DEVELOPMENT AND IMPROVEMENT OF THERMODYNAMIC UNDERSTANDING FOR THE NUCLEAR WASTE DISPOSAL SAFETY CASE

Thermodynamic and geochemical model calculations are important tools used in the context of Safety Cases of repositories for nuclear waste disposal. Understanding and properly quantifying the thermodynamic driving forces controlling the mobilization and retention of radionuclides, as well as the degradation of waste matrices and technical barriers, underpins the long-term performance assessment of such disposal systems and increases its credibility. Thermodynamic approaches can sometimes add time-independent boundary conditions into long-term predictions. Further developing thermodynamic databases (TDBs) and underlying scientific understanding of key processes arises as a research need in the context of geological disposal of radioactive waste. In the framework of the JOPRAD Programme Document “The Scientific and Technical Basis of a Future Joint Programme on Radioactive Waste Management and Disposal”, the sub-domain “Chemical Thermodynamics” was rated with the highest level of common interest within the category of “Radionuclide and Chemical Species Migration”. This contribution outlines the specific need for experimental and theoretical investigations providing information for the prediction of processes over long timescales based upon fundamental scientific constants, i.e. via chemical thermodynamics, in key fields for geological disposal of radioactive waste. As a starting point for further discussion, the following topics of interest have been outlined: (i) data gaps identified within the Thermochemical Database project of the Nuclear Energy Agency (NEA-TDB), as well as for other elements / systems of relevance for waste disposal and not covered by NEA-TDB; (ii) radionuclide-organics complexation, including cement additives (beyond CORI), degradation products and small organic ligands disposed of with the waste; (iii) TDB for elevated T conditions and the need of developing advanced methods for the estimation of thermodynamic properties; (iv) solid solutions including relevant end-members for waste disposal, e.g. clay or cement systems, alteration products of waste packages etc.; (v) the interplay of thermodynamic and kinetic effects, in particular with focus on ill-defined solid phases, Ostwald ripening and description of redox processes; and (vi) the link between local equilibrium at small scale or between few components and global disequilibrium. This contribution is intended to trigger the development of a working group on these topics, where of course the list of topics is susceptible to be modified or extended to better adjust to the needs of WMO, TSO, regulators and RE. The review books within the NEA-TDB and its quality assurance procedures represent a key anchoring point, which provides the most comprehensive international effort for building up a high-quality TDB in the context of nuclear waste disposal, and it is at the core of most national and trans-national TDB initiatives in this field, e.g. ThermoChimie (France, UK, Belgium), THEREDA (Germany), JAEA-TDB (Japan), WIPP-TDB (US), among others. Beyond the need of maintaining the know-how in the area of thermodynamics, this initiative supports present and future capabilities to perform reliable use of thermodynamic concepts in predictions underpinning the performance of various disposal configurations and the Safety Case.