The Aircraft Reactor Experiment was designed for operation at temperatures in the region of 1500°F at a power of 1 to 3 Mw with a fluoride-salt fuel circulating in a heterogeneous core. The moderator was hot-pressed BeO blocks cooled by circulating sodium. The heat produced was dissipated in water through hot liquid-to-helium-to-water heat exchange systems. All sodium and fuel circuit components were made of Inconel fabricated by inertgas (Heliarc) welding. The sj^stem was heated to design temperature by means of electrical heating units applied over all parts of the system. Instrumentation and control of the experiment were fairly conventional. For the most part, standard instruments were modified slightly for the high-temperature application. The reactor system was constructed and operated in a building specifically provided for the purpose.