Chloride diffusion in pore water in Olkiluoto veined gneiss and pegmatitic granite from a structural perspective

Abstract

Spent nuclear fuel from Finnish power plants is planned to be deposited deep in the crystalline bedrock in Olkiluoto, Finland. The bedrock and more specifically the elemental composition of ground water, which is composed of the fracture water and the matrix pore water, needs to be well characterized to assess the risks inherent to the long term safety of the site. To this end, it is valuable to investigate elemental composition of the matrix pore water since it tends to conserve hydrogeological signals for longer time spans compared to open fracture waters.In this study, the chloride concentration of matrix pore water in veined gneiss (VGN) and pegmatitic granite (PGR) samples were investigated. Chloride was out-diffused from the naturally saturated rock cores into deionized water. Chloride pore diffusion coefficients were derived by modelling the chloride breakthrough curves obtained from the out-diffusion experiments. Two component modelling gave best fit to the experimental results. There two diffusion coefficients were (9±2)×10-11 m2/s and (0.5±0.1)×10-11 m2/s for PGR and (2.5±0.5)×10-11 m2/s and (0.4±0.1)×10-11 m2/s for VGN. Porosity distribution and total porosities of the rock samples were studied with the C-14-PMMA autoradiography. Porosity for PGR was found to be 0.6 % with large mineral transecting fissures, and porosity for VGN was found to be 0.7 % with highly porous mineral clusters connected to each other via grain boundaries and intragranular pores. The findings here show that heterogeneity has to be taken into account in modelling to find better agreement with the experimental results. C-14-PMMA autoradiography results indicate dual-component behavior for diffusion in PGR and VGN which were used in the modelling.

Comparison of water phase diffusion experiments in laboratory and in situ conditions

“…In both measurements, the early part of the breakthrough curves were identical for both of the tracers. However, measurement performed in VGN showed higher late part of the breakthrough curve for HTO than for 36 Cl. This indicates that transport of 36 Cl is retarded less by the rock matrix than the transport of HTO.…”

“…The spacers divided the flow into three identical flow paths which were connected into a single annular channel using the curved spacers. The tracer pulse, containing HTO and 36 Cl, was injected into the flow in the beginning of the experiment using an injection valve (9725i, Rheodyne) that contained a injection loop with a volume of 0.5 ml. Previously, [34] performed successfully similar experiments for unsaturated samples in gas phase and [45] have reported preliminary results for the measurement performed for VGN sample.…”

“…The injected tracer solution in WPDE1 comprised the radioisotopes HTO, 22 Na, 36 Cl and 125 I, of which 22 Na was expected to be slightly sorbing. In WPDE2, 125 I was excluded and 85 Sr and 133 Ba (divalent cations with more sorbing properties than 22…”

“…Collected samples were first measured with a HPGe gamma spectrometer (either Canberra GX8021-7500SL Xtra with 80% relative efficiency or Canberra GC4519 with 45% relative efficiency) for the activities of all gamma-emitting nuclides ( 22 Na, 85 Sr, 125 I and 133 Ba). This was followed by a separation of pure beta-emitter 36 Cl by silver chloride precipitation. The precipitate was dissolved in strong ammonia, mixed with liquid scintillation cocktail (Ultima Gold T M AB, PerkinElmer) and measured by liquid scintillation counting (TriCarb 2910TR, PerkinElmer).…”

“…The aim of the REPRO-project is to quantify the distribution coefficients, effective diffusion coefficients and connected porosity of the unaltered rock matrix in the repository level stress conditions. The in-situ experiments are performed in drill holes [21,22] while the drill cores from the same sections are used in laboratory experiments [33,34,12,35,36,37,38,20,39].…”