International benchmark on radionuclide solubility and speciation for the Long-term In-situ Test (LIT) at Grimsel Test Site (GTS, Switzerland)

Thermodynamic benchmark calculations have been performed to better understand the behavior of $^{75}$Se(VI), $^{99}$Tc(VII), $^{233}$U(VI), $^{237}$Np(V), $^{241}$Am(III), Th(IV) and $^{242}$Pu(IV)) in the evolving geochemical conditions of the Long-term In-situ Test (LIT) at the Grimsel Test Site (GTS) and corresponding mock-up experiment. It also aims to identify the status of the geochemical speciation models and databases for these elements. The experiments are simulating the near-field conditions in some radioactive waste repository concept including a bentonite engineered barrier emplaced in crystalline rock and the findings are contributing to the long-term safety assessment of these facilities.
In general, the calculations of all six modelling groups who participated in the benchmark agree well and differences in results could always be traced back either to differences in input data or to modelling assumptions. Such assumptions concern for example the characteristics of solid phases, especially the selection of a crystalline, aged or fresh mineral phase, which directly impacts the predicted radionuclide (RN) solubility as illustrated in the results for thorium and plutonium. ... mehrIn a similar way, the inclusion/exclusion of aqueous species leads to an increase/decrease of the solubility like for the polyselenides in case of selenium. In particular, the selection of recently approved species such as ternary silica actinide complexes or ternary uranyl carbonate complexes are impacting, for example, the speciation and solubility of U, Pu, Am or Np in the systems investigated here. It is crucial that all such data selection and modelling decisions need to be transparent and well justified or at least documented.The results from the thermodynamic benchmark contribute to a better understanding of the mobility of the respective RNs in the LIT and accompanying mock-up test. The observed mobility of significant fractions of $^{99}$Tc, $^{237}$Np and $^{233}$U in the mock-up experiment, but not in the LIT, together with the results from the thermodynamic benchmark calculations, suggest that conditions in the mock-up experiment are less reducing, since they are not buffered by surrounding minerals in the shear zone as is the case in the LIT. Rather low mobility $^{241}$Am(III) and $^{243}$Pu(III/IV) redox states are predicted in the calculations for all waters in accordance with the fact that no release of these nuclides is observed in either LIT or in the mock-up experiment. Ongoing analysis of the over-cored material of the LIT, namely the characterization of RN distribution profiles in the bentonite, will contribute to a further verification of the benchmark results.