The Aircraft Reactor Experiment (ARE) was an experimental nuclear reactor designed to test the feasibility of fluid-fuel, high-temperature, high-power-density reactors for the propulsion of supersonic aircraft. It operated between November 8-12, 1954 at the Oak Ridge National Laboratory (ORNL) with a maximum sustained power of 2.5 megawatts (MW), and generated a total of 96 MW-hours of energy. The ARE was the first reactor to use circulating molten salt fuel. The hundreds of engineers and scientists working on ARE provided technical data, facilities, equipment, and experience that enabled the broader development of molten salt reactors as well as liquid metal cooled reactors. Background The concept of nuclear-powered aircraft was first formally studied in May 1946 by the US Army Air Forces. It was supposed that the unique characteristics of nuclear power could be applied to long-range supersonic flight, which was considered highly valuable in terms of military strategy. Challenges in the proposal were understood immediately, and by 1950 the Atomic Energy Commission joined with the Air Force to study the possibilities via technology development in the Aircraft Nuclear Propulsion (ANP) program. The ORNL staff of the ANP project decided that technical information and experience needed to support the objective of nuclear-powered flight could most economically be obtained from building and operating the ARE. They considered the task of flying a supersonic airplane on nuclear energy exceedingly complex, and thought more than one experimental reactor may be necessary before sufficient information was obtained to design and construct a reactor for flight. Originally, the ARE was conceived as a liquid sodium metal-cooled Beryllium oxide (BeO)-moderated solid-fuel reactor. The BeO moderator blocks were purchased with the solid-fuel design in mind. However, concerns regarding chain reaction stability related to xenon in solid fuel at very high temperatures were serious enough to warrant abandoning solid fuel and replacing it with circulating fluid fuel. A fluid-fueled option with molten fluoride salt was worked into the original design. Reactor The ARE was designed to be a prototype of a 350-megawatt, BeO-moderated, circulating-fuel aircraft reactor. It used a fuel composed of 53.09 mole % NaF, 40.73 mole % ZrF4, and 6.18 mole % UF4. The reactor was a BeO cylinder with bent tubes directing flowing fuel through the core in both directions. It was surrounded by an Inconel shell. The ARE operational life targeted 1000 hours, with as much time as possible at the full power level of 3