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. CanUO2(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.

Microanalysis (micro-XRF, micro-XANES, and micro-XRD) of a tertiary sediment using microfocused synchrotron radiation.

Micro-focused synchrotron radiation techniques to investigate actinide elements in geological samples are becoming an increasingly used tool in nuclear waste disposal research. In this article, results using mu-focus techniques are presented from a bore core section of a U-rich tertiary sediment collected from Ruprechtov, Czech Republic, a natural analog to nuclear waste repository scenarios in deep geological formations. Different methods are applied to obtain various, complementary information. Elemental and element chemical state distributions are obtained from micro-XRF measurements, oxidation states of As determined from micro-XANES, and the crystalline structure of selected regions are studied by means of micro-XRD. We find that preparation of the thin section created an As oxidation state artifact; it apparently changed the As valence in some regions of the sample. Results support our previously proposed hypothesis of the mechanism for U-enrichment in the sediment. AsFeS coating on framboid Fe nodules in the sediment reduced mobile groundwater-dissolved U(VI) to less-soluble U(IV), thereby immobilizing the uranium in the sediment.

Confocal micrometer-scale X-ray fluorescence and X-ray absorption fine structure studies of uranium speciation in a tertiary sediment from a waste disposal natural analogue site.

Investigations by micrometer-scale X-ray fluorescence and X-ray absorption fine structure (micro-XRF and micro-XAFS) recorded in a confocal geometry on a bore core section of a uranium-rich tertiary sediment are performed in order to assess mechanisms leading to immobilization of the uranium during diagenesis. Results show uranium to be present as a tetravalent phosphate and that U(IV) is associated with As(V). Arsenic present is either As(V) or As(O); we found no evidence for As(III). The As(O) is observed to be intimately associated with the surface of Fe(II) nodules and likely arsenopyrite. A hypothesis for the mechanism of uranium immobilization is proposed, where arsenopyrite acted as reductant of groundwater-dissolved U(VI), leading to precipitation of less soluble U(IV) and thereby forming As(V).