Finnish radioactive waste management company Posiva has received permission from the Radiation & Nuclear Safety Authority (STUK – Säteilyturvakeskus) to produce backfill material, which is bentonite clay of the right composition, for the final disposal tunnels for used nuclear fuel.
In March Posiva completed the procedure qualification test on the backfill installation at the Onkalo used nuclear fuel repository in Olkiluoto. During the test, almost 12 metres of tunnel were filled with backfill clay using the GBIS backfill installation system. The minimum goal for the test that took about three weeks was to fill a length of at least 10 metres of the tunnel.
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.
In fact, the production plant for the tunnel backfill material has been ready for operation for some years. At the recent start-up meeting for the production plant, the input data for the production phase of the backfill material for the final disposal facility were reviewed together with the producer (Lux Mineral Services, LMS), in order to establish the production capability of the plant.
The start-up meeting was followed by an inspection of start-up readiness STUK. Based on the inspection, STUK concluded that Posiva can start full production of the tunnel backfill for the first disposal tunnel.
“This is a significant decision and an important milestone for Posiva, as this is the first of the several engineering barriers designed for the isolation of the final disposal facility that can now be moved to the production stage,” said Posiva’s Mika Niskanen who is the system owner of the buffer, the backfill, and the tunnel sealing.
Posiva’s clay plant is located in an industrial estate in western Finland. At present, the clay factory, or the production plant of granular backfill, has a stock of some 4,000 tonnes of backfill raw material waiting to be processed. The production process will be started shortly and is estimated to continue until the end of the year.
The next major points with the tunnel backfill material include the construction inspection of the finished material after production and, as the Trial Run of Final Disposal proceeds underground, the test installation of the bentonite batch that has already been produced and undergone the construction inspection.
The purpose of the Trial Run of Final Disposal in Olkiluoto is to test the complete final disposal process starting with the encapsulation of the spent nuclear fuel through to the emplacing of the canister in the deposition hole and up to the backfilling and closing of the tunnel. However, short trial tunnel comprises only four holes and instead of real used nuclear fuel, dummy elements will be packed in the canisters.
