Overview of Andra's R&D Program: Is R&D Still Needed Once the Construction License Application Has Been Submitted? 

Stéphan Schumacher

Scientific Director, Andra, France

On January 16, Andra submitted the construction license application (DAC) of Cigéo, the French project for the deep geological disposal facility for the most highly radioactive waste. This is a crucial step marking both a culmination and a new start for the project.

First and foremost, it is the result of 30 years of progressive development under regular evaluation and a process of iteration between knowledge acquisition, design and safety assessment. Over time, the scientific and technological aspects of the project have been refined into well‑defined design principles and a robust safety demonstration. This design is underpinned by a scientific and technological knowledge base resulting from the following elements:

• A thorough understanding of the phenomenology involved in establishing behaviour models, a characterization of the physico-chemical and mechanical properties and associated parameters (values and uncertainties depending on the siting area and mineralogical variability), as well as the implementation of numerical modelling of the various components of the disposal at different spatial and temporal scales.

• A detailed engineering phase of the stepwise design development integrating demonstrators of components (concrete or steel disposal container) and processes (handling demonstrator, funicular demonstrator, etc.) as well as in-situ demonstrators at the Meuse/Haute-Marne underground research laboratory (various gallery lining construction methods, 1-scale MA-VL and HA cells, ½-scale for sealing structures, etc.).

If Cigéo is authorized, this stage should lead to the start of the pilot phase (once the decree has been issued) followed by the start of construction by 2030. Cigéo will be operated for about 100 to 120 years.

Given the considerable scientific and technological knowledge base supporting the construction license application, the aim of this presentation is to answer the question of whether or not R&D should be maintained and if so, in what areas and for what purposes. To answer this question, we need to make the link between R&D needs, the incremental development of Cigeo and the long-term operation of the disposal. To do so, we will use several examples such as the dimensioning of the architecture, based on T and THM criteria, the design and the building of the tunnel junctions, and gas migration in the disposal and the host rock. This will involve linking R&D needs with the incremental development of Cigéo.

Coupled Processes in Fractured Rock for Deep Geothermal Energy and Less Deep Geological Repositories

Ki-Bok Min, PhD

Professor, Department of Energy Resources Engineering, Seoul National University (SNU), Seoul, Republic of Korea

Deep geothermal energy and deep geological repositories (DGR) are two important applications that are applied in fractured rock with shared and distinctive characteristics. Deep geothermal energy through the enhanced geothermal system (EGS) is targeted at a depth of ~5,000 meters, which is one order deeper than the typical depth of DGR at ~500 meters. In EGS, the main source of perturbation is the injected fluid and extraction of heat, whereas in DGR, excavation and heat release are the primary sources of disturbance. While some EGS sites are operational, other attempts in various locations have failed due to insufficient or reversible permeability enhancement and significant induced seismicity. While some countries have made decisions on site selection and construction for DGR, many others are still in the process. While public acceptance is an important factor, the site-specific nature of these applications is evident in their current status. This presentation will focus on some of the findings and challenges associated with EGS as an application in deep fractured rock, particularly in terms of coupled hydromechanical processes. Comparable challenges for DGR will be addressed in the context of thermally driven fracture shearing and fault reactivation. The importance of coupled processes for the two applications will be compared and discussed, and site specific nature of the two applications will be discussed in terms of their prospect.

On the HM and THM Behaviour of Argillaceous Rocks

Antonio Gens, PhD

Centre Internacional de Mètodes Numèrics en Enginyeria (CIMNE)
Universidad Politècnica de Catalunya (UPC)

The lecture starts by providing some geological background that underlies the wide variety of argillaceous sedimentary rocks that exist in nature. It then examines some aspects of the hydro-mechanical (HM) and thermo-hydro-mechanical (THM) behaviour of these materials concerning structure development, uplift and swelling, compressibility, strength, microfabric, anisotropy, and hydraulic properties. Those features are examined in the context of appropriate behaviour frameworks, developed for argillaceous materials. 

Two themes are then selected for special consideration. The first one is the response of argillaceous weak rocks to deep underground excavations. It will be shown that it is possible to reproduce the development of localised deformations (fractures) around the excavation using an appropriate model and a non-local formulation that avoids the pathological mesh dependence normally present when dealing with brittle materials. The second theme examines the thermal generation of pore water pressures when the rock is subjected to temperature increases. The main factors and parameters underlying this phenomenon are identified, based on a conceptual analysis. Both themes are illustrated with reference to field behaviour observed in large-scale tests performed in underground research laboratories.