Over the past 10 years at the Hanford Site, Fluor has successfully completed several radioactive materials packaging campaigns, all of which have included container closure welding. These campaigns utilized current, fusion welding processes and were performed on a semi-remote basis; that is, the Welding Operator had access to the weld joint to make repairs and equipment adjustments as needed, but performed the welding via cameras and remote video. Upcoming packaging activities will be performed under fully-remote operations (no human access to the weld joint), making weld repair difficult. Fluor believes a more robust joining process, one that can make high-quality, defect free-welds on a consistent basis, will be needed to successfully complete this work. In addition, improved long-term degradation properties, associated with container welds, may be required.
Current United States (US) Used Nuclear Fuel (UNF) containers are made of austenitic stainless steel (S/S) and fabricated using fusion welding processes. Fusion welds in this material can be sensitive to environmental degradation, and in particular, Stress Corrosion Cracking (SCC), depending on conditions and length of service. With uncertainty surrounding the status of a US national repository and its impact on the disposal of UNF (significant delay), the nuclear industry is preparing for extended, on-site storage. Because of sensitivity to SCC and the need to consider extending container storage terms, there is concern regarding the performance of UNF container welds.
In an effort to address these two issues, container weld quality (process robustness) and long-term corrosion performance, Fluor, along with the Pacific Northwest and Savannah River National Laboratories, are evaluating the use of Friction Stir Welding technology for the fabrication of UNF containers.