The Nuclear Assembly Test is currently being designed to demonstrate the performance characteristics of a 100‐kWe version of the power source for the SP‐100 Generic Flight System. Particular emphasis will be placed upon the operation of the prototypical ground test reactor under conditions of high‐working temperatures and long life. The key features of the reactor include a small, compact core with component materials consisting of refractory metals and alloys. Because of the unique features of the SP‐100 system, extensive use is made of Monte Carlo methods in the design and analysis of the reactor configuration. In addition, detailed testing of the reactor design has been carried out in the Zero Power Physics Reactor facility to provide calibration factors for the principal performance parameters. The key features of the test reactor design are described in this paper.