The fate of radionuclides during bentonite erosion: insights from the Grimsel Lit Experiment and comparison to a laboratory mock-up test

In the framework of the Grimsel Test Site (GTS) Colloid Formation and Migration (CFM) Project, a Long-term In situ Test (LIT) over the duration of 4.5 years under well-controlled hydraulic conditions has been performed simulating the direct contact between a compacted bentonite (FEBEX) geotechnical barrier and a water-conducting fault (MI shear zone) [1]. The compacted bentonite was emplaced as 16 rings, the central four rings were fabricated from a mixture of FEBEX (90%) and structurally labelled Zn-montmorillonite (10%). Within these four rings 16 glass vials filled with a structurally labelled Ni-montmorillonite slurry containing a suite of radionuclides (241Am, 237Np, 242Pu, 99Tc, 137Cs, 75Se, 233U) were emplaced. An identical labelled bentonite ring was investigated in a laboratory Plexiglas set-up with a 1 mm parallel plate fracture intersecting the bentonite ring. Controlled flow rates and original Grimsel groundwater under argon atmosphere in a glove box were used to establish geochemical conditions comparable to those of the in situ test.
Very low concentrations in the ppq range of 99Tc only detectable via AMS [2] could be determined in near-field observation boreholes in the course of the LIT, whereas the mock-up test showed mobilization of 99Tc, 237Np and 233U at levels detectable with SF-ICP-MS. ... mehrThe data will be discussed in light of the redox conditions established in the near field and the mobilized compacted bentonite derived colloid concentrations. Especially in the LIT different sampling configurations highlighted the heterogeneity of the flow field and complexity of the water-conducting feature. Furthermore, the experimental data is compared to results from diffusion calculations assuming planar or circular geometry and different radionuclide oxidations states. Finally, the LIT experiment, after preserving the near-field integrity via resin injection, was overcored with a 300 mm diameter drill core to examine inter alia (a) the expansion of the bentonite gel layer into the shear zone and (b) the fate of the radionuclides within the compacted bentonite, the bentonite-granodiorite interface and the MI shear zone.
The LIT experiment was compared to the laboratory mock up experiment. The Plexiglas set up containing the mock-up test was opened after ca. 5 years from the end of the experiment for post mortem analysis. The bentonite ring and gel were preserved as two samples of the same size that adhere onto the two Plexiglas plates. Two of the vials were found intact and the two others broken due to the pressure rise during the experiment. Autoradiography was performed on the ring to detect gamma emitters radionuclide tracers (namely 241Am and 137Cs). Two different types of diffusion distances were identified. For the intact vial, the radionuclides were still mostly contained in the vial, with a diffusion at a millimeter scale around the vial. In the case of the broken vial, the radionuclides were diffusing further away from the source at a centimeter scale. Cutting of the sample into ca. 100 μm slices by abrasive peeling and desorption of the radionuclides with concentrated nitric acid will be performed in order to investigate with (SF)ICP-MS the diffusion profiles of the radionuclides through the bentonite.
[1] www.grimsel.com [2] Quinto et al. (2019) Anal. Chem. 91(7), 4585-4591.