Modeling the Operation of Long-Life Thermionic Converters with Nuclear Heating

Abstract

Predicting the operation of thermionic converters (TEC) with nuclear heating is a high-priority problem given their long service life [1]. Operational failures of TEC are usually associated with a reduction of the generated power [2]. Ordinarily, statistical methods as well as computational and experimental approaches based on mathematical modeling are used to make predictions and assessments of the probabilistic characteristics of the predictions. Such an attempt at prediction was made in [3] for the electrodes of a single-element TEC. The present article is concerned with validating the accelerated-testing method in order to confi rm the stability of the output characteristics of long-life ventilated thermionic converters. The method is based on the temperature dependence of the adsorption processes occurring at the electrodes. Models of the Variation of the Emission Properties of TEC Electrodes.The required service life is now approaching 10 years. This precludes direct demonstration experiments to confi rm service life; methods for predicting it are needed. In the absence of threshold processes, a method of forcing a process-determining parameter is needed. Changing from measuring the total effect to determining the rate of a corresponding process followed by integration of the functional dependence for the proposed lifetime affords greater possibilities. This method does not require special validation if the only process present is the one in question. However, in the event that several processes are superposed model validation is required because the processes can interact.For TEC the main factor determining the service life and degradation of the output characteristics is the state of the emitter and collector surfaces which the following processes can affect:collector -adsorption of the gaseous and volatile fi ssion products of the fuel, deposition of the emitter material by evaporation and cyclic mass-transfer processes, contamination of the surface as a result of the condensation of the components of the residual gases as well as segregation of the impurities from inside the collector, and interaction of the impurities reaching the surface with the collector materials and the adsorbed layers of cesium; emitter -segregation of the impurities on the surface and faceting of the (112) faces as a result of vacuum etching of the surface with formation of facets with the most stable (110) faces.The distribution shown in Fig. 1 of the primary elements W, Mo, C, O, and Cs in a surface layer of the collector in a single-element converter operating on a thermal stand for 2500 h is typical and characteristic for all single-element TEC, right up to the maximum testing time for 3 years [4]. The individual particularities of the service-life tests brought about changes in the ratios of the primary components of the surface layer of the collector, leaving its composition unchanged. For this reason, these data can also be used to analyze the serviceability of long-life converters.