Finland starts building plug for repository tunnel

Posted by World Nuclear News on 1 April 2015 in WNN

Posiva, the Finnish waste management company, said today it has started construction of a heavy plug for the demonstration repository tunnel at its final repository and waste encapsulation plant at Olkiluoto. The underground characterisation facility is known as Onkalo.

The plug is required because a deposition tunnel must remain water-tight. Once completed, the plug will undergo pressure testing.

Work is under way at Olkiluoto, Posiva said, to construct “a massive reinforced concrete plug, the likes of which will be placed at the mouths of all the deposition tunnels. At this point, however, the technology is just being tested. The purpose of the testing is to ensure that everything in the deposition tunnel works as intended when final disposal begins.” Posiva is jointly owned by Finnish nuclear utilities Fortum and TVO. That means the bentonite clay, with which the tunnels will eventually be backfilled, remains in the tunnel as it becomes wet. It also means that the tunnel remains water- tight, it added.

Construction of the tunnel plug began in February and will take about six months. After its final completion in December, the plug will be tested.

“The pressure test allows us to establish whether water is able to flow through the connecting surface between the plug and the bedrock. The test pressure will be identical to ground water pressure at the test depth of 420 metres,” Posiva development engineer Petri Koho said.
The full-scale test plug will be constructed in the Onkalo demonstration tunnel. Some 6 m of the 7 m long, 6 m wide, plug lies within the tunnel.

Following pressurisation of the space behind the wedge-shaped plug, its ‘behaviour’ will be monitored using sensors placed in an adjacent tunnel. The test results will be compared with the results of a plug test jointly carried out by Sweden’s SKB and Posiva in the Äspö bedrock laboratory in Sweden.

“It is possible that the next development version of the plug will be similar to the one currently under construction in Onkalo. It is also possible that it will be similar to SKB’s plug, or a combination of the two,” said Koho.

In the course of the project, lasting until autumn 2016, Posiva will verify that the plug meets established requirements and that the selected plug design can be implemented on an industrial scale.

The deposition tunnel plug test is known as Poplu and is part of DOPAS, a joint European project to carry out full-scale testing of final repository sealing technology. That project – which is jointly funded by the European Atomic Energy Community (Euratom), companies with nuclear waste management responsibilities and various research organisations – focuses on the development of plugs and sealing structures for final repositories for used nuclear fuel.

Switzerland has a similar facility. The country’s national radioactive waste disposal cooperative Nagra announced earlier this month that an underground tunnel, in which the Full-scale Emplacement Experiment will be carried out at the Mont Terri Rock Laboratory, has been sealed and monitoring has begun. The experiment aims to simulate the conditions within a repository containing high-level radioactive waste.