Spread of Plutonium in GPHS Modules: Evaluation of the Qualification RTG Graphites

Abstract

As the General Purpose Heat Source (GPHS) clads are vented to permit exiting of helium resulting from the alpha decay of the plutonium‐238 radioisotope fuel, questions always arise as to how well the radioactivity is retained by the iridium vent and subsequent double layer of porous carbon composites. The ability to immobilize the plutonia can affect facility use, facility safety systems, launch safety analyses results, and the ability to acquire shipping license for a fueled radioisotope thermoelectric generator (RTG). The 18 GPHS modules used to power the Qualification (Qual) RTG had experienced almost 85,000 hours (over 300 Ms) of ground storage, launch, and space vacuum temperatures and qual‐level mechanical environments. Upon disassembly of the RTG and removal of the aeroshells from the graphite impact shells (GISs) which contain two fueled clads each, there was no loose contamination which could be attributed to the contained fuel. Direct counting of the aeroshells using the Mound precision counting facility verified that the quantity of plutonium‐238 that migrated beyond GISs was very small; there were no detectable fuel nuclides nor daughter products. This study demonstrates that the GPHS system of vented fuel clad/GIS/aeroshell is an effective double containment scheme for plutonium‐238 even after years of high‐temperature operation.