Radiochemical studies in development of deep geological repository in the Czech Republic

The coordinated development of a deep geological repository (DGR) started in the Czech Republic as early as 1993. The first stage of DGR development programme was focused mainly on gathering information from foreign programmes. The next stage, which is now in progress, consists in preparation of methodologies and laboratory research focused on acquiring data needed for proving safety of the Czech concept of the deep geological repository. Radiochemical studies play a crucial role in the safety assessment of a deep geological repository. Solubility of radionuclides in pore water of repository materials and sorption of radionuclides on these materials present primary parameters needed for the repository safety evaluation. In this contribution the main achievements of radiochemical studies performed in the framework of the Czech DGR development programme are summarized and further plans outlined. The results of selection of critical radionuclides of spent fuel from WWER 440 and 1000 reactors, based on spent fuel inventory calculations and review of solubility of radionuclides in groundwaters are presented in the first part of the contribution. Sorption experiments have been focused primarily on the development of methodology and understanding the problems connected with description of transport of radionuclides from spent fuel through bentonite buffer to geosphere. 137Cs was used in most of sorption experiments devoted to studying methodology problems mainly due to the simplicity of its species and easy detection. It is shown that even evaluation of sorption of this radionuclide, occurring only in single valence state and not forming complexes, is not straightforward and cannot be described by simple Kd model. Some of the results of modelling of sorption of Cs using a surface complexation model are shown. Sorption of technetium, as a representative of redox sensitive radionuclides, has been studied under various conditions. It is shown that the presence of some species in bentonite can significantly change sorption and solubility behaviour of technetium in bentonite and thus completely change the level of its hazard. Since diffusion is the only possible way of radionuclides transport in the repository buffer and backfill materials, a high number of experiments have been devoted to developing methodology of the diffusion coefficients determination. Some of the results with non-sorbing 3H and 36Cl used for determination of effective diffusion coefficients and effective porosity of bentonite are shown. The future radiochemical studies will be focused on the determination of migration parameters of all radionuclides important for DGR safety in materials considered to be used in the deep geological repository and the development of reliable models, describing their behaviour on the basis of well established mass transport theory. (author)