Radionuclide geochemistry evolution in the Long-term In-situ Test (LIT) at Grimsel Test Site (Switzerland)

The Long-term In-situ Test (LIT) of the Colloid Formation and Migration project (CFM) at the Grimsel Test Site, investigates the generation of bentonite colloids and, hence, radionuclide mobilization within a well-defined and controlled shear zone in a crystalline rock. In this context, the determination of radionuclide aqueous speciation is essential to understand whether radionuclides are easily transported or immobilized by precipitation or uptake processes in the bentonite barrier included in a repository concept for nuclear waste, and mimic in the LIT experiment. The objective of this work is to determine the aqueous speciation of seven radionuclides (i.e. ⁷⁵Se(VI), ⁹⁹Tc(VII),²³³U(VI), ²³⁷Np(V), ²⁴¹Am(III), Th(IV) and ²⁴²Pu(IV)) by thermodynamic calculations in different water compositions representing the geochemical evolution through the LIT. A comparison of the results obtained from two different modelling groups allows the identification of the geochemical key parameters affecting radionuclide mobility in this context and the corresponding numerical and conceptual uncertainties. Particularly, silicate complexes of trivalent actinides and uranium(VI) carbonato complexes (i.e. Ca_nUO₂(CO₃)₃^(4-2n) n = 1 or 2) seem to be crucial in these environments, even at reducing conditions. Conceptual uncertainties like inclusion/exclusion of tetravalent actinide-bearing colloids formation and polyselenides have clearly been identified.

Multiactinide Analysis with Accelerator Mass Spectrometry for Ultratrace Determination in Small Samples: Application to an in Situ Radionuclide Tracer Test within the Colloid Formation and Migration Experiment at the Grimsel Test Site (Switzerland)

The multiactinide analysis with accelerator mass spectrometry (AMS) was applied to samples collected from the run 13-05 of the Colloid Formation and Migration (CFM) experiment at the Grimsel Test Site (GTS). In this in situ radionuclide tracer test, the environmental behavior of ²³³U, ²³⁷Np, ²⁴²Pu, and ²⁴³Am was investigated in a water conductive shear zone under conditions relevant for a nuclear waste repository in crystalline rock. The concentration of the actinides in the GTS groundwater was determined with AMS over 6 orders of magnitude from ∼15 pg/g down to ∼25 ag/g. Levels above 10 fg/g were investigated with both sector field inductively coupled plasma mass spectrometry (SF-ICPMS) and AMS. Agreement within a relative uncertainty of 50% was found for ²³⁷Np, ²⁴²Pu, and ²⁴³Am concentrations determined with the two analytical methods. With the extreme sensitivity of AMS, the long-term release and retention of the actinides was investigated over 8 months in the tailing of the breakthrough curve of run 13-05 as well as in samples collected up to 22 months after. Furthermore, the evidence of masses 241 and 244 u in the CFM samples most probably representing ²⁴¹Am and ²⁴⁴Pu employed in a previous tracer test demonstrated the analytical capability of AMS for in situ studies lasting more than a decade.