Posiva chose friction stir welding for the encapsulation of spent nuclear fuel
Copper - cast iron canister is the most important engineered barrier in the KBS-3 disposal concept for spent nuclear fuel adopted in Finland and Sweden. The copper canister consists of a copper tube with an integrated or a welded bottom and of a lid which is welded in the encapsulation plant after the spent fuel has been assembled into the canister. The properties of the weld shall not deviate from the properties of the copper components.
Posiva and the Swedish nuclear waste management company Svensk Kärnbränslehantering AB (SKB) have over the years developed two welding methods: electron beam welding (EBW) and friction stir welding (FSW). In 2005, SKB selected the FSW for the application for the construction license of the encapsulation plant in Oskarshamn. Last year, Posiva made an evaluation of the two methods and decided as well to select the FSW as the welding method for the encapsulation plant, which will be built in Olkiluoto.
Friction stir welding method 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. The method results in better long-term properties of the weld than the other method, EBW, which as well has proven to produce acceptable welds.
The advantages of the FSW are the weld with smaller and more even grain size, isotropic grain structure and good mechanical properties. In other words, the properties of FSW weld are closer to the mechanical properties and grain structure of the copper components. In very aggressive conditions, the FSW weld has proven to have better properties compared to EBW. The same applies for stress corrosion and creep properties of the welds, where better results have been gained with FSW.
Now, since Posiva has settled on the same welding method with SKB, the companies have signed a specific technology agreement on friction strir welding to enhance the cooperation that is already on-going. The companies combine their forces in a joint programme to develop the FSW equipment and method further, until they are qualified. During the programme, Posiva plans to procure and qualify FSW equipment for its use and SKB will gather valuable experience in licensing the welding equipment and method.