As part of the TOPAZ International Program, the performance of single cell thermionic fuel elements (TFE) was evaluated durng unfueled ground testing of individual TEF’s and TOPAZ II space nuclear power systems. Experiments are coordinated between the TFE test rig and the TOPAZ II system tests to correlate TFE performance. The experiments conducted to date include optimum power data, low power operations in the ‘‘station keeping’’ mode and high output power performance. In addition, the data from these experiments are currently being evaluated by the U.S. Naval Post‐graduate School for computer thermal modeling of the heat transfer characteristics of a thermionic power system (Benke and Venable 1995). This testing is being conducted at the New Mexico Engineering Research Institute (NMERI) by the Air Force Phillips Laboratory (PL) for the Ballistic Missile Defense Organization (BMDO). © 1995American Institute of Physics