Aktiviteetti: Tutkimustyypit › Väitöskirjan ohjaaja tai sivuohjaaja
Sorption and matrix diffusion in crystalline rocks – In-situ and laboratory approach The geological disposal of spent nuclear fuel might be good solution when all the aspects of risk assessment have been taken into account. In Finland and in Sweden, the host rock for geological disposal is crystalline rock and repository in based on the Swedish KBS-3 multi barrier design. When radionuclides from the spent nuclear fuel are possibly released into the bedrock, they will be transported by advection along water conducting fractures. Retardation of radionuclides can occur by molecular diffusion into the stagnant pore water and/or by chemical retardation onto the mineral surfaces in the rock matrix. Assessing the risks of the spent nuclear fuel deposition, it is important to understand the transport behaviour of radionuclides in the bedrock i.e. in the rock/groundwater interface.
Most of the parameters; like diffusion coefficient, distribution coefficient and rock porosity, covering the radionuclides’ migration in the rock are obtained from the laboratory scale experiments. In order to upscale the results from the laboratory experiments to the real scale of the nuclear fuel deposition, in-situ experiments are performed. For example the Swiss National Cooperative for Disposal of Radioactive Waste (Nagra) have been conducting extensive in-situ experiments at the Grimsel test site. In the first in-situ test, tritiated water (HTO), 22Na, 134Cs and 131I as well as non-radioactive isotopes 127I and 23Na, were circulated in a borehole interval isolated by packers 70 cm apart from each other. The second ongoing Long Term Diffusion experiment uses mainly the same radionuclides as well as nonradioactive element selenium.
This thesis presents the laboratory analysis of HTO and iodine from in-situ diffusion experiment and supporting laboratory studies for determining the sorption and diffusion of cesium and selenium on Grimsel granodiorite. Cesium sorption was studied with batch sorpion experiments using the crushed rock and selenium diffusion and sorption with batch and block scale experiments using Kuru grey granite and Grimsel granodiorite rock blocks. HTO and iodine diffusion was modelled with Time domain diffusion method (TDD) in in-situ rock blocks as well as selenium at laboratory with rock blocks.
Outleaching method was found to be successful for analyzing the non sorbing radionuclides from the connected pore network of the Grimsel granodiorite. TDD modelling of the results lead to the apparent diffusion coefficient of 3×10-10 m2/s for both HTO and iodine. No significant difference between in-situ and laboratory diffusion coefficient was detected. Cesium sorption was 0.107 ± 0.003 m3/kg on Grimsel granodiorite at 10−8 M Cs concentration. Sorption was found to be highest on biotite Kd = 0.304 ± 0.005 m3/kg which explains the in-situ diffusion results of cesium where it was found tofollow biotite veins in Grimsel granodiorite
Sorption of selenium was significantly overestimated when the determination was conducted with the crushed rock using batch sorption method compared to the studies conducted with intact rock. Outleaching method was found to be successful for analyzing the non sorbing radionuclides and flexible TDD modelling was found to be very useful tool in handling the measured data from both block scale experiments and in-situ experiments of non- and weakly sorbing tracers.