The leaktightness of the tunnel is ensured with a heavy plug
Massive reinforced concrete plugs are installed at the mouths of the deposition tunnels. The upcoming plugging test will serve to produce data on the functioning of the plugs – each of which is approximately six metres long and over 5.5 metres in diameter – in conditions similar to those occurring in the deposition tunnels.
What is the purpose of the plugs installed at the mouths of the deposition tunnels, Posiva's R&D coordinator Johanna Hansen?
“The plugs ensure the leaktightness of the deposition tunnels and hold the tunnel backfilling material in place. Furthermore, they prevent the ingress of water into the central tunnels during the operational phase of the repository. The plugs are cast in reinforced concrete using low-pH concrete specially developed for the final disposal conditions.”
Have such plugs been cast before in Finland?
“No full-sized reinforced concrete plugs have been cast or tested before in Finland. However, according to current plan, one will be cast in the Olkiluoto underground research facility ONKALO in spring 2014. The test plug will have two parts: the actual cast concrete structure and an insulation part placed behind the concrete part. This will enable us to test and measure the functioning of the plug in conditions similar to those occurring in the final disposal environment.”
What types of tests will the test plug be subjected to after it has been cast?
“Utilising an adjacent tunnel, the pressure inside the tunnel sealed by the plug will be increased until it corresponds to the conditions occurring inside the deposition tunnels. The plug will be subjected to pressure caused by groundwater and the expanding tunnel backfill material. Sensors transmitting data on the temperature, humidity, pressure and stress conditions, as well as changes in the concrete structure of the plug, will be placed inside the test plug. The plug monitoring measurements will be started immediately after the casting phase, and they will be continued until 2015.”
Have similar plugs been constructed before, and if, where?
“The company responsible for final disposal in Sweden, SKB, has constructed four full-sized tunnel plugs in its Äspö rock laboratory. The Canadians have also constructed plugs deep under ground. Furthermore, concrete plugs have been constructed under challenging conditions at various underwater sites all over the world.
The design of the plug cast in Finland will differ from the Swedish concept. This will enable us to gain experience on the suitability of different plugs for different conditions and to exchange data with the Swedes, complementing each other's research.”
What type of international cooperation has Posiva participated in in plug development?
“In Finland, the deposition tunnel plugging test is known as POPLU. It is a part of a joint European development project, the aim of which is to carry out full-scale testing of final repository sealing technology. The international DOPAS project includes the performance of complete or partial full-scale plugging tests in Finland, Sweden, France, the Czech Republic and Germany.
The particular focus of the DOPAS project, jointly funded by the European Atomic Energy Community (Euratom) and the members of the European Technology Platform for Implementing Geological Disposal of Radioactive Waste, is on the development of plugs and sealing structures for final repositories for spent nuclear fuel. The project has a budget of EUR 15.7 million and is participated in by 14 nuclear waste management companies and research institutes from eight European countries. The other Finnish project partners in addition to Posiva are VTT and B+Tech Oy. Posiva acts as the coordinator of this EU project.”