01.04.2015 09:01

Test plug will be subjected to high pressure

A deposition tunnel must remain water-tight under all circumstances. This year will see the construction of a heavy plug in ONKALO’s demonstration tunnel. Once completed, the plug will undergo pressure testing.

In the ONKALO demonstration tunnel has been excavated the plug-shaped slot. On the right is picture of the plug when it's ready.

In Olkiluoto, work is under way 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.

In this case, this means that the bentonite clay, with which the tunnels will eventually be backfilled, remains in the tunnel as it becomes wet. This also means that the tunnel remains water-tight.

The construction of the tunnel plug began in February. The actual construction work will take approximately six months. After its final completion in December, the plug will be subjected to testing.

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Petri Koho, Posiva's development engineer
“The space behind the plug will be pressurised in order to determine its water-tightness. 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,” says development engineer Petri Koho from Posiva.

The full-scale test plug will be constructed in the ONKALO demonstration tunnel. The length of the plug in the direction of the tunnel is six metres. It is six metres wide and nearly seven metres high.

Finnish-Swedish cooperation

Following the pressurisation of the space behind the wedge-shaped plug, its behaviour is monitored with the help of numerous measuring sensors placed in an adjacent tunnel. The testing results will be utilised for the further development of the deposition tunnel plug. The 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 some sort of these two,” says Koho.

The deposition tunnel plug test is known as POPLU. In the course of the project, lasting until autumn 2016, Posiva will verify that the plug meets the set requirements and that the selected plug design can be implemented in industrial scale.

POPLU is a part of DOPAS, a joint European project, the aim of which is to carry out full-scale testing of final repository sealing technology. The particular focus of the DOPAS project, jointly funded by the European Atomic Energy Community (Euratom), companies with nuclear waste management responsibilities and various research organisations, is on the development of plugs and sealing structures for final repositories for spent nuclear fuel.


Posiva ensures the functionality of its disposal concept

In 2015, Posiva will test prototypes of equipment required in final disposal operations. Following initial equipment tests carried out above ground, testing will continue in ONKALO. The first piece of equipment to be tested is the bentonite buffer installation vehicle, followed by the disposal canister transfer and installation vehicle and finally, the deposition tunnel backfill material installation vehicle.

“The purpose of the equipment tests is to support the verification of the feasibility of our disposal concept. At the same time, we keep developing manufacturing capabilities for bentonite buffer blocks and tunnel backfill material,” says Tiina Jalonen, Posiva’s Senior Vice President, Development.

At Posiva, work is continuously underway to prepare for the construction of the final disposal repository and the encapsulation plant. Posiva specifies the matters which must be completed by the commencement of the construction operations.

Among the tasks scheduled for this year is the production of a description of the plans for the work to be carried out in the coming years. A detailed description will be provided for the 2016–2018 period. The description for the following three years will be more general in nature.

“The description will be prepared in accordance with the statutory three-year plan. We will submit it to the Ministry of Employment and the Economy in September,” says Jalonen.

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