Performance analysis and an assessment of operational issues of Ya-21U

Abstract

Extensive testing of the Soviet made TOPAZ‐II space nuclear power system unit designated ‘‘Ya‐21U’’ was conducted both in the USSR (1989–1990) and in the US (August 1993 to March 1995). The unit underwent a total of 15 tests for a cumulative test/operation time of almost 8000 hours. These tests included steady‐state operation at different power levels, fast startups and power optimizations. Leaks were detected in some of the Thermionic Fuel Elements (TFEs) after the first test in the US. These leaks that facilitated air incursion into the interelectrode gap caused operational changes in both electric power and conversion efficiency and changed the optimum cesium pressure and load voltage. Additional changes in operational performance were detected following shock and vibration tests performed in August 1994. Test data was examined and analyzed to assess the performance of not only individual TFEs, and also the whole Ya‐21U unit, and identify causes for measured operational performance changes; most probable causes were identified and discussed. The Ya‐21U unit remained operational throughout extensive testing for 8000 hours at conditions far exceeding the design limits of the TOPAZ‐II system. No single TFE was damaged during testing and measured operational performance changes were uniform among working section TFEs. In addition to providing a unique knowledge base for future development and operation of thermionic power systems, the test results testify to the reliability and ruggedness of the TOPAZ‐II system design.