Hydrogen-tolerant heat pipes for biomodal systems

Bimodal space nuclear systems which combine both thermal propulsion and electric power generation capabilities in a single integrated power and propulsion platform have recently emerged as strong candidates for future military and commercial space applications. For propulsion, the reactor heats hydrogen which is exhausted through a nozzle to provide propulsive thrust. Thermal energy from the reactor is also converted to electricity by in-core or out-of-core energy converters. Hydrogen permeation must be considered in heat pipes designed for bimodal systems. Potential effects include depriming of the heat pipe arteries, blockage of the heat pipe condenser, hydride formation, and effects on the thermal-to-electric energy converters. Two methods of minimizing hydrogen effects are hydrogen barrier coatings, and a trilayer hydrogen barrier. Aluminum oxide, beryllium oxide and aluminum oxide/molybdenum aluminide were identified as potential hydrogen barrier coatings, however, to date no coating has been demonstrated under bimodal conditions. A 2--3 order of magnitude reduction in hydrogen permeation is predicted when compared to uncoated molybdenum. The trilayer hydrogen barrier contains a porous layer vented to vacuum. The porous layer provides good thermal conductivity between the two containment walls. At the same time, hydrogen permeation is reduced because the large surface area of the porous metal vents any permeating hydrogen. A trilayer hydrogen barrier experimentally demonstrated a 25--170 fold reduction in hydrogen permeation between 873 K and 673 K, respectively. A greater reduction can be achieved by installing capillary tunnels in the porous metal layer