Compatibility of Materials with Alkali Metals for Space Nuclear Power Systems

The alkali metals lithium, potassium, and cesium are of principal interest as heat transfer and working fluids in high-performance space-nuclear reactors employing single- and multiloop Rankine cycle conversion. The compatibility of these alkali metals with structural alloys has been investigated for more than a decade in both laboratory- and engineering-scale tests. It is demonstrated that reliable engineering systems involving potassium and cesium can be constructed and operated at 2000°F with first-generation niobium-base alloys like Nb-1% Zr and at 2200°F with advanced niobium-base alloys. Niobium-base alloys can be used at higher temperatures in lithium systems. Tantalum-base alloys appear to be satisfactory for potassium or cesium to at least 2200°F and probably a few hundred degrees higher with lithium. Very advanced systems designed for temperatures of 2500°F and higher probably will require a new group of alloys; tungsten-base alloys appear to be strong candidates for these applications.