Migration Case Study: Transport of radionuclides in a reducing Clay Sediment (TRANCOM-II)

In Europe, clay formations become more and more important as candidate geological formations (e.g. Boom Clay, Callovo-Oxfordian, Opalinus.) for the deep disposal of High-Level radioactive Waste (HLW). In demonstrating the suitability of a geological site for the disposal of radioactive waste, it is essential to consider the potential mobility of critical radionuclides through the relevant "rock" types. This project addresses the migration behaviour of radionuclides, identified as important for the long term safety (U, Se, Pu, Am), in a reducing clay environment, with special emphasis on the role of the Natural Organic Matter (NOM). In such reducing environments, the solubility limit is considered as the most important mechanism to lower the mobile concentration of these radionuclides and the speciation is most likely neutral or negatively charged so a low retardation is expected. However, the presence of NOM may jeopardise the expected low concentration and sorption: by solubility enhancement due to complexation/colloid formation with NOM or by influencing the sorption behaviour. The objective is to develop and demonstrate a conceptual model for the description of the migration of radionuclides in a reducing, NOM rich clay environment that can be implemented in performance assessment (PA) models. The main questions are: (i) does the NOM increase the radionuclide concentration in solution due to complex formation and (ii) what is the possible role of mobile NOM as radionuclide carrier? To answer these questions, following research strategy was adopted taking the Boom Clay Formation as a Case Study. Prior to any experimental work, speciation and solubility's for the considered radionuclides were calculated not accounting for NOM. The mechanisms determining the overall behaviour of the radionuclides in the clay environment are then studied in solution in presence of NOM (solubility, complexation) and with respect to the solid phase (retention by immobilisation, sorption). Modelling is used to interpret and derive interaction constants which can be implemented in a geochemical "clay" database. Understanding and quantifying the interaction mechanisms in the clay environment should allow to model and interpret migration experiments with mixtures of the radionuclides and 1 4 C-labelled NOM. Separately, methodologies were developed to identify the source of mobile organic matter. The obtained results are translated into conceptual models which can be used to evaluate the performance of Boom Clay as potential host rock Batch experiments revealed that the solubility of amorphous UO 2 is ∼10 - 8 mol dm - 3 with no effect of NOM complexation. However, it was evidenced that the presence of NOM facilitated the formation of uranium colloids upon dissolution of UO 2 . The formation of colloids, with molecular size between 2 nm and 0.45 μm, accounts for a total uranium concentration, three orders of magnitude higher than the solubility of the amorphous UO 2 . Uranium colloids were also found being dominant in leaching experiments of natural uranium from Boom Clay samples. These colloids, only evidenced in batch experiments, are unlikely to be mobile in the compact natural Boom Clay. Electromigration experiments showed that U(VI) reduces to U(IV) and precipitates. This U(IV) precipitated phase constantly releases positive or neutral charged U-species. Furthermore, classical migration experiments with U(IV) mixed with 1 4 C-labelled NOM showed that U(IV) migrates independently of the NOM through the Boom Clay. The migration of uranium in the Boom Clay is governed by strong retention due to precipitation (solubility limit) and sorption and is not enhanced by the mobile organic matter.