Optimization will be in focus of advanced programmes as first repositories enter in construction and operation. Optimization provides benefits regarding technical, economical, long-term safety related aspects as well as flexibility and robustness. The need of optimization is justified by the long running character of repository projects and thus by expected changing boundary conditions (e.g. new waste types) or evolution of technology and/or the adaptations of processes due to operational experiences. Further, optimization is a process that shall involve all stakeholders of a RWM programme, including civil society and the regulatory bodies. Within the frame of the EURAD 2 program a work package “HLW Repository optimisation including closure – OPTI” was initiated. The objectives of the WP are to develop a mutual understanding and provide recommendations about methodologies and further activities for design and optimization of specific HLW deep geological repository systems, structures and components (SSCs) and procedures. Within the WP 23 parties from 11 European countries are involved. The participants represents waste management organizations (WMO), technical support organizations (TSO), research entities (RE) as well as representatives from the civil society. To open the discussion for additional representatives from the civil society as well as the regulatory body or other interested parties a session dealing with optimization is proposed. The session goal would be to open the discussion about optimization such as What is it? Why? When? Who is involved? etc.

In the multi-barrier concept for the safe long-term storage of high-level nuclear waste in deep geological repositories, the stability of the individual barriers is a key factor.
The stability of the individual barriers, such as the outer and inner canister materials, and the buffer materials (clays, cementitous materials), depend on a variety of hydro-bio-geochemical factors, but also on the materials used in each case. Material stability and forecast of material degradation phenomena in the long-term are important aspects to be adressed in the safety case and the design of a deep geological repository. This session adresses the interplay between timely and rapid research and development of canister and disposal concepts versus the reliable assesment of the long-term material behavior used in a multi-barrier concept, with a special focus on corrosion issues. Topics of interest are:
- Corrosion studies on potential canister materials
- Application orientied material and canister development and testing
- Stability and degradation of buffer and filler materials affected by corrosion
- Modelling of material behavior on long time-scales with respect to hydro-bio-geochemical induced corrosion phenomena
This session covers by an interdisciplinary approach the challenge to adress and forecast material behavior for the timely implementation of a safe deep geological repository.

Final disposal containers for HLW like spent nuclear fuel and vitrified high-level waste from reprocessing are a crucial component both for the handling and emplacement of the waste and (depending on the safety concept) as a barrier for ensuring post-closure safety of the repository. As the inventory, geology and safety concept differs from country to country, various types of containers are being developed worldwide.

The session should provide insights to national and international developments in e. g., container design, material selection, safety and design requirements, both from the regulator’s and the implementer’s perspective, on the way to long-term safe HLW repositories worldwide.

Potential contributions:
- Presentations about research projects related to HLW final disposal containers in Germany, e.g. by BASE, BAM, BGE, GNS
- Presentations about international container concepts (e. g. from Switzerland, Sweden, Czech, EURAD-2 InCoManD)

This session addresses the geomechanical behavior of host formations and engineered barriers in nuclear waste disposal (NWD), emphasizing long-term integrity and containment under thermo-hydro-mechanical (THM) conditions. A robust understanding of these behaviors is critical for repository design, site selection, and ensuring long-term safety. Advancements in evaluating the hydromechanical state of host rock formations, characterizing geomaterials, and developing modeling techniques are of central importance in enhancing the short- and long-term predictions and safety assessments of repositories.


Perspective contributions include:
1. Barrier properties of natural and engineered materials:
• Mechanical response and fluid transport mechanisms in host rocks (e.g., salt, clay, shales) and barrier materials (e.g., bentonite, crushed salt, concrete).
• Impact of THM coupled phenomena on rock integrity and containment safety.
• Multiphase flow, gas transport, and long-term performance, including creep behavior.
• Recent advancements in laboratory and in situ testing techniques for geomechanical and hydraulic characterization.
• Methods for assessing barrier integrity and failure implications.
2. Field-scale hydromechanical conditions of host rocks:
• Measuring and modelling the in situ (effective) stress state of host rocks
• Prediction of hydromechanical properties of host rocks from field data (e.g. geophysical surveys, geophysical borehole measurements)
• Integration of field and laboratory data to enhance understanding and prediction.
3. Numerical Modelling and Simulation:
• Advanced numerical models to predict THM behaviour during operational and post-closure phases
• Model calibration using laboratory and field data.
• Long-term geomechanical response prediction.
4. Case studies
• Examples – natural or man-made (e.g., caverns, mining) – of barrier integrity and/or failure
• Insights from sealing systems in repository and mining applications.

