Radioisotope Thermoelectric Generators (RTGs) are highly reliable solid state devices that produce electricity through the flow of heat generated by the decay of a radioisotope, primarily plutonium-238, through a device called an unicouple. RTGs have been used for over thirty years to provide electrical energy to satellites and deep space probes. The three most recent deep space missions are being powered by the General Purpose Heat Source (GPHS) RTG, which utilizes 572 unicouples to produce 300 watts of electricity, nominal. Each of these unicouples has a C-seal, which is designed to prevent air from entering the interior of the RTG. This is necessary because oxygen degrades the performance of the unicouples and the multi-foil insulation embrittles at the continuous high temperatures in the interior of the RTG. Despite maintaining a positive pressure of argon in the RTG, oxygen will slowly back diffuse past the C-seals and gradually degrade the interior components. To further minimize oxygen from entering the RTG, the RTG was placed within a chamber and the air in the chamber was replaced with inert gas during anticipated long periods of non-activity.
Four GPHS RTGs were assembled and tested during the mid 1980’s to support the Galileo and Ulysses missions. The Galileo mission required two RTGs and the Ulysses mission required only one RTG. The fourth RTG served as a spare RTG for both missions. More recently, three new RTGs were fabricated for the Cassini Mission with the previous spare also serving as the spare RTG for this mission. Following the Cassini launch in October 1997, the spare RTG was returned the Mound Facility and is being maintained for use on possible future missions.
This paper presents the two different Long Term Processing methods, which have been employed on the spare RTG since it was assembled and the data it produced during Long Term Processing. Test results show that Long Term Processing in an inert atmosphere was and still is a viable method of processing RTGs as this unit is still capable of providing reliable electrical energy.