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). Internal heat generation of the in-core section of the heat pipes and sheath insulator specimens provides the heat input to the evaporator portion of the heat pipes, while heat is rejected from the heat pipes by radiation and by conduction via a copper fin placed on the condenser section of each heat pipe. The position of the copper fins and approximate neon filling conditions must be determined for each pipe such that the desired operating temperature can be achieved. The thermal models were used to evaluate the effects of variations of internal nuclear deposition heating, emissivity, and reactor coolant temperature on heat pipe operation. The models verify that the fins on the heat pipes will compensate for the parameter variations of the reactor core. This paper contains a description of the test section of the IFAC-SI experiment which includes the sheath insulator samples, heat pipes, fins, and enclosing container. The thermal models developed are described and their results and effects on the experiment design are presented.