(GRS 239) Geochemical Modelling in the Near-Field of a HLW Repository in a High-Saline Environment

The overall objective of the R&D projects ISIBEL and ISIBEL-II was to summarise the state of the art in disposal of heat-generating radioactive waste in salt formations and to evaluate whether a demonstration of technical feasibility and repository safety was possible. For the first time, a concept that took full credit of the favourable properties of salt formations, reflecting the concept of safe containment, was developed and tested. Geochemical modelling is an important input into the development of a comprehensive safety case for a waste repository. Based on laboratory experiments it provides the solubility limits or concentration limits of every considered element as the major parameter to represent its mobility in the geochemical environment after mobilization. In order to be conservative, this parameter should be derived from the maximum expected concentration of the considered element. In order to demonstrate conservatism, a key objective of the
safety assessment is to obtain a thorough knowledge of the geochemical processes, especially in the near field. For a high saline environment a dataset of solubilities for the long-term safety analyses for a repository for heat-generating waste was documented in the long-term safety assessment “Sicherheitsanalyse Mischkonzept (SAM, /BUH 91/)”. Significant progress in the development of numerical tools, its underlying databases and their application for geochemical modelling has been made in the last 25 years. Dozens of research projects were carried out in this period improving the thermodynamic databases. What is lacking is a comprehensive analysis of the expected species in a highsaline environment and the maximum expected concentration of the relevant radionuclides for the long-term safety. This report is a fundamental part for this kind of analysis summarizing available thermodynamic data and assessing geochemical calculations performed at GRS. Due to the high complexity of the geochemistry in a repository the work described in this report cannot give the comprehensive picture but intends to start  a process aiming at an appropriate set of solubility data for high saline environments. According to the envisaged time frame of the site selection process in Germany
/KOM 16/ this process has to be initiated immediately.