In all nuclear power generating countries, spent nuclear fuel and long lived radioactive-waste management is an important environmental issue today. Disposal in deep clay geological formations is one of the promising options to dispose of these waste. An important item for the long-term safety of underground disposal is the proper evaluation of the Damaged Zone (DZ) in the clay host rock. The DZ is defined here as the zone of the host-rock with Thermo Hydro Mechanical and Chemical (THMC) modifications induced by the repository, with major changes in the transport properties of radionuclides. These transport properties are the low permeability of clays, a slow diffusive transport combined with the absence of preferential migration pathways for solutes and some sealing capacity. The DZ can therefore be considered as the part of the disturbed zone (dZ), which could impact the Safety Functions of the host-rock.
The priorities of the TIMODAZ project have been set on the study of the combined effect of the Excavation Disturbed Zone (EdZ) and the thermal impact on the host rocks around a radioactive waste disposal. More specifically the extent and the evolution of the Damaged Zone inside the disturbed zone will be assessed.
The TIMODAZ project will study the inter-related processes involved in the evolution of the clay during the thermal transient. Three types of clay will be investigated: the Boom Clay, the Opalinus Clay and the Callovo-Oxfordian argilitte. Even if the characteristics of these clays are different, the THM processes governing the fracturing and the sealing present some similarities.
In order to strengthen our knowledge of fracturing and the sealing processes under evolving thermal conditions, specific laboratory tests will be performed. In particular, the effects of temperature on damaged clay as well as on clay properties will be investigated. Special attention will be given to study the possibility of the creation of an irreversible damage. The tests include the study of desaturation/resaturation processes at ambient and higher temperatures. The recent developments in transmission/emission tomography are promising to study heterogeneities and the evolution of fractures, density and water content of samples during temperature-controlled geomechanical tests. Some tests will be complemented with a radionuclide migration test, in order to evaluate any possible relict of preferential migration along the sealed fracture. Different chemical conditions will be considered: chemical undisturbed, oxidised and alkaline environment. Mineralogical analyses will be performed and linked to the hydromechanical observations. These laboratory tests will provide the necessary data for the numerical models to be used in TIMODAZ.
Results from previous and current THM in-situ tests will be available for the project. An additional small scale in-situ THM test will be conducted at Mont Terri as an extension of the SELFRAC test configuration. The results of these in-situ tests will be very useful to provide the link between pure lab testing and full scale tests. Different numerical codes will be evaluated by participation in benchmark tests for modelling of THM processes in clays. Sealing and chemical processes induced by THM phenomena will also be incorporated in some computer codes.
The thermal impact on the stability of the gallery lining will also be investigated. This issue is particularly important in term of operational safety when the retrievability of the radioactive waste is considered.
The modelling work together with the results of the lab and the in-situ tests should give clear indication on the evolution of the DZ with time upon temperature evolution. An important objective of the project will be to perform predictive simulations of the large scale heater experiment PRACLAY that will be performed in the HADES underground research facility at Mol in Belgium.
The results of the TIMODAZ project will be situated in the context of the long term performance of a repository. All of the experimental works to be performed in TIMODAZ will contribute to a better understanding of the processes occurring within the clay around a disposal system for heat-emitting waste in clay during the thermal transient phase. As this transient should span over several centuries, the development and testing of sound, phenomenology-based models is an essential step towards meeting the Safety Case requirement of adequate understanding of the long-term evolution.
The TIMODAZ project will answer to the following key questions:
- What will be the clay evolution around a disposal system of heat-emitting waste during the thermal period?
- What are the uncertainties on the evolution of the EDZ and TDZ? How can these uncertainties be dealt with?
- How are the clay properties modified as a result of the thermal, mechanical and chemical conditions during the thermal period?
- Under which temperature conditions do the changes in clay properties become irreversible?
- What is the extent of the alterations from a PA point of view?
The knowledge gained within the TIMODAZ project will allow to assess the significance of the TDZ (Thermal Damaged Zone) in the safety case for disposal in clay host rock and provide direct feedback to repository design teams. In order to ensure an appropriate and continuous link between the end-user needs and the priorities of the TIMODAZ project, the following end-user group has been constituted: ONDRAF/NIRAS (BE), NAGRA (CH), ANDRA (FR), RAWRA (CZ), ARAO (SI) and RATA (LT). This group will be active throughout the duration of the project.