The latest results on colloid associated radionuclide mobility from the CFM project, Grimsel (Switzerland)

Thorsten Schäfer1, Ingo Blechschmidt2, Jocelyne Brendlé3, Wolfgang Hauser1, Stephanie Heck1, Florian Huber1, Markus Lagos1, Andrew Martin2, Paul Reimus4
1 Institut für Nukleare Entsorgung (INE), KIT
2 NAGRA, Wettingen, Switzerland
3 Lab. Mat. Mineraux, UMR CNRS, Mulhouse, France
4 Los Alamos National Laboratory , NM, USA

O 2.6 in Hydrogeologie von Hardrock und Kluftgesteinen / Grundwasser und Karst

29.05.2014, 12:00-12:20, H17, NW II

The influence of colloidal/nano-scale phases on the radionuclide (RNs) solubility and migration behavior is still one of the uncertainties in repository safety assessment [1]. Within the Colloid Formation and Migration (CFM) project at the Grimsel Test Site (GTS Switzerland) a huge geo-technical effort was taken to isolate hydraulically a shear-zone from the artificially introduced hydraulic gradient due to the tunnel construction. The construction is a combination of polymer resin impregnation of the tunnel surface and a steel torus to seal the tunnel surface. Natural outflow points of the MI shear zone were localized prior to the construction and sealed by surface packers. This design gives the opportunity to adjust the flow velocity in the fracture.

After optimization of the experimental setup and injection procedure through a number of conservative tracer tests a license was granted in January 2012 by the Swiss regulator (BAG) to perform the first radionuclide tracer test under these low-flow conditions. The injection cocktail of 2.25L volume consisted of 101.4 ± 2.5 mg/L montmorillonite clay colloids, whereas 8.9 ± 0.4mg/L were present as synthetic montmorillonite with structural incorporated Ni. For details on the structural characterization of the Ni-montmorillonite phyllosilicate, see [2]. Beside the colloids and the conservative tracer Amino-G (1646 ± 8ppb) the radioisotopes 22Na, 133Ba, 137Cs, 232Th, 237Np, 242Pu and 243Am were injected. The trivalent and tetravalent actinides were quantitatively associated with the colloids present as well as a part of the Cs, whereas Np(V) and Na are not bentonite colloid bond.

For on-site colloid analysis a mobile Laser- Induced Breakdown Detection (LIBD) system similar to the one used in the CRR experiments [3] was transferred to Grimsel and installed in-line at the “Pinkel” outlet to directly monitor the mobile colloid fraction throughout the experiment.

The conservative tracer Amino-G was recovered quantitatively and for the weakly sorbing tracers analyzed by gamma-spectrometry recoveries for 22Na, 137Cs and 133Ba of 64%, 10% and 1%, respectively, were found. The clay colloid recovery determined by LIBD and HR-ICP-MS analyzing Al and Ni as structural components of the clay particles provided 48-52%.

For the initial quantitatively colloid associated actinides Am(III) and Pu(IV) a recovery of 21-22% and 30-35%, respectively, could be determined. Np recovery is significantly reduced to ~4 %, which hints to a kinetic controlled Np(V) reduction. The data obtained so far clearly show the mobility of bentonite derived montmorillonite colloids under near-natural flow conditions in the MI shear zone of the Grimsel Test Site [4]. The experimental data will be discussed in detail in the presentation.

[1] T. Schäfer, et al. Appl. Geochem., 27 (2012) 390-403.

[2] Reinholdt, et al., Nanomaterials, 3 (2013) 48-69.

[3] H. Geckeis, et al., Radiochim. Acta, 92 (2004) 765-774.

[4] www.grimsel.com

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Letzte Änderung 31.10.2013