"Finnish waste management company Posiva has completed the procedure qualification test on backfill installation at the Onkalo used nuclear fuel repository. During the test, almost 12 metres of tunnel were filled with backfill clay using the backfill installation system.

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.

The procedure qualification test on backfill installation was designed to demonstrate the operability of the systems and equipment in tunnel conditions at a depth of 435 metres in the bedrock. Another requirement was to achieve the specified result by deploying the operating methods and procedures that will be deployed during operation of the repository.

The minimum goal for the test, which took about three weeks, was to fill at least 10 metres of tunnel. The actual test acceptance criterion was to achieve a tunnel backfill degree of a minimum of 98%. Posiva noted that analyses that are still to be verified indicate that the achieved backfill degree exceeded this criterion.

"The large, long combination machine almost fills the entire tunnel and compacts clay with large screws while pushing it into the tunnel," said Posiva Development Manager Ari Maarni. "At the same time, it reverses slowly in order to achieve a uniform backfill degree. During the test, the transfer vehicle ran autonomously between the discharge station and the installation machine supplying more clay to the installation machine."

Maarni noted that the biggest challenges during the test were caused by the uneven floor of the tunnel. "The docking of the transfer vehicle to the installation machine was difficult in points with large surface height differences. This slowed down the work. However, as the test proceeded a lot was learned and we were able to adjust the equipment to operate faster."

The qualification test was carried out in the 50-metre-long tunnel used for the Trial Run of Final Disposal. The remaining 40 metres of the tunnel will be filled in the actual Trial Run of Underground Operation, before the tunnel is finally sealed.

The next installation procedure qualification tests will focus on testing the buffer installation system as well as the canister transfer and installation vehicle. These installation procedure qualification tests will be carried out as the last ones, and in practice parallel with each other, before the actual underground stage of the Trial Run of Final Disposal.

During the Trial Run, the operation of the complete final disposal facility is tested. Instead of actual used nuclear fuel, the trial run is conducted using non-radioactive test elements.

The Finnish government granted Posiva a construction licence for the project in November 2015 and construction work on the repository started in December 2016. 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 nuclear power plants. The operation will last for about 100 years before the repository is closed."

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https://www.world-nuclear-news.org/articles/posiva-completes-backfilling-trial-run-at-repository