Near Field Radionuclide Concentrations: Sorption or Solubility Constrained?
Authors: Kienzler Bernhard , Vejmelka Peter , Metz Volker 
Published: April 2020

Abstract
The amount of mobile radionuclides is controlled by the geochemical isolation potential of the repository. Many investigations are available in order to determine the maximal radionuclide concentrations released from different waste forms of specific disposal strategies for disposal in rock salt formations. These investigations result in reaction (dissolution) rates, maximum concentrations, and sorption coefficients. The experimental data have to be applied to various disposal strategies.
The case studies presented in this communication cover the selection, the volumes, and the composition of backfill materials used as sorbents for radionuclides. As an example, for brown coal fly ash (BFA) - Q-brine systems, sorption coefficients were measured as well as solublilities of several actinides and other long-lived radionuclides. Dissolved CO32− was buffered to negligible concentration by the presence of high amount of Mg in solution. In the sorption experiments Pu, Th, Np, and U concentrations close or below detection limit were obtained. Concentrations in the same ranges are computed by means of geochemical modeling, if precipitation of “simple” tetravalent hydroxides (An(OH)4(am) phases) is assumed. In the case of U in a Portland cement dominated geochemical environment, measured U(VI) concentration corresponds to the solubility of hexavalent solids, such as Na2U2O7. A similar behavior of U was observed in high-level waste glass experiments.
Experiments investigating sorption behavior of corroded cement showed that in the case of application of a sufficient large inventory of actinides, measured concentrations were found to be independent of the inventory. In this case, measured concentrations were controlled by solid phases. If smaller actinide inventories were applied, resulting concentrations were found to be below concentrations constrained by well-known solids. Here, a more or less pronounced sorption of the radioactive elements was observed.
The radionuclide concentrations determined in the BFA “sorption” experiments are found to be close to the detection limits. For this reason, it is not possible to extrapolate the radionuclide behavior to lower concentrations. We cannot distinguish, if sorption or precipitation controls measured radionuclide concentrations. However, in the presence of reducing materials such as BFA, solubilities of tetravalent actinides and of Tc(IV) represent a realistic estimation of the maximal element concentrations needed in performance assessment studies. The concentrations of these redox sensitive elements are controlled by precipitation of An(OH)4(am) phases for disposal concepts considered in German salt formations. Under this assumption, quantities such as solid-solution ratios used in (sorption) experiments do not affect the mobilization of the radionuclides.
Additional conclusions can be drawn from comparison of the findings for the redox sensitive elements in the BFA / portland cement brine systems: We can assume that expected actinide and technetium concentrations in the near-field of radioactive wastes are affected by the total inventory of radionuclides in the disposal room. Sorption will be relevant, if the total dissolved radionuclide concentration remains below the maximal solubility defined by the solid radionuclide phase which is stable in the geochemical environment. In contrast to the portland cement system, the relevant radionuclide phase are most probably tetravalent hydroxides in the BFA systems.
These conclusions are of high importance to performance assessment for the radioactive waste repository systems, because they restrict the applicability of sorption models in the near field of the waste.