Eect of oxygen on the operation of a single-cell thermionic fuel element Dmitry V. Paramonov * , Mohamed S. El-Genk Institute for Space and Nuclear Power Studies/Chemical and Nuclear Engineering Department, The University of New Mexico, Albuquerque, NM 87131, USA Received 11 November 1997; accepted 12 March 1998 Abstract The operation of thermionic converters (TICs) has been shown to be in¯uenced by the presence of tracer amounts of oxygen in the cesium vapor ®lled interelectrode gap. The induced changes, however, depend on the oxygen and cesium partial pressures and the materials and temperatures of the electrodes. The eects of oxygen on the loss rate from a tungsten emitter surface, by sublimation and formation of volatile tungsten oxides, as a function of oxygen partial pres- sure were investigated. Results showed that the deposit of the tungsten oxides on the cooler collector aect the converter performance, owing to the induced changes in the collector's oxygenated±cesiated work function and the eective em- issivity of the electrodes. Parametric analyses were performed which investigated the eects of introducing tracer amounts of oxygen in the interelectrode gap on the performance parameters of a ®ssion-heated single-cell Thermionic Fuel Element (TFE). These parameters are the electrodes work functions and eective emissivity, the TFE volt±ampere characteristics, the axial distributions of the current density and the electrodes temperatures, and the tungsten loss rate from the emitter surface. Ó 1998 Elsevier Science B.V. All rights reserved. 1. Introduction Thermionic converters (TICs) are being sought for space electric power applications and in topping cycles in terrestrial electric power generation, because they have no moving parts, high reliability and relatively high heat rejection temperature. They operate at typical emit- ter temperatures of 1800±1900 K and collector temper- atures of 800±1000 K. Because the high temperature in TICs is limited to the emitter, commercial steel and su- per steel alloys can be used as structural materials for the cooling loop of the collector. Many investigations have attempted to increase the conversion eciency of TICs, beyond the typical 12±14%, by either increasing the emitter temperature up to 2100 K and/or introducing tracer amounts of oxygen in the cesium vapor ®lled in- terelectrode gap (typically 6 0.5 mm wide). When oper- ating at such high emitter temperature, however, lifetime issues, such as the gradual loss of the emitter material and the eects on TICs performance of the emitter ma- terial deposits on the collector surface should be ad- dressed. The results of laboratory investigations of TICs with isothermal tungsten emitters have demonstrated that the introduction of a tracer amount of oxygen or cesium ox- ides in the interelectrode gap could signi®cantly improve the converter performance [1±4]. The measured im- provement was attributed to two eects: (a) An increase in the emitter oxygenated bare work function [3], increasing the emission current due to the low oxygenated±cesiated work function of the emitter. Such a low collector work function not only decreases the optimum cesium pressure, corresponding to the maximum TIC electric power output, but also permits the use of a larger inter- electrode gap, therefore, increasing the converter reliability. Journal of Nuclear Materials 256 (1998) 218±228 * Corresponding author. Present address: Westinghouse Electric Corporation, Science and Technology Center, 1310 Beulah Road, Pittsburgh, PA 15235-5098, USA. Tel.: +1-412 256 1656; fax: +1-412 256 2444; e-mail: paramodv@westing- house.com. 0022-3115/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 1 1 5 ( 9 8 ) 0 0 0 5 7 - 9