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The Instrumented Fast-Reactor Accelerated Component-Sheath Insulator test (IFAC-SI) is a key experiment of the Thermionic Fuel Element (TFE) Verification Program designed to allow continuous monitoring of sheath insulator specimens with an applied voltage during the in-reactor test. The insulators are cermets, an alumina matrix with a dispersion of niobium particles, sandwiched between two cylinders of niobium. The IFAC-SI experiment is designed to monitor the applied voltage, resistance, and temperature of two sets of three specimens while maintaining the sets at two desired operating temperatures. The temperature control is achieved using two separate vapor controlled heat pipes contained in the vacuum space of a stainless steel capsule. The capsule will be tested in a modified In-Core Instrument Test facility (INCOT) wet thimble design at the Experimental Breeder Reactor II (EBR II).

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.