Optimization can be defined as the process of making a system as good or effective as possible. In the context of radioactive waste management (RWM), continuous improvement and optimization are inherent ambitions of every disposal program. However, the terminology and understanding of optimization vary across programs, countries, and stakeholders. Relevant RWM stakeholders include implementers (waste management organizations), regulators, technical support organizations, research entities, and civil society organizations. These stakeholders exhibit distinct views of optimization, attributable notably to their heterogeneous roles and needs in a RWM program. The EURAD-2 work package OPTI provides a platform for interactions between members from the three EURAD colleges as well as CSOs, on the optimisation of HLW GDFs. These views about optimisation were discussed in a workshop held January 2025. A paper summarizing the conclusions of this workshop was prepared and discussed further. The intention of the contribution is to present the content of the paper and open the discussion to a larger group.

The exchange was centred around the “WHY?”, the “HOW?” and the “WHEN?” there are needs and priorities for optimisation. For instance, views on the possible scope for optimisation at the different phases of a RW disposal programme, the main challenges for optimisation and the best or existing approaches to optimisation were collected during the workshop. Some key observations on optimization emerge. Optimization is relevant for all stages of a repository program and optimisation must be considered holistically. It means that the disposal system has to be optimised as a whole, considering several optimisation processes that take place in parallel, such as the optimisation of the protection (radiological and non-radiological) and the optimisation of the resources required (and therefore the costs). The process of optimisation must be carried out taking account of prevailing circumstances, including the regulatory framework, the status of knowledge and the available resources (e.g. financial). However, the priorities shift over the lifetime of the program. At the start of a disposal programme, the focus of the disposal system optimisation is mainly on optimising the protection. Optimization in earlier stages, e.g. preparation of the safety case and licensing, is an established engineering process ensuring compliance with regulatory requirements and enhancing the overall safety of repository systems. Then, once the safety strategy and safety concept have been optimised and agreed with the regulator, the programme enters its next phases and the focus shifts progressively towards optimising the resources needed to implement the safety strategy and safety concept. At later stages, optimization after licensing or during construction and operation may have a different focus, as safety is already demonstrated (e.g. focus on reduction of conservative assumptions, optimization of the procurement of materials). Thus, various approaches and limits to optimization can be defined at different program stages; the prevailing circumstances of each national program set a frame in which optimization can be done. Furthermore, economical drivers, developments in science and technology, and interactions with the society can outmanoeuvre the optimization process.

How to cite: Herold, P., Dieudonné, A.-C., Detilleux, V., and Svoboda, J.: Optimization in the context of high-level radioactive waste repositories, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-127, https://doi.org/10.5194/safend2025-127, 2025.

Introduction

The Federal Company for Radioactive Waste Disposal (BGE) is responsible for the search and selection of a site with the best-possible safety for the disposal of high-level radioactive waste (HLW) in Germany. The federal Repository Site Selection Act, (Standortauswahlgesetz – StandAG) [1] regulates the Site Selection Procedure. It requires the site selection to be participatory, transparent, learning, and self-questioning based on scientific expertise. In the current Step 2 of Phase I, representative preliminary safety analyses (rvSU) are carried out for the sub-areas identified in Step 1 [2] in order to determine potential regions for surface exploration (Phase II). In accordance with section 6 (4) EndlSiUntV [3], the rvSU comprise preliminary designs of the geologic disposal facility (GDF) for the three considered host rocks: claystone, crystalline rock, and rock salt. The areal footprint of a GDF is an important result of the design work that influences the judgement of suitability of areas for disposal.

GDF design and areal footprint

The type and amount of radioactive waste and the way in which it shall be emplaced are among the important basic elements for the development of a GDF design. One of the key aspects of the design is the underground layout, which is central for the evaluation of areal requirements. The main parts of the layout are the arrangement of disposal and access galleries, an infrastructure area, the cross-sections, and the diameters of the galleries including rock support and pillar widths. The required pillar widths can vary over depth and are one of the most sensitive factors for the areal footprint of the GDF. Consequently, rock mechanical calculations have been carried out to determine the aforementioned parameters. Based on this, thermal simulations were used to determine the maximum thermal loading of the canisters respecting predefined maximum temperatures on the surface of the canister. 

Figure 1: Schematic representation of a repository mine in crystalline rock with close-ups of sealings and galleries.

