THEREDA: Thermodynamic reference database for geochemical modelling of nuclear waste disposal under saline conditions – Application, overview, and new developments

The Thermodynamic Reference Database (THEREDA) is designed for geochemical calculations in the context of repositories for radioactive waste under high-saline conditions. For this purpose, it adopts the Pitzer ion-interaction approach. THEREDA is currently the only database worldwide that allows comprehensive polythermal calculations (up to at least 100 °C) of the hexary system of the oceanic salts Na-K-Mg-Ca-Cl-SO$_4$-H$_2$O, including acids, bases, and CO$_2$/carbonates. Its validity is documented primarily, yet not exclusively, by application to solubility data. THEREDA's potential to predict the development of the geochemical environment, e.g. in the event of an intrusion of solution, constitutes a prerequisite for the engineering design of a nuclear waste repository with regard to the selection and placement of plug and sealing system components.

The focus of THEREDA lies on the calculation of solubilities of radionuclides (actinides, fission and activation products), chemotoxic and matrix elements e.g., canister materials, and compounds having an impact on the overall geochemical milieu in the near field of a repository under high-saline conditions.
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Special features of THEREDA besides its focus on high-saline solutions are procedures for testing prior to any release combined with intercode-comparison, extensive and publicly available validation against published experimental data, and systematic application of a scheme to mark data with regard to quality, reliability, and origin. Traceability of data and validated experimental results to published sources is also emphasised. This documentation is in part realised in ready-to-use parameter files for users, and in part on the website (www.thereda.de). Another feature is the possibility for operation of THEREDA by several institutions in a net-worked manner.

To highlight THEREDA's potential, examples for applications of the database are given. Additionally, ongoing efforts for the further development of THEREDA are described in the outlook at the end of this article.