High-temperature performance of an integral cesium reservoir in a thermionic converter

Abstract

IntroductionT HE inclusion of cesium reservoirs in thermionic converters has played a vital role in the production of converters that achieve optimum performance. 1 Cesium vapor in the interelectrode space of a thermionic converter reduces the negative space charge effect on the emitter surface and the work functions of the electrodes due to the adsorption of cesium atoms on their surfaces. 2 The dependence of the cesium vapor pressure p cs on the liquid reservoir temperature T, is given empirically as 1 p cs = [(2.45 x 10«)/vTjexp(-8910/r r ), Torr (f or T r in Kelvin. Because of the exponential dependence of the cesium pressure on the liquid reservoir temperature, a high degree of temperature control is necessary for the reservoir to maintain a constant cesium pressure. As the liquid reservoir temperature typically ranges from 525 to 625 K (-0.5 to 6 Torr), and the minimum temperature in the core of the thermionic fuel element (TFE) of the advanced thermionic initiative (ATI) reactor design is approximately 880 K, the liquid reservoir would have to be located external to the core, complicating the overall design of the power system with intricate plumbing and valve arrangements. 3 Due to the problems cited above, integral, solid sorption reservoirs have been evaluated as an alternative to liquid reservoirs for use in the ATI-TFE design. The potential advantages of such a reservoir are as follows:1) The reservoir could be designed to operate at a temperature intermediate to the emitter and collector temperatures, thus eliminating the electronic hardware associated with liquid reservoir heaters and temperature controllers.2) Plumbing and other problems associated with the use of liquid cesium could be eliminated.3) Optimum cesium pressure could be maintained over a wide range of emitter temperatures due to direct temperature feedback from the converter. Harbaugh and Basiulis 4 investigated the storage characteristics of molybdenum and tungsten reservoirs and tested the performance of these reservoirs in actual converters. They demonstrated optimum converter performance with the reservoirs above 973 K, and also found that the cesium pressure in an adsorption reservoir is less sensitive to changes in reservoir temperature than the cesium pressure in a liquid reservoir. The metal reservoirs were ultimately found to be impractical because of their limited storage capabilities and the variation of vapor pressure, at constant temperature, due to loss of cesium.Other investigators identified graphite as a potential sorption medium. The intercalation of cesium in graphite forms compounds ranging from C (S Cs to C^Cs. Salzano and Aronson 5 characterized these compounds and measured the effect of the graphite reservoir temperature on the cesium vapor pressure. The compounds were found to supply cesium vapor at pressures of 0.5-10 Torr via two-phase equilibrium reactions at temperatures ranging from 700 to 1200 K. The cesium pressure was determined to depend only on the graphite temperature and the reservoir compound. Yates...