Los Alamos National Laboratory and Glenn Research Center with the help of National Security Technologies demonstrated the use of a nuclear fission system as a power source that transferred heat via a water-based heat pipe to a small Stirling engine–based power converter to produce electricity. This experimental setup demonstrated that a small reactor based on heat pipes and Stirling engines is possible and produces a system with well-characterized nuclear feedback between the reactor and the power conversion system. This paper describes the experimental setup, modeling of the system, and results that confirm the basic physics of the experiment.