THEREDA - Thermodynamic Reference Database. Summary of Final Report

A long term safety assessment of a repository for radioactive waste requires evidence, that all relevant processes, which might have a significant positive or negative impact on its safety, are known and understood. In the case of brine intrusion into the disposal area, it has to be demonstrated, that the initiated chemical reactions don’t lead to an undue release of radionuclides into the biosphere. The starting point for this is to assess the solubility of contaminants in the solutions interacting with the waste. Solubility estimations can either be based on experimental data determined at conditions closely resembling those in the repository or on thermodynamic calculations.
A so called “thermodynamic database” created from experimental data is the basis for thermodynamic model calculations. Several research institutions in Germany are working on an improvement of the thermodynamic database. This work comprises investigations into fundamental thermodynamic data (such as vapour pressures or solubili-ties) as well as the application of sophisticated analytical or spectroscopic tools, which allow insight into aqueous speciation or structural details of surface complexes as basis for correct chemical and thermodynamic models.
Experience teaches that thermodynamic equilibrium calculations performed by different experts readily become difficult to compare and evaluate. This is only in part due to ill-defined (and -documented!) boundary conditions imposed on the calculations, but is frequently related to the use of different thermodynamic data or different conceptual models underlying them. Further difficulties arise by the fact that thermodynamic data used for a calculation actually are strongly interrelated; modification of an individual value without adapting the dependent values leads to “inconsistent” data. If applied in a calculation, this may lead to erroneous results, often unnoticed by the user.
As a result, in different institutions various databases exist that are appropriate for spe-cific tasks. However, they might lead to different results when they are applied to the same problem. This situation is unacceptable, both from a scientific point of view and considering the special public awareness for the final disposal of radioactive waste.
In 2002, a working group of five institutions was established for the creation of a com-mon thermodynamic database for nuclear waste disposal in deep geological formations (HZDR, GRS, TU BAF, KIT, AF-Consult). The common database was named THEREDA: Thermodynamic Reference Database.
It was agreed that the newly created database should be operated jointly by all members of the working group. In the mean term it is intended, that its usage becomes mandatory for geochemical model calculations for nuclear waste disposal in Germany. Furthermore, it was agreed that the new database should be developed along the guidelines long-term usability, easy access, applicability, internal consistency, comprehensiveness, documentation.
Activities within the time for which this report is valid cover a wide range of aspects. At first, a data model had to be designed from scratch which allows for the storage of thermodynamic data, at the same time facilitating export into code-specific parameter files. Creating the data model emphasis was laid upon its long term usage. Thus, a degree of abstraction was chosen which exceeds todays necessities and allows for future extensions. Technically the databank is implemented on a web server. Programs were created, which permit reading and writing access to the data. From the created webpages programs can be called that produce code specific parameter files ready for download upon specific request by the user.
THEREDA can thus be thought of as a databank in conjunction with a suite of peripheral programs, which aims at administrating, processing and extracting data. The data export is intended for the use in programs that calculate thermodynamic equilibria in aqueous solutions at temperatures which are of potential interest for hydrogeochemical systems in general and solutions containing hazardous contaminants like radionuclides or heavy metals in particular. As such, THEREDA is not designed to hold primary experimental data, neither data concerning any liquid other than aqueous solu-tion, e. g. melts or other substances which are stable under conditions beyond those where aqueous solutions may exist. Emphasis is laid on the correct calculation of ex-perimentally determined solubilities and aqueous speciation. Accompanying the above mentioned activities the working group agreed upon guide-lines which are to be followed upon selection and assessment of data. A system of quality assuring measure was set up; this comprises technical aspects relating to the databank as well as criteria determining how data are to be internally reviewed prior to release (auditing). As an external measure of quality assurance an internet forum was established to feedback questions and requirements from realistic problems into the project. A handbook was written to guide users in the handling of THEREDA (for the time being in german only).
Finally, thermodynamic data were entered. They comprise the system of oceanic salts as well as species and solid phase of a variety of radio-toxic and chemo-toxic ele-ments. This piece of work is on-going. At present, benchmark calculations are prepared. The first release of data will cover the system of oceanic salts (apart from C).