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03.07.2009 14:53

The consideration of permafrost in final disposal

According to Professor Matti Saarnisto, Posiva’s estimates on future periods of permafrost are in conflict with empirical observations and estimates from other studies.

Saarnisto is referring to Posiva’s report on the expected evolution of the spent fuel repository (“Expected Evolution of a Spent Fuel Repository at Olkiluoto,” Posiva Report, 2006–2005) published in 2007, which, based on modelling calculations, estimates the future penetration depth of permafrost in Olkiluoto to be about 170 metres. According to Saarnisto, this figure is in conflict with observations made in Canada and with modelling results reported elsewhere.


The estimates presented by Posiva are based on conclusions made about the last ice age, projections on future climate development by the Intergovernmental Panel on Climate Change (IPCC), and scenarios developed based on them by the international BIOCLIM project funded by the European Commission. Of course, these estimates are subject to considerable uncertainties, and the report in itself does not pass judgement on the probability of the scenarios presented. The scenarios studied were selected to represent current expert opinion on possible evolutions without prejudice to other alternative scenarios. Professor Saarnisto has publicly stated that he considers IPCC estimates on climate change and its causes incorrect. Therefore, it is no wonder that he does not approve of any scenarios constructed on the basis of IPCC estimates.

 

The estimates on the penetration depth of permafrost presented in the report are derived directly from a mathematical model. A similar model was used by the Swedish SKB in the evaluation of the proposed final disposal sites in Sweden. According to SKB calculations, penetration depth at the proposed Swedish final disposal sites would vary between 160 and 250 metres. Model-based calculations carried out at the Whiteshell Research Area in Canada have produced significantly higher penetration depths mainly due to different assumptions on future mean temperatures. Differences between these modellings are mainly explained by geographical location, but to some degree also by the characteristics of the local bedrock. These factors, in addition to climate change, have had an effect on the penetration depth of permafrost in the past as well: it is well-known that permafrost has, in some parts of the world, penetrated to depths of over 700 metres; but on the other hand, there are no clear indications that permafrost has ever reached these depths in Finland.

 

Posiva is currently defining future climate scenarios together with the Finnish Meteorological Institute, and in this context, we will be also estimating the length and nature of future periods of permafrost. The scenarios defined will be used as the basis for the safety argument to be prepared to support the construction license application of the spent fuel repository. The characteristics and hydrological effects of permafrost are also being addressed in the international Greenland analogy project partially funded by Posiva. On the other hand, Posiva is also investigating what really happens in the repository if the temperature of the bedrock falls under the freezing point of water. The “enormous” pressures caused by permafrost mentioned by Professor Saarnisto have, in any case, been taken into account in the planning of the repository, since the system is designed to withstand the weight of a continental glacier and any resulting hydraulic pressure. The most important permafrost-related question is what will happen to the bentonite clay surrounding the canisters. This matter is being currently studied with experimental investigation in, for example, B+Tech, a Helsinki-based company specialised in bentonite research. Based on preliminary results, bentonite appears to retain its distinctive characteristics in low temperatures.

 

For more information, please contact: Timo Seppälä, Senior Manager, Communications, Posiva, tel. +358 50 351 1193



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