Bedrock protects and retains
Spent nuclear fuel will be stored in the future in the bedrock, which has been studied at Olkiluoto for more than 30 years. Bedrock modelling has become more and more accurate, and there are only very few minor surprises in store at this point. Most of the information has been obtained from the research facility ONKALO that extends 437 metres below the ground level. The ability of the bedrock to protect and retain is being studied at ONKALO as we speak.
"Of course, you cannot say that anything is absolutely certain until you have seen it with your own eyes, but we have made huge advances in modelling over the years," says Lasse Koskinen of Posiva, head of the long-term safety team. Koskinen models the groundwater flow and the migration of radionuclides, and describes long-term changes, such as those caused by climate change.
Strong, stable and predictable
The bedrock at Olkiluoto meets all the criteria laid down for a repository for spent nuclear fuel, and there is enough intact bedrock to house the entire planned volume of spent fuel. The most important thing is that the fuel will be placed inside shield canisters in final disposal holes with no cracks in the rock that could conduct water and no long cracks.
"The canister could, in principle, be broken during an earthquake. The probability of such an earthquake is highest when a glacier is melting and drawing away. Such damage is only possible if there is a large crack close to the canister, however," Koskinen explains.
The bedrock has cracked more than a billion years ago, either when the rock crystallised or a little while after crystallisation. No new cracks can occur, but the old cracks may move if the stress to which the bedrock is subjected suddenly changes. A couple of centimetres of the rock may flake away when the hole is being drilled. This is of no significance in terms of safety, however. "In practice, the canister cannot be damaged inside a disposal hole that meets all the criteria. We nevertheless study the consequences of damage because only in such a situation could people or animals be subjected to radiation, even in principle," Koskinen explains.
Retention of radionuclides
Even if a canister were damaged, the fuel would dissolve into the groundwater very slowly. A small portion of the radioactivity could dissolve into the water very quickly, however. This means that most of the freed radionuclides would not travel far from the canister. A very small portion of the radionuclides could come to the surface through the cracks in the bedrock, but the limit values determined by the Radiation and Nuclear Safety Authority would not be exceeded even in such a case.
A test on radionuclide retention, managed by Koskinen, is currently ongoing at ONKALO. Water is being led into a test hole drilled to the final disposal depth and the water causes radioactive tracers to move a distance of around two metres. The delay in the tracers when compared to the flow is used to study how the tracers would be retained in the bedrock. "The results of a previous test confirmed our previous assumptions. The rock effectively retains radionuclides. The radioactive substances spread into the bedrock, and it takes a long time for them to travel further with the flowing water. Naturally, during the test most of the tracers will move with the flow in the 'test crack' because such a crack excellently conducts water," Koskinen explains.
The significance of bedrock in final disposal is high. It will protect the disposal capsule from external dangers and effectively limit the migration of radionuclides. The significance of the bedrock will be clearly lower if there is a large crack in the final disposal hole. Cracks influencing final disposal will be eliminated already at the planning stage, however.
From an instant to infinity
Licentiate in Philosophy and physicist Lasse Koskinen (aged 49) has studied the groundwater flowing in the bedrock, which is 1.9 billion years old, for the past 19 years. His typical 10-hour working day is mainly taken up with meetings related to safety studies and groundwater modelling. The research area is the 24 km³ of bedrock on Olkiluoto Island with approximately two to three cracks per one metre of drilled holes, and where the groundwater flow rate in the bedrock is around 200 litres per minute. The period of time being studied is 100,000 years – the time important in terms of the safety of final disposal – which is being modelled and forecasted based on previous, historical developments in the area.
Five obstacles to releases are the key in these safety studies: the fact that the spent nuclear fuel is in a solid state, the canister manufactured from copper and cast iron, a bentonite buffer, the tunnel filling material and the bedrock. These barriers will limit and slow down the release of radioactive materials from the fuel inside the canister. The key point is that the disposal canisters will be placed in areas where the bedrock is intact. The allowed maximum flow rate in a final disposal hole is 0.1 litres per minute. Furthermore, significant geological structures and long single cracks will be avoided when choosing the locations of the holes.
The final disposal holes will be 8 metres deep and 1.75 metres wide. The holes will be drilled in final disposal tunnels that are 200 to 300 metres long and 10 metres apart. Excavation of the tunnels will start in the early 2020s. The estimates state that around 200 tunnels will be made. The tunnels will be located around 420 metres below ground level.
The groundwater in the bedrock will probably be modelled for at least as long as the final disposal activities take place, i.e. for the next 100 years. There is an extensive observation network at Olkiluoto, and issues like the groundwater pressure will be monitored through dozens of drilled holes. Lasse Koskinen says that his work is very challenging and extremely interesting.
Written by Johanna Aho