the 1950s, however, no nuclear-powered aircraft has ever taken flight. The best the United States ever did was fly a plane that carried an operating nuclear reactor (see Fig. 1). 
The ANP was abandoned in 1961. While this was primarily because of the development of intercontinental ballistic missiles (ICBMs), several serious technical concerns remained through its termination. First and foremost, despite the large quantity of energy contained in nuclear fuels, a nuclear-powered engine capable of delivering the power density necessary to fly a large aircraft has yet to be successfully designed. Until that happens, any discussion of a future with nuclear-powered aircrafts is purely speculation. In addition, due to the close quarters of an aircraft, the crew could be exposed to radiation from the nuclear reactor. Because thick lead shielding would be too heavy, a distributed shielding method was being developed at the time of the ANP to eliminate specific radioactive particles. Of course, to commercialize a nuclear-powered passenger plane, this technology would need to be engineered to perfection to avoid any radiation poisoning. And finally, the event of a crash could be catastrophic for a nuclear aircraft. Extensive testing would be required to ensure that, in the event of a crash, the radioactive materials would be contained. 
While there have been recent attempts to revive the nuclear-powered airplane, the field remains underdeveloped.  Assuming the aforementioned power density and safety hurdles can be overcome, however, nuclear energy can potentially service the transportation industry in the form of environmentally-friendly aircrafts with extraordinarily high performances.
© Sam Dull. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 "Aviation and the Environment," U.S. General Accounting Office, GAO/RCED-00-57, February 2000.
 F. Schlachter, "Has the Battery Bubble Burst?," APS News 21, No. 8 (August/September 2012).
 A.C. Madrigal, "A Single Boeing 777 Engine Delivers Twice the Horsepower of All the Titanic's," The Atlantic, 30 Dec 11.
 J. Bernstein, Nuclear Weapons: What You Need to Know. (Cambridge University Press, 2007.
 A. M. Weinberg, The First Nuclear Era: The Life and Times of a Technological Fixer. (American Institute of Physics, 1997).
 S. Dowling, "Could This Be the First Nuclear-Powered Wirliner," BBC, 14 July 16.