5. Challenges and Future Directions:
• Addressing uncertainties in the short and long-term predictions.
• Innovations in system characterization and modelling practices.
• Future research needs in the domain.

The session will focus on the processes, their modeling and parameterization that are governing the radionuclide transport through the buffer, backfill and host rock / cap rock. Contributions providing thermodynamic data of (bio)geochemical systems and fundamental understanding of the underlying molecular processes relevant for the long-term safety assessment of nuclear waste disposal are welcome in this session. Presentations are invited on both numerical and analytical approaches to reactive transport processes at the pore scale and beyond.
Themes:
- Identification of significant deficits in process understanding and strategies for advancing the state of knowledge
- Gaps in thermodynamic databases: relevance, use of estimation methods (primarily related to speciation, solubility, sorption)
- Methodological developments (detection limits, data processing and interpretation, multilateral procedures, upscaling in complexity, time and space)
- Uncertainty and sensitivity analyses, geostatistics
- Analysis of material heterogeneities that critically control sorption processes in geomaterials
- Parameterization of reactive transport models, validation of numerical approaches
Goals:
The contributions shall focus on those phenomena that are either in general essential to the radionuclide transport or that have currently large uncertainties rendering them especially critical. Strategies and methodologies to treat them in a way to make safety assessments more realistic and less uncertain shall be derived and discussed.

This session is aimed towards different aspects of permeability, including its laboratory and in situ measurement and its critical upscaling required for modeling geological systems. We welcome keen laboratory and in-situ researchers as well as numerical modelers to discuss the challenges in and relevance of permeability assessment, regardless of the type of host rock or barrier material.

Key Laboratory Topics:
• Challenges in Measuring Low Permeability: Explore the pitfalls of measuring the lowest permeability values and how to overcome them.
• Unverified Method Comparability and Round-Robin Tests: Discuss the variability across measurement techniques and how collaborative round-robin testing can pave the way for greater comparability.
• Standardization Procedures for Site Selection: Debate the feasibility of developing standard procedures for permeability measurements, which are crucial for reliable evaluation and eventual site selection.
In Situ Measurement Topics:
• Distinguishing Flow Processes Across Scales: How to measure, distinguish and map fracture and matrix flow?
• Spatial and Volumetric Extrapolation: Discuss how to extrapolate localized permeability measurements to larger, reservoir or disposal-site scales, ensuring representativeness and reliability in assessments.
Numerical Modeling Topics:
• Upscaling Micro-Scale Processes: Delve into how small-scale measurements can be scaled to accurately represent larger geological systems.
• Fracture Networks to Equivalent Porous Media: Examine methods to modify discrete fracture networks into equivalent porous media to support the development of large-scale, realistic digital twins of geological systems.
• Future developments of barriers and geosphere: How to deal with changes of porosity due to mechanical load or precipitation of secondary phases, how (un)certain is pore-clogging?

The session will also provide room to discuss two-phase flow, relative permeability and the material dependency of environmental controls, which might dynamically change throughout the long-term evolution of a repository.

The second European Programme on Radioactive Waste Management EURAD-2 has been launched in October 2024. Under the umbrella of this international cooperation in the field of RWM, more than 100 organisation from 52 countries conduct ten R&D projects, a valuable range of Knowledge Management activities and six Strategic Studies.
For the symposium the outcomes of EURAD-2 would be very valuable with respect to the scientifically excellent works and further development of the state of the art throughout all phases of a RWM programme and covering the broad scope of our EURAD-2 roadmap. All this within a unique international cooperative framework bringing together the different key actors and stakeholder in the field of RWM plus civil societal representatives. Whereas the R&D and KM activities have a duration of five years, all Strategic Studies will be finished after two years by september 2026. One year in future all StS will already have produced relevant and interesting results within their gap analyses, white or green papers.
It is proposed to have a few detailed talks from interested and selected work packages, supplemented by an overview talk about the rest of the EURAD-2 strategic studies, e.g.
1. EURAD-2 – programme overview and mission of the conducted strategic studies
2. Presentation of objectives and results/recommendations from WP FORSAFF (Waste management for SMRs and future fuels), WP CLIMATE (Impact of climate change on nuclear waste management), WP DITUSC (Development and Improvement of Quality Assured Thermodynamic Understanding for use in Nuclear Waste Disposal Safety Case)
3. WP ASTRA (Alternative RWM strategies)
4. WP DITOCO2030 (Next generation Digital Twins to support Optimisation, Construction and Operation of surface and subsurface radioactive waste management facilities)
5. WP OPTI (HLW repository optimisation including closure)