Test elements used in trial run of Finnish repository

Tuesday, 8 October 2024

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 announced in late August the start of a trial run - expected to take several months - of the operation of the final disposal facility, albeit still without the used fuel. Four cannisters are to be deposited in holes which are eight metres deep and located in a 70-metres long final disposal tunnel. The final disposal tunnel will then be filled with bentonite clay and sealed with a concrete plug. The trial run also covers the retrieval of a damaged cannister back above ground. Posiva noted that the equipment and systems of the final disposal facility will be tested together for the first time in accordance with planned processes during the trial run stage.

The first two encapsulations were carried out using only weights. In this third trial, elements looking like actual fuel were packed inside the canister.

The used nuclear fuel is placed in the final disposal canister made of copper and spheroidal graphite cast iron. This takes place in the fuel handling cell which features concrete walls that are about 1.3 metres thick. After all the fuel assemblies have been placed in the canister, it is filled with Argon and closed tightly with the inner steel lid. The fuel assemblies are not disassembled. The weight of one fuel assembly is about a quarter of a tonne.

The topmost copper lid of the canister is sealed by friction stir welding in the welding chamber and the leak-tightness of the joint is verified by means of visual as well as eddy current and ultrasound inspections. Posiva said the sealing joint produced by the selected friction stir welding method has an integrity comparable to that of the canister mantle.

The first three canisters have also undergone the welding and machining processes successfully in the trial run.

The trial run now proceeds to the packing and sealing of the fourth canister.

The encapsulation plant is connected to the underground final disposal repository with a canister lift which transports the canisters down to the underground reception station on the final disposal level at a depth of 430 metres. There they are transferred into the deposition tunnels with the transfer and installation vehicle.

Posiva has applied for an operating licence for the repository for a period from March 2024 to the end of 2070. The government will make the final decision on its application, but a positive opinion by Finland's Radiation and Nuclear Safety Authority (STUK) is required beforehand. The regulator began its review in May 2022 after concluding Posiva had provided sufficient material. The ministry had requested STUK's opinion on the application by the end of 2023. However, in January this year, STUK requested the deadline for its opinion on Posiva's operating licence application be extended until the end of 2024.