Finnish waste management company Posiva has contracted Bond Technologies of the USA to manufacture, install and commission a friction stir welding system for sealing copper disposal canister lids at its used fuel encapsulation plant under construction at Olkiluoto.
Posiva has developed a friction stir welding system in collaboration with Sweden’s Svensk Kärnbränslehantering (SKB), which will also use the system for the planned Swedish repository, based on the same technology as the Finnish one.
The contract includes the manufacture, installation and commissioning of the friction stir welding equipment. The system is an important and critical part of the production process of the encapsulation plant. Posiva and Bond Technologies designed and developed the welding device, which is now under manufacture, during an earlier design stage of the project.
Used nuclear fuel is packed in copper and cast-iron canisters before its deposition in bedrock. The metal canister is the most important engineered barrier of the disposal concept. The canister’s copper shell consists of a copper tube with an integrated or welded bottom, depending on the manufacturing method, and a copper lid that is welded to the canister at the encapsulation plant. The properties of the weld must not significantly deviate from the properties of the base material.
Friction stir welding is based on hot forming, where the required temperature is caused by friction of a tool rotating with high speed in the material to be welded. Friction stir welding meets the requirements of long-term safety very well, since its benefits include small grain size in the weld area, even and isotropic grain structure as well as good mechanical properties and microstructure that are suited to the disposal canister.
Photo Credit: Posiva