Nuclear reactor thermionic space power systems incorporating thermionic fuel element generally use refractory metal emitters, which contain the nuclear fuel. The purpose of the current work is to determine the effect, If any, of the diffusion of uranium oxide fuel through chemically vapor deposited (CVD) tungsten on converter performance. The test vehicle Is a modified, planar variable-spaced research converter. The emitter, with an effective area of 1 cm2, contains four depleted uranium oxide fuel pellets, 4 mm in diameter. The converter equipped with a cesium graphite reservoir, was charged with cesium during the preliminary testing. This paper describes the preliminary testing of the converter to assess the converter performance before any significant diffusion takes place.The testing was carried out with the usual protocol. The emitter temperature was 1800 K and the collector temperature was varied from 1000 K to 1070 K. The range of interelectrode spacing was from .25 to .5 mm. The cesium pressure was varied from 1 to 6 torr in a geometric progression. The graphite reservoir was charged with cesium so that the resulting compound yielded 4 torr cesium pressure at about 973 K graphite temperature. Experiments also examined pressure versus loading characteristics of the graphite. At the conclusion of this preliminary testing the converter was removed from the test stand in preparation for mounting in a life test stand.