Microreactors, also known as special purpose reactors (SPRs), have been developed worldwide for use in unique applications such as electricity production in remote areas, military installations, and various industrial needs. Among all the microreactor designs, the liquid metal filled heat pipes are being considered as one of the promising concepts for the reactor core cooling system of SPRs. The heat pipe in this design concept offers several advantages, such as very high conductivity, the high operating temperature under isothermal condition, and entirely passive heat removal feature. However, the application of heat pipe technology into the nuclear system is new and requires the development and validation of heat pipe codes to perform the safety analysis for the reactor design and licensing. With the purpose to generate high-fidelity
experimental data in the ECMF lab at the University of Michigan, we have designed and constructed a state-of-the-art experimental facility with a single sodium-filled heat pipe (MISOH1) made of a 1,016 mm long and 25.4 mm outer diameter stainless steel tube. In this paper, the heat pipe start-up process and performance under various operation conditions, including heat transfer rate, cooling condition of condenser section, and heat pipe orientation were investigated. The test section was composed of low Z materials with a purpose to be installed inside Michigan High-Resolution Tomographic Imaging (CHROMA) system. With the X-ray radiography, visualization of the sodium two-phase flow was attempted to understand the start-up and ideal heat transfer cycle of the sodium heat pipe.