The first copper casting blank has been produced for fabrication of a final disposal canister to be used in the Onkalo used nuclear fuel repository, according to Finnish radioactive waste management company Posiva. The casting was reviewed by quality control engineer Teppo Uusi-Uola, canister system owner Terhi Glas and Posiva canister manufacture coordinator Jouko Lammi. The process was also overseen by Finnish Radiation & Nuclear Safety Authority (STUK – Säteilyturvakeskus).
The casting campaign for the first six blanks began at Luvata’s plant in Pori. Luvata is a subsidiary of Japan’s Mitsubishi Materials. The casting of the initial blank was completed in a single shift. A preliminary analysis sample was taken of the casting before it was measured and weighed. The cast blank will now be transferred to the sawing process where its front and rear end are sawn to measure, and official sample slices are cut for a material analysis.
Next, the casting blank will have its surfaces machined to size before undergoing non-destructive testing (NDT), a visual inspection and a dye penetrant inspection. The final step will be Posiva’s inspection of the casting and the associated documentation. After passing inspection, the casting will be packed ready for transport for delivery to the cylinder manufacturer. The original weight of the casting blank is over 17 tonnes of copper and the final delivery weight for cylinder manufacture is about 12 tonnes.
“The casting process was a great success,” said Jouko Lammi, canister manufacture coordinator at Posiva. “A good casting quality is important for the subsequent canister production phases. New investments have been made in the foundry to improve the quality and the processing of the casting. One of the most important accomplishments has been the increase in the diameter of the casting with a larger gravity die.”
The copper cylinder produced from the casting blank, together with the copper bottom and cover that are welded onto it, forms a leak-tight copper canister with five-centimetre-thick walls. In final disposal, the canister acts as a corrosion barrier for the canister insert made of cast iron. The insert can hold 12 fuel elements.
At the repository, used fuel will be placed in the bedrock, at a depth of about 430 metres. The disposal system consists of a tightly sealed iron-copper canister, a bentonite buffer enclosing the canister, a tunnel backfilling material made of swellable clay, the seal structures of the tunnels and premises, and the enclosing rock.
Posiva, jointly owned by Finnish nuclear utilities Fortum and TVO, applied for a construction licence for a repository to the Ministry of Employment and the Economy in 2013 and the government granted the licence in November 2015. The site for a repository at Eurajoki, near the Olkiluoto NPP, was selected in 2000 and parliament approved the decision-in-principle on the repository project in 2021.
Once it receives the operating licence, Posiva can start the final disposal of the used fuel generated from the operation of TVO’s Olkiluoto and Fortum’s Loviisa NPPs. The facility will operate for about 100 years before it is closed.
