18.06.2015 12:21

Impact of permafrost on final disposal

Posiva has conducted thorough research on the impact of permafrost on the final disposal of spent nuclear fuel in the bedrock conditions of Olkiluoto. The Radiation and Nuclear Safety Authority of Finland (STUK) has assessed Posiva's research and in the safety assessment submitted by STUK in February to the Finnish Ministry of Employment and the Economy for the purposes of the construction licence of the final disposal facility, it concludes that final disposal can be implemented safely in accordance with Posiva's plan.

Posiva has after the submittal of the construction licence application continued modelling analyses related to climate, permafrost and Ice Ages, in terms of both past and future scenarios. The results of the modelling studies, which have been conducted in compliance with Posiva's nuclear management programme YJH-2012, are to be reported in a few years' time and used as part of the Safety Case in the operating licence application.

In addition to the modelling process, Posiva has in collaboration with the Swedish and the Canadian nuclear waste management companies carried out a joint project, the Greenland Analogue Project (GAP). In this project, investigations have been conducted in the Kangerlussuaq area in Western Greenland since 2008 to study the impact of the Ice Sheet and permafrost on crystalline bedrock near the edge of the glacier at a depth of up to 600 metres. The project has served as an analogue study of future Ice Ages in Olkiluoto. The research results from the project will be published during 2015 in the Posiva reports series.

Posiva's studies show that the bedrock is more fractured in its surface part than deeper down. The fracturing of the surface parts is usually associated with the impact of glaciations. The influence of repeated glaciation can be seen, for example, as the occurrence of horizontal fractures near the ground surface. Deeper down there are fewer fractures; age determination also shows that they are very old and there are no indications of the development of new fractures due to glaciation.

The potential extension of permafrost down to the final disposal depth has been taken into account in Posiva's latest safety assessments. According to studies of the strength of the final disposal canister, freezing temperatures have no effect on the mechanical strength of the canister.

Based on the Safety Case (Posiva 2012-041)), permafrost is not expected to impact the performance of the buffer, i.e. the bentonite clay or the backfill either. This is supported by laboratory analyses carried out on the buffer and backfill materials (Schatz & Martikainen 20102)) and (Schatz & Martikainen 20133)), which have shown that even several repeated freeze-thaw cycles do not have an adverse effect on the properties of the materials used in final disposal.

Professor Matti Räsänen et al.4) published in June 2015 an article about the impact of permafrost on final disposal. According to the article, the results of the basic research that has now been conducted will change radically the premises of successful implementation of the final disposal of nuclear waste in Olkiluoto. The input data used by Posiva in its own research as regards the length of the non-glacial period can be made more specific based on the published research article. In addition to permafrost, also several other factors are taken into consideration in the determination of the final disposal depth.

1) Posiva, 2013. Safety Case for the Disposal of Spent Nuclear Fuel at Olkiluoto -Performance Assessment 2012. Eurajoki, Finland: Posiva Oy. POSIVA 2012-04. 520 p. ISBN 978-951-652-185-8

2) Schatz, T. & Martikainen, J., 2010. Laboratory Studies on the Effect of Freezing and Thawing Exposure on Bentonite Buffer Performance: Closed-System Tests. Eurajoki, Finland: Posiva Oy. POSIVA report 2010-6. 58 p. ISBN: 978-951-652-177-3.

3) Schatz, T. & Martikainen, J., 2012.Laboratory tests and analyses on potential backfill materials. Eurajoki, Finland: Posiva Oy. Posiva working report 2012-74. 170 p.

4) Matti E. Räsänen, Janne V. Huitti, Saroj Bhattarai, Jerry Harvey III, Sanna Huttunen (2015): The SE sector of the Middle Weichselian Eurasian Ice Sheet was much smaller than assumed, Quaternary Science Reviews 122, 131-141.

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