THEREDA – Thermodynamic Reference Database for the nuclear waste disposal in Germany

1 Introduction
The disposal of nuclear waste including the assessment of long-term safety is still an open question in Germany. In addition to the pending decision about the repository host rock (salt, granite, or clay) and the associated site selection, the basic necessity of a consistent and obligatory thermodynamic reference database persists. Such a database is essential to assess potential radionuclide migration scenarios accu-rately and to make well-founded predictions about the long-term safety up to one million years. Specific challenges are comprehensive datasets covering also elevated temperatures and high salinities. Concern-ing the required elements (actinides, fission products as well as matrix and building materials), no other thermodynamic database is available that is compatible with the expected conditions. Due to these defi-ciencies THEREDA [1,2], a joint project of institutions leading in the field of safety research for nuclear waste disposal in Germany and Switzerland, was started in the year 2006.

2 Database features
THEREDA offers evaluated thermodynamic data for many compounds (solid phases, aqueous species, or constituents of the gaseous phase) of elements relevant according to the present state of research. In particular, all oxidation states expected for disposal site conditions are considered. In the present release, THEREDA includes data for actinides and their chemical analogues (Th, U, Np, Pu, Am, Cm & Nd), fission products (Se, Sr, Tc & Cs) and matrix elements (Na, K, Mg, Ca, Al, Si | Cl, SO₄, CO₃). For the calculation of cementitious phases the current version of CEMDATA (18.1) was integrated [3].
THEREDA is based on a relational databank whose structure intrinsically ensures the internal consisten-cy of thermodynamic data. Data considered respond to the needs of both Gibbs Energy Minimizers (ChemApp, GEMS) and Law-of-Mass-Action codes (Geochemist’s Workbench, PHREEQC, ToughReact). The database is designed generically so that it can store interaction parameters for various models. Namely, the PITZER ion interaction approach to describe activity coefficients of hydrated ions and molecules in saline solutions [4] as well as ideal and non-ideal solid solution approaches are consid-ered in the actual dataset.
After free registration, THEREDA is accessible via internet through www.thereda.de. This is not only a portal to view the data itself, their uncertainties and the primary references of the data; it provides also additional information on issues concerning the database. Ready-to-use parameter files are available for download in a variety of formats (geochemical code specific formats and generic ASCII type). They are also used for internal test calculations – one essential element of the quality assurance scheme. The capa-bilities of THEREDA are demonstrated using approx. 400 application case calculations, whose results were compared with experimental values published in literature.

References
[1] Altmaier, M. et al., “THEREDA - Ein Beitrag zur Langzeitsicherheit von Endlagern nuklearer und nichtnuklearer Abfälle”, atw, 53, 249–253 (2008).
[2] Moog, H.C. et al., “Disposal of nuclear waste in host rock formations featuring high-saline solutions – Implementation of a thermodynamic reference database (THEREDA)”, Appl. Geochem., 55, 72–84 (2015).
[3] Lothenbach, B. et al., “Cemdata18: A chemical thermodynamic database for hydrated Portland cements and alkali-activated materials”, Cem. Concr. Res., 115, 472–506 (2019).
[4] Pitzer, K.S., Activity Coefficients in Electrolyte Solutions, 2nd Ed., pp. 542, CRC Press, Boca Raton (1991).
[5] Ryan, J. L., et al. “The solubility of uranium(IV) hydrous oxide in sodium hydroxide solutions under reducing conditions”, Polyhedron, 2, 947 (1983).
[6] Rai, D. et al. “The Solubility of Th(IV) and U(IV) Hydrous Oxides in Concentrated NaCl and MgCl₂ Solutions” Radiochim. Acta, 79, 239–247 (1997).
[7] Neck, V. et al. “Solubility and hydrolysis of tetravalent actinides”, Radiochim. Acta, 89, 1–16 (2001),