Heat rejection systems are under study by NASA Glenn Research Center for various nuclear electric power and surface applications. Heat rejection systems are designed for long term use and need to be durable to space environments. One threat to such systems is that of micrometeroid impact on tubing with a subsequent loss of coolant. To combat this problem, system design is driven towards the use of individual heat pipes. A heat pipe in its simplest form is a passive two-phase heat transfer device in a sealed tube. The evaporator region of the heat pipe is in thermal contact with a heat source. The envelope of the heat pipe is typically made of a high conductivity material and transports the heat to the liquid inside. The only fluid inside the heat pipe is the working fluid in thermodynamic equilibrium with its own vapor. Liquid in the evaporator region evaporates to vapor and is transported to the other end of the tube where the vapor condenses to release the latent heat to the tube wall. The condensation process can be augmented by various external means such as provision of fins. The condensed fluid is transported passively back to the evaporator region by the capillary action of a wick that lines the inner wall of the tube.
Heat rejection systems for power conversion systems utilizing a fission heat source need to reject heat at high temperatures, approximately 500 K. One heat pipe system that showed promising results is the titanium-water heat pipe system. Titanium was chosen as the envelope material for its potential long-term compatibility with water, high strength, low density, and ability to be easily machined into needed heat pipe cross sectional geometry.
In Spring 2006, NASA Glenn Research Center requisitioned nine titanium water heat pipes from three vendors. Through an in-house effort, a heat pipe test bed was designed and built and the heat pipes were performance tested. The heat pipes are currently undergoing life tests for five or more years to evaluate long term chemical compatibility between the titanium envelope, titanium wick material and water. Life testing is conducted at 500 K. Operation at the lower temperature of 360 K is done periodically to check for the presence of non-condensable gases that may have been generated due to reactions between heat pipe materials or possible contaminants.