Design of the Hanford Multi-Canister Overpack (MCO) and Development and Qualification of the Closure Welding Process

Processing more than 2,100 metric tons of metallic uranium spent nuclear fuel (SNF) into large stainless steel containers called Multi-Canister Overpacks (MCOs) is one of the top priorities for the Department of Energy (DOE) at the Hanford Site, located in southeastern Washington state. The MCOs will be temporarily stored on site and eventually shipped to the federal geologic repository for long-term storage. MCOs are constructed and “N” stamped in accordance with the requirements of the American Society of Mechanical Engineers (ASME) Section III, Division 1, Class 1 Components. Final closure welding poses a challenge after the fuel is loaded. Performing required examination and testing activities (volumetric examination and hydrostatic leak testing) can be difficult, if not impractical. An ASME Code Case N-595-3, was written specifically to allow code stamping by addressing such closures and providing alternative rules. MCOs are the first SNF canisters within the DOE complex to successfully use this Code Case for receiving ASME Code stamps. This paper discusses the design of the MCO, application of the N-595-3 Code Case, and development and qualification of the final welded closure. The MCO design considers internal pressure and handling loads, as well as processing and interim storage activities. The MCO functions as the primary or innermost containment as part of an overall transportation package so the design also considered interface features with secondary and transport containers. The MCO, approximately 2 feet in diameter and nearly 14 feet tall, is constructed primarily of Type 304/304L stainless steel and the final pressure boundary is of all-welded construction. The closure-weld is made with the Gas Tungsten Arc Welding (GTAW) process, using an automatic, machine-welding mode. Examination and testing of the closure includes the N-595-3 specified requirements—progressive Liquid Penetrant testing (PT) and final helium leak testing. At completion of the closure, the MCO is “N” stamped as a 450 pounds per square inch (design pressure) vessel. To ensure the process consistently achieves the required weld penetration, a series of developmental tests was performed to identify an optimum and robust set of welding parameters. Testing included test welds made on plate mockups and then actual MCO mockups. With the primary welding parameters (welding current and travel speed) established, a simple two-factor, two-level, factorial experiment with replication at high and low heat input conditions was conducted. Evaluation of the results included weld photomicrographs, which helped establish process range limits for these parameters broad enough to cover typical equipment and measurement variations and provide additional operating margin. To date, over 316 MCOs have been loaded, dried, and transported to the Canister Storage Building (CSB), where the welding is done. Of those, 161 MCOs have received final welded closure and ASME Code “N” stamps. All cover cap final closure welds have met specified requirements without incident.