The alkali metals are excellent high-temperature coolants because of their relatively low vapor pressures and very high heat-transfer coefficients. These properties of the alkali metals suggest their use in nuclear reactors where very high heat fluxes are generated by the nuclear fuel. Recently, alkali metals have been proposed as coolants and working fluids in high-temperature, compact, space power plants utilizing the Rankine cycle. In the temperature range of interest (900 to 1200°C), only the refractory metals and their alloys have adequate strength and can be considered as suitable container materials. There has been particular interest in the Nb-1%Zr alloy. The alkali metals Li, Na, K, Rb, and Cs were tested in Nb-1%Zr capsules at 1150°C for 6000 h in a vacuum chamber at a pressure of <5 × 10−8 torr. It was clearly demonstrated that by maintaining low oxygen levels in the alkali-metal Nb-l%Zr systems, there was no corrosion detected in any system. However, when Na was tested in Nb-1%Zr containing in excess of 1500 ppm O at 1094° C for 10 000 h, significant corrosion was detected.