TEST-RESULTS OF YA-21U THERMIONIC SPACE POWER-SYSTEM

The Soviet-made TOPAZ-II space nuclear power system unit designated Ya -2lu underwent a total of 15 tests both in the Union of Soviet Sociali st Republic (USSR) (1989-1990) and in the United States (August 1993 t o March 1995) for a cumulative test/operation time of 7681 h at condit ions far exceeding design limits. These tests included steady-state op eration at different power levels, fast start-ups and power optimizati ons, and shock and vibration tests. Test results are presented and ana lyzed. Results indicate a gradual change in the performance parameters such as the optimum cesium pressure and optimum load voltage. The ele ctric power and conversion efficiency of the unit at an input thermal power of 105 k W decreased from 3.7 k W(electric) and 4% in the test i n the USSR to 2.13 kW(electric) and 2.3% in the last test in the Unite d States. A discussion and qualitative assessment of potential causes of the performance changes of the Ya-2lu unit are given.