Filling and sealing sodium heat pipes

This paper describes facilities for filling and sealing high purity sodium heat pipes. The purpose of the work is to define a process for filling heat pipes with sodium that contains less than 10 ppm of oxygen. Small, cylindrical heat pipes were used to define the process. The work was performed for the NASA Lewis Research Stirling Space Power Converter program and was conducted under NASA’s Civil Space Technology Initiative (CSTI). Once the low‐oxygen filling process is defined, a large, complex heat pipe for a Stirling Space Power Converter (SSPC) will be loaded with sodium. The heat pipes were processed at a fill station on a High Temperature Sodium Loop circulating 180 kg (400 lb) of liquid sodium. The oxygen content of the sodium in the loop was below the detection limit of the analytical technique (the detection limit is about 6 ppm). The heat pipes were baked out and subjected to multiple flushes and extended soaks with liquid sodium before the final filling operation. A final purity of 8 ppm of oxygen was achieved. A process for performing the final closure of the fill tube was developed which maintained vacuum tight conditions in the heat pipe.