A tracer study was launched in ONKALO
A study has been launched at final disposal depth in ONKALO, where radioactive tracers were injected in a hole drilled in intact rock. The extensively prepared REPRO project (Rock matrix rEtention PROperties) has progressed to the field research stage. It will investigate the retention of radioactive substances in the microscopic porous network of intact rock.
Research coordinator Lasse Koskinen, what is the research all about?
"We are investigating the retention of radioactive substances in rock that does not have any significant water conducting fractures. In such conditions, substances migrate extremely slowly, which is partially due to their diffusion in the microscopic porous network of the rock. Diffusion refers to distribution caused by the thermal movement of particles towards a lower concentration. In practice, the porous network retains nuclides and slows down their movement.
How is retention measured under ONKALO conditions in practice?
"The research is conducted in a research niche excavated at final disposal depth. A bore hole about 20 metres long has been bored in the rock wall. A two-metre section of the hole has been plugged for the test. A tracer pulse is conducted into the plugged section with a continuous but very slow flow."
The goal is to finally dispose of the final disposalanisters in good rock that is as intact as possibleThe bore hole in the REPRO research largely corresponds with the good-quality rock in the disposal hole of the final disposal canister; the rock does not have any significant structures that conduct water. If the canister were to, against expectations, break and the bentonite barrier surrounding it fail in retaining the radioactive substances, they would be restrained by the porous network of the rock. In the test, our goal is to assess how powerfully the radionuclides are retained in the porous network of the rock.
What are the benefits of the study for the implementation of final disposal?
The goal is to finally dispose of the final disposal canisters in good rock that is as intact as possible. The bore hole in the REPRO research largely corresponds with the good-quality rock in the disposal hole of the final disposal canister; the rock does not have any significant structures that conduct water. If the canister were to, against expectations, break and the bentonite barrier surrounding it fail in retaining the radioactive substances, they would be restrained by the porous network of the rock. In the test, our goal is to assess how powerfully the radionuclides are restrained by the porous network of the rock.
Are there previous research results available concerning the retention of radionuclides, or is this study a test case that attempts to find out how the retention works?
Retention research has been conducted extensively throughout the world, e.g. in Switzerland and Sweden, but not with such test preparations and arrangements as in ONKALO. The retention studied now has been modelled mathematically in advance, taking into account the test arrangements, such as the flow rate in the bore hole and the level of radioactivity in the tracer. The experiment conducted now will find out whether the retention takes place as per the advance assumption and its mathematical model.
This test will indicate the strength of retention in good rock, which means that the information obtained from the test supports these estimates. Retention is radionuclide-specific and the movement is also affected by the groundwater flow conditions. In addition, the dissolution of spent fuel into groundwater is very slow. The largest share by far of the radioactivity of spent fuel will remain near the canister hole even if the canister and bentonite barrier were to lose their integrity.
What radioactive tracers are being used in the study ?
The tracers used in the test represent weakly absorbing radionuclides. Therefore, we have selected radioactive isotopes of iodine (125I), chlorine (36CL), natrium (22Na) and hydrogen (3H) for the test. The radioactivity of each tracer can be measured separately in the test and the strength of retention can thus be assessed radionuclide-specifically.
How long will the study last?
The flow rates used in the research are extremely small (microlitres per minute) which means that radioactive tracers travel rather slowly to the plugged bore hole and back to the measurement device outside the bore hole. Depending on the flow rates, the study may last up to three years.