An example for the repository layout for crystalline rock is depicted in Fig. 1. It is primarily based on the KBS-3-concept of Posiva [4] and consists of two parallel access galleries, which are connected at regular intervals by crosscuts. The disposal galleries branch off perpendicularly from the access galleries. In the disposal galleries, the HLW-canisters are each placed in vertical boreholes, which are filled with bentonite blocks. The backfill material used for the disposal galleries and sealing structures consists of bentonite and concrete. The area required for the GDF is made up of an area for infrastructure, the disposal area, and the area in between.

References

[1] “Standortauswahlgesetz vom 5. Mai 2017 (BGBl. I S. 1074), das zuletzt durch Artikel 8 des Gesetzes vom 22. März 2023 (BGBl. 2023 I Nr. 88) geändert worden ist“, 2023.
[2] BGE, “Zwischenbericht Teilgebiete gemäß § 13 StandAG”, 2020.
[3] “Endlagersicherheitsuntersuchungsverordnung vom 06. Oktober 2020 (BGBl. I S. 2094, 2103)“, 2020.
[4] Posiva Oy (2023): Safety Case for the Operating Licence Application (SC-OLA). Hg. v. Posiva Oy (Website). Available online at: https://cms.posiva.fi/, checked on 26.09.2023.

How to cite: Lohser, T., Bertrams, N., Schlüter, F., Werres, M., Fahrendorf, F., Gawletta, D., Klimke, S., Gottron, D., Keller, A., Seidel, D., and Rühaak, W.: Preliminary design of a disposal facility for high-level radioactive waste in claystone, crystalline rock, and rock salt, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-94, https://doi.org/10.5194/safend2025-94, 2025.

The closure of Cigéo, the future deep geological disposal facility for radioactive waste in France, will rely on the use of swelling clay-based seals. Their main safety functions depend on the development and stability of swelling pressure that is transferred to the surrounding materials and host rock. Under such conditions, the seal has low permeability to water, while the connected fractured zone in the host rock is under compression. If bentonite-sand mixtures are used, gas permeability can be engineered so that gas pressure build-up due to hydrogen generation in the repository remains below pre-defined threshold values. It is clear that the design of such closure systems need not only to consider engineering aspects of construction and short-term effects, such as tunnel convergence during operation, concrete liner loading, or bentonite swelling upon hydration. Long-term effects become a key aspect, such as the effect of gas generation and pressure build-up on the long-term saturation of the seal, the geochemical interaction between different barriers and the host rock, which can impair the longevity of the engineered barriers, or long-term creep effects in concrete and host-rock.

In this contribution, we present a methodology that is developed as an informative tool for optimizing different closure systems while considering long-term performance as a central aspect. A numerical modelling framework that accounts for the governing complex physical and chemical processes involving hydro-chemo-mechanical couplings, which can result in changes in the swelling pressure, has been implemented in iCP, an interface between Comsol Multiphysics and Phreeqc. With this framework, different engineering designs and alternative materials of the sealing systems for horizontal, decline, and shaft seals have been simulated, considering their evolution over long time scales, until up to 100,000 years. The results are used as an informative tool for decision making, with long-term performance as a key aspect of the engineered solution.

These models encompass fluid flow under partially saturated conditions, nonlinear solid mechanics, and especially coupling with realistic geochemical models of the key reactive transport processes. Special emphasis on the coupling and interfacial processes between the seal and the surrounding materials (concrete supports, concrete liner, compressible layer, and the surrounding Callovo-Oxfordian claystone) is placed. The effects of the geochemical evolution of bentonite on its swelling pressure at the seal scale are quantified. The mechanical response of the system obeys not only to the hydro-mechanical couplings, but also to the impact of geochemical interaction of the bentonite-based seal with surrounding materials.

We focus here on the setup of the framework and an overview of the outcomes when applied to several alternative designs and scenarios. Long-term performance assessment covers the post-closure period until up to 100,000 years. Furthermore, a set of sensitivity analyses is conducted to quantify the uncertainty of several modelling assumptions and simplifications.

How to cite: Idiart, A., Laviña, M., Cabrera, V., de la Iglesia, M., Cochepin, B., Michau, N., and Talandier, J.: The role of coupled hydro-chemo-mechanical modelling in the optimisation of a repository closure system, Third interdisciplinary research symposium on the safety of nuclear disposal practices, Berlin, Germany, 17–19 Sep 2025, safeND2025-37, https://doi.org/10.5194/safend2025-37, 2025.