PARFREI – A research project to determine parameters of release, sorption and solubility of radionuclides

The German site selection procedure according to the Repository Site Selection Act (StandAG) is an iterative process to select a site for a high-level radioactive waste repository in Germany. The Bundesgesellschaft für Endlagerung (BGE) is developing the site selection procedure (Standortauswahlverfahren), which is implemented in a step-wise approach. With the identification of potential sub-areas (Teilgebiete), preliminary safety analyses (vorläufige Sicherheitsuntersuchungen, vSU) are carried out. In step 2 of phase 1 (Section 14 of the StandAG) of the site selection procedure, representative preliminary safety analyses (repräsentative vorläufige Sicherheitsuntersuchungen, rvSU) are to be performed. A particular challenge lies in the description and assessment of the potential radionuclide mobility in the repository systems. In addition to processes such as advection, diffusion, sorption and radioactive decay, the corrosion of the waste forms and subsequent dissolution and precipitation processes of radionuclides are of particular relevance and interest.

The research work performed in PARFREI has provided quantitative data on the release, solubility and sorption of radionuclides as input for the rvSU. PARFREI was jointly carried out by the Institute for Nuclear Waste Disposal of the Karlsruhe Institute of Technology (KIT-INE), the Institute for Resource Ecology at the Helmholtz Center Dresden-Rossendorf (HZDR) and the Institute of Fusion  Energy and Nuclear Waste Management of the Research Center Jülich (FZJ). PARFREI provides comprehensive lists of parameters to quantify potential radionuclide mobility, and discusses the available underlying process understanding, also critically reflecting the international state of the art in science and technology.

In a first work package (WP), PARFREI has analysed available data and derived representative geochemical conditions for the different host rocks systems considered. According to potential host rock formations for a repository in Germany, the systems (i) crystalline rock, (ii) claystone (separated for conditions in southern Germany and in northern Germany), and (iii) rock salt were considered. In further work packages, PARFREI has used the hydrochemical boundary conditions obtained in WP1 to (a) quantify the radionuclide release from spent nuclear fuels and high-level waste glass (WP2), (b) determine the solubility limits of the radionuclides in solution (WP3), and (c) provide data to describe radionuclide sorption on the host rock materials within a Kd- approach (WP4). This was done for each of the potentially relevant geochemical boundary conditions and host-rock systems. The list of radionuclides considered in PARFEI includes several actinides (and their decay products) as well as relevant fission and activation products, addressing different potential oxidation states when required. The work in PARFREI is based on scientific publications, national and international project reports and recognized and transparent thermodynamic databases. The quantitative parameters from PARFREI are usually derived based on experimental evidence and model calculations or, whenever necessary, considering chemical analogy and expert judgement. This contribution presents the basic methodological and scientific approach adopted in PARFREI and illustrates the key project results by selected examples, also discussing main remaining uncertainties. 

Acknowledgements: The PARFREI project was commissioned and financed by the Bundesgesellschaft für Endlagerung (BGE).