Dr. Thomas Lautsch, BGE, Germany The bedrock of the Konrad repository is characterized by a complex geology. While the emplacement horizon is in competent iron ore strata, the infrastructure of the repository is situated in different layers of clay. One of the many clay strata of the Konrad stratigraphy is the “Fladentonsteinserie”, where the inlet of shaft 2 into the second level is located. In comparison to other clay layers the Fladentonsteinserie shows a distinctive squeezing behavior. While crystalline systems and sandstone become stable after years of rock mass movement the Fladentonsteinserie stays mobile for many decades. This effect is enhanced at the inlet of shaft 2 into the second level because there are a lot of tectonic fractures nearby. The system of the outer lining is installed directly after excavation in multiple steps. It is complex and includes rock bolting, shotcreting and injections to control the ground. But due to the longterm mobility of the Fladentonsteinserie also the design of the inner lining is extremely strong with high quality concrete combined with lots of steel to deal with the abutment stress. We have learned, when building a repository in clay, you have to consider the characteristics of the different clay formations to find a suitable ground control system. This ground control system should not only consists of rock bolts and shotcrete but also of intelligent design of geometry and stratigraphical setting.

Christophe Bruggeman, SCK CEN, Belgium

Radioactive waste management spans the entire lifetime of nuclear installations and facilities, from their initial conceptual design up to their final decommissioning and beyond. Competence building in this field is challenged by multiple dimensions: knowledge transfer in the back-end of the nuclear fuel cycle will have to take place over multiple generations due to the very long licensing and implementation times of deep geological repositories; newcomer countries and countries with small inventories face challenges of maintaining the pace and standards compared to those leading the way in pre-disposal and disposal of (mostly) low- and intermediate-level wastes; a special but important challenge to ensure continuity of knowledge and competence is faced by ‘exiting’ countries, i.e., countries that phase out nuclear power but have not finally disposed of their spent nuclear fuel and radioactive waste. This presentation will reflect on these challenges and will draw from case studies, lessons learned and perspectives from both national and international (EURAD) programmes.

Liange Zheng¹, Radhavi Samarakoon¹, Chun Chang¹, Sharon Borglin¹, Chunwei Chou¹, Yuxin Wu¹, Jens T. Birkholzer¹

¹Energy Geoscience Division, Lawrence Berkeley National Laboratory, Berkeley, 94720, U.S.A.

Email Corresponding Author: lzheng@lbl.gov

Understanding the THMC (thermal-hydrological-mechanical-chemical) behavior of bentonite buffer in a geological repository is key to the evaluation and prediction of its long-term performance. Studies on the THMC process have in the past been focused on conditions under or just over 100 ºC since most design concepts impose a thermal limit of 100 ºC in bentonite. Understanding the bentonite behavior under high temperature (up to 200ºC) will benefit the confidence-building of repository with low thermal limit and could open the possibility of raising the thermal limit. In this presentation, we first employed generic coupled THMC model to explore bentonite alteration under high temperature, focusing the impact of illitization on the change of swelling stress. While modeling work deepen our understanding, further tests, particularly large scale in situ test was warranted for further evaluation of this issue. In the second part of the presentation, we introduce the HotBENT test that was launched at the Grimsel Test Site and a series of supplementary laboratory column tests and modeling work. Observations from the tests and the corresponding model work help us understand the early perturbation of bentonite buffer under high temperature and data from these tests improve the calibration of key constitutive hydrological and mechanical models and therefore enhance the modeling capability for calculating the long-term evolution of bentonite buffer. How to model the water flow in granulated bentonite and handle the temperature-dependency of key constitutive relationships such as water retention curve are remaining key challenges for models.

Irina Gaus, Nagra, Switzerland

Optimization of geological disposal projects for radioactive waste has gained attention across all developmental phases. Given the extensive timeframe and substantial financial commitments involved, ongoing optimization is imperative for ensuring their ultimate success. While the initial emphasis of Deep Geological Repository (DGR) concept development primarily centered on post-closure safety, the current scope has significantly broadened to encompass various key aspects, including design, engineering, cost, environmental impact, and societal acceptance. Clay-based repository programs are presently progressing through all developmental stages, from the preliminary pre-site selection stage to the pivotal stage of construction license submission. Site selection marks the initial significant stride toward optimization. Subsequent phases of repository construction and operation offer extensive opportunities for optimization, ranging from thermal loading and repository footprint optimization to innovative tunnelling concepts, sealing and closure methodologies, and advancements in robotics and remote handling. Throughout the presentation, reference will be made to prominent clay-based repository programs.