THermodynamic Experimental Research and Methods for Overcoming Gaps in Analysing Performance (THERMOGAP)

Objective

To advance thermodynamic understanding for safety-relevant issues—such as low-sorbing radionuclides, complex chemical interactions (ternary-quaternary complexes), and overlooked beneficial processes like anion uptake in solids—by integrating experiments with model development to fill gaps and provide reliable methods and actionable practices that reduce uncertainty and support robust safety assessments
 

Description of the WP

ThermoGAP delivers clear added value by complementing, rather than duplicating, existing thermodynamic database initiatives (e.g. NEA TDB, Thereda, ThermoChimie, Nagra-PSI, etc.), with a strong focus on improving interoperability, traceability, and consistency between TDBs in support of Safety Case applications. It aims to facilitate the practical use of thermodynamic data across different modelling platforms and national contexts, thereby reducing discrepancies in model outcomes and strengthening confidence in safety assessments.
Building on the DITUSC Strategic Study, it targets key scientific gaps through focused research on under-addressed processes relevant for long-term safety (e.g. solid solutions, higher-order complexes, and less well-characterised redox systems), combining experimental investigations with advanced modelling approaches. This integrated strategy ensures that new data and concepts are not only generated but also effectively translated into forms usable within TDBs and performance assessment models.
Best practices and harmonised guidelines will be promoted to ensure compatibility with ongoing TDB developments, while advancing methodologies and geochemical tools to better assess, quantify, and propagate thermodynamic uncertainties in process-based models. Particular attention is given to improving transparency in data selection and evaluation, and to enabling more systematic treatment of uncertainty across different modelling frameworks.
Overall, ThermoGAP strengthens the robustness, transparency, and credibility of Safety Cases, while establishing practical, implementable methodologies for End Users. In parallel, it contributes to capacity building within the community and fosters dialogue with civil society by clarifying the role, limitations, and added value of thermodynamic models in supporting safety demonstrations.
 

Outcomes

Performance assessment and system models: ThermoGAP will combine experimental and modelling approaches to close key thermodynamic data gaps (e.g. U–Si–OH, Nb–ISA) and assess their impact on disposal system performance. Using simplified reference cases, it will quantify the effects of uncertainties and knowledge gaps, and develop practical implementation methodologies, including treatment of conceptual uncertainty/assumptions in PA abstraction and propagation of thermodynamic uncertainties in process-based models.

EBS systems: ThermoGAP will improve understanding of retention and mobility of low-sorbing, safety-relevant elements (Se, I, Cl) in cementitious materials through new datasets and models. It will better characterise corrosion products of iron-based components and assess the role of secondary phases and degradation products (e.g. zeolites), ensuring a more consistent representation of EBS evolution in Safety Case models.

Geo-datasets and conceptual models: ThermoGAP will evaluate the impact of potentially detrimental ternary and higher-order complexes on radionuclide retention, currently treated conservatively. It will improve understanding of solubility under saline and high ionic strength conditions and advance SIT-based methodologies, leading to more realistic, better justified geochemical models, and ultimately allowing a reduction of unnecessary conservatism in safety assessments.