An analysis of Ya-21U thermionic fuel elements test results

Abstract

The Soviet made Ya‐21U unit of TOPAZ‐II space nuclear power system underwent numerous tests for almost 8000 hours, both in the USSR and in US. A change of the Ya‐21U unit performance was measured during these tests. In an attempt to identify the most probable causes of this change in performance, test results of the working section Thermionic Fuel Elements (TFEs) were examined and their volt‐ampere characteristics analyzed with the help of the Thermionic Transient Analysis Model (TITAM). Results showed significant increase in the effective emissivity and the cesiated work functions of the electrodes which caused both the emitter temperature and the output electric power to decrease. Impurities and air incursion in the interelectrode gap had changed surface properties of the electrodes, causing the average effective emissivity of the Ya‐21U unit TFEs to be ∼0.03–0.035 higher than for as fabricated TFEs. The cesiated work functions of the electrodes were higher than that for clean electrode materials, causing the Ya‐21U unit load electric power to decrease.