Thermo-hydro-mechanical (geochemical) processes in the engineered barrier system (RTDC3)
In most repository concepts under investigation of EU Member States, the engineered barrier system includes buffer materials, which are emplaced around the disposed waste canisters. Clay-based buffer materials will progressively hydrate and act as a low permeability barrier and play an essential role in ensuring the overall performance of the repository since they control and limit the migration of radionuclides released from the disposed waste after closure of the repository. However, various factors and processes such as thermal gradients in the area surrounding heat-producing waste, changes in the mechanical conditions of the near-field, groundwater inflow-outflow and the movement of gases produced as a result of radioactive decay and the corrosion of steel will induce changes in the mechanical conditions of the near-field. An important part of the work performed by NF-PRO concentrates on processes affecting the properties and the behaviour of the buffer. Main emphasis is on the investigation of the combined effects of thermal, geomechanical and hydrodynamic processes on the performance of the buffer. To this end, experimental work is carried out on different types of buffer material (bentonite, salt), at different scales (from laboratory to real scale), and under different degrees of saturation and thermal conditions.
The Engineered Barrier System (EBS) of a geological repository encloses the disposed radioactive waste by isolating it physically and chemically from the natural environment. The EBS controls the in- and outflow of groundwater, the dissipation of heat generated as a result of radioactive decay and stress and strain conditions in the near-field environment. Accordingly, thermal, hydraulic and mechanical processes are (in addition to chemical) principal factors to be taken into account when evaluating the overall performance of the engineered barrier system. In geological repository systems, these processes are coupled and their relative importance and impact evolves with time.
NF-PRO's RTD Component 3 investigates the impact of thermal, hydraulic and mechanical processes as well as their interactions or couplings on the barrier performance of the EBS system. An important part of R&D performed as part NF-PRO involves the hydration of bentonite buffers and its effect on the barrier performance. In a geological repository for high-level waste disposal, gases will be released from the disposed waste and the waste container due to radiolysis and corrosion of metals. To this end, data will be obtained from a large-scale gas migration test and used for validating numerical models that were developed for predicting the long-term effects of the released gases on THM properties of clay buffer. These investigations are performed within NF-PRO as part of Work Package 3.3.
Work performed in the framework of the Integrated Project NF-PRO includes the operation of the FEBEX in situ test at Grimsel Test Site (Switzerland). FEBEX is a full scale integrated experiment investigating the hydraulic-mechanical behaviour of the bentonite buffer and the surrounding rock of a geological repository for the disposal of heat-producing radioactive waste. In combination with experimental work, mathematical models are developed and tested in view of validating and refining models for calculating the saturation and swelling of the bentonite buffer, heat dissipation in the near-field and alterations of the backfill.
Crushed salt is commonly applied as a backfill material in geological repositories for radioactive waste disposal in salt formations. The mechanical performance of the crushed salt buffer, in particular its response to compaction as well as its behaviour during interaction with high saline brines are key factors with regard to the overall performance of the disposal system. NF-PRO’s Work Package 3.5 includes experimental and modelling studies in support of these investigations.
Finally, NF-PRO’s RTD Component 3 includes a phenomenological analysis of the THM (C) behaviour of clay and salt buffer materials (Work Package 3.6). This analysis will allow for an in depth understanding of the evolution of the properties of buffer materials under various hydrological, thermal and mechanical conditions relevant for different disposal systems. The outcome of this WP is of particular importance for the evaluation of the overall near-field system performance.