Study holes in ONKALO now complete
The ONKALO facility now comprises a total of five research niches, which are approximately 30-metre branches of the access tunnel. One of them, known as the POSE niche, contains three study holes with a diameter of 1.5 metres and a depth of approximately seven metres (7.26 m exactly). These holes emulate the disposal canister hole, even though their diameter is slightly smaller than that of the actual hole.
Why were these holes made, geologist Kimmo Kemppainen?
"We have started an experiment with which we survey the strenght of the bedrock on the surface of a round hole. The normal bedrock stress is concentrated on the surface of the hole, which means that the rock with lower strength may begin to spall, i.e. chips of rock may come off the surfaces of the hole. In the next phase, we will increase the stress field by means of heating."
What kinds of results have you achieved – has the inner surface of the holes cracked or spalled due to the bedrock stress?
"By now, we have drilled two holes at a distance of approximately 90 cm from each other, and the natural stress has concentrated on the pillar formed between them. The stress on the hole surfaces clearly exceeds the anticipated level of strength but, so far, no spalling has occurred. This is a good sign indicating that the rock is stronger than anticipated. Minor changes in the mica streaks were detected on the sides of the first survey hole. In the next phase, we will start heating the pillar, which will increase the pillar's stress field. We will also study the impact of heat on the hole and heat conduction around the hole. Our goal is to make the rock spall in order to define its spalling strength. In the third hole, we will install thermal elements inside the hole to emulate the disposal canister, which generates heat."
What kind of thermal capacity are you using in the experiment?
"The nominal capacity of the thermal elements is 4 kW, and they can be adjusted to make the bedrock temperature as even as possible. The temperature is measured both from the heating holes and on the surfaces of the large study holes."
How is the thermal conductivity of Olkiluoto bedrock in comparison with bedrock elsewhere in Finland?
"The thermal conductivity of Olkiluoto rocks varies between types; the average thermal conductivity value of the tested samples is 2.91±0,51 Wm-1K-1; the values of rocks vary between 2 and 7 (for soapstone, it is 6.4 Wm-1K-1). The thermal capacity is 712±32 J kg-1 K-1, and the average for natural stones is 840 J kg-1 K-1 (for soapstone, approximately 980 J kg-1 K-1 )."
How long will the study continue?
"We will continue until at least the middle of 2011, which is when, according to the preliminary plan, the heating experiments in the third hole will be concluded. As the survey proceeds, we will surely encounter new factors that require closer examination and may postpone the conclusion of the experiment."
Is there water leaking into the holes? If so, how much?
"A small amount of water is leaking into the holes, but at this point the water seems to contain tracer substances, which means that it is water used in the drilling of the holes."
What will happen to the holes after the study?
"It remains to be seen how much of them will survive the experiments. It is possible to maintain some of the holes for long-term research, follow-up and monitoring use. If visitors are not allowed in the demonstration tunnels, these holes will remain as visitor sites."