Modular Rtg Technology Status

MOD-RTG represents the design configuration for the next generation of Radioisotope Thermoelectric Generator (RTG), aimed at improving specific power over current General Purpose Heat Source Radioisotope Thermoelectric Generators (GPHS-RTGs). As part of the MOD-RTG development program sponsored by the Office of Special Applications of the U.S. Department of Energy, testing has been performed on multicouple thermoelectric devices. This paper presents a discussion of the performance of these devices including diagnosis and resolution of test anomalies observed. Plans to verify long life multicouple performance and prepare MOD-RTG for flight readiness are also discussed in the paper. Eight multicouples were tested in a series string at Fairchild Industries. Six of the multicouples operated on a positive voltage bias. They were operated for approximately 6000 hours and continued to perform normally when testing was terminated. Good agreement was shown with performance predicted from the data base of standard SiGe thermoelectric material. Two multicouples operated with a negative voltage bias, and they both exhibited anomalous behavior. Performance was erratic and degradation in open circuit voltage and internal resistance were observed. Failure analysis of the two negatively biased multicouples indicated the failure was caused by alkali contamination which entered into the multicouple inter leg glass from external sources within the test module. The alkali ions were drawn into the multicouple by the negative bias and produced degradation of the insulating glass. Additional multicouples are being fabricated with improved cleanliness procedures and high purity, low alkali glass and their long term performance will be verified using newly refurbished test modules. A MOD-RTG Ground Demonstration System (GDS) will be fabricated and tested following verification of multicouple performance.