Initiation and evolution of the Excavation Disturbed/Damaged Zone (RTDC4)
The characteristics of the host rock may be locally altered due to the excavation of disposal galleries and access shafts. Parts of the rock that have been affected by the excavation are known as the Excavation Damaged Zone (EDZ) and form the transition between the engineered barriers and the intact host rock. NF-PRO’s RTD Component 4 investigates the key process leading to the development and the evolution of the Excavation Disturbed/Damaged Zone including their potential impact on overall performance of the disposal system.
Repository construction will impose changes in the mechanical conditions of the near-field environment and will lead to the perturbation of the host rock. Induces stresses may cause creep or initiate the formation of cracks and fractures in the host rock surrounding the disposal galleries in a zone called Excavation Damaged Zone (EDZ). Non permanent effects can also be observed in the host rock, beyond the damaged rock, zone called Excavation disturbed Zone (EdZ).
The loss of integrity of the host rock in the EDZ may lead to the formation of an interconnected network of fractures resulting in the formation of a preferential pathway for pore fluids, dissolved radionuclides and gases. Consequently, the impact of the EDZ is an important aspect to be taken into account when assessing the overall performance of the near-field system. The extent of this EDZ is restricted and will depend on a number of factors including the applied excavation techniques, the host rock type (crystalline rock, salt, indurated or plastic clay) and hydromechical properties properties; the repository design and dimensions (depending on the type of waste) and the nature and properties of the engineered barrier and lining material.
During repository operation, part of the rock surrounding the disposal galleries will become partially de-saturated as a result of the ventilation of shafts and galleries. De-saturation can result in the hardening of clay, creep or the formation of micro- and macro-scale fractures in the host rock surrounding the underground galleries.
In addition to changes induced by mechanical stresses, repository excavation may also lead chemical modifications of the host rock. Chemical alterations mainly result from the oxidation of reduced minerals present in the host rock, in particular sulphur bearing minerals like pyrite or minerals containing ferrous iron. On the other hand, hyper-alkaline pore fluids equilibrated with cement-based materials (for example the concrete gallery liner and cement-based components of the engineered barrier system) are chemically highly reactive with most host rock types. Consequently, interactions of this kind of fluids in the EDZ may have significant effects on the host rock properties, their role fluid transfer and the potential radionuclide transport...
The characteristics and properties of the EDZ will evolve with time. Consequently, R&D included in the Integrated Project NF-PRO covers various phases in EDZ initiation and development and has been structured according to the following time-dependent evolution of the Excavation Disturbed/Damaged Zone.
- the initial phase, corresponding to the development of the damaged zone induced by tunnel excavation ;
- the transient phase, corresponding to the operational phase of the underground facility and during which the tunnels and disposal galleries are ventilated and exposed to the atmosphere. This will lead to desaturation processes followed by resaturation processes after emplacement of the canisters and sealing of the cells. Predominant processes during the transient phase include hydro-mechanical phenomena coupled with geochemical processes (principally due to oxidation) and thermal effects.
- the long term phase, corresponding to phase after closure and complete re-saturation of the repository. EDZ evolution during this phase will be mainly determined by the thermal impact on the hydro-mechanical EDZ properties and chemical changes on the retention capacity of the damaged zone (effect of the “alkaline plume”).
Work performed as part of NF-PRO includes the application of and the development of advanced methods for the in situ identification and characterisation of the initial phase of EDZ development (in the Underground Research Laboratories in Bure, Tournemire (France) and Mont Terri (Switzerland). In addition to in situ measurements, laboratory tests are conducted and mathematical models are developed to calculate short-term EDZ behaviour and to support the interpretation of in situ observations.
The effects of the ventilation (desaturation / oxidation processes) and the resaturation of the EDZ during the phase of repository operation and waste emplacement, are assessed through in situ experiments carried out in the URLs Mont Terri (Switzerland) and HADES (Belgium); Conceptual and numerical models are developed in order to understand and simulate the role of these key processes, determining the short term evolution of the EDZ.
After repository closure, resaturation can lead to the partial or complete closing of fractures and discontinuities that formed during the initial phase and the transient period. This process, known as "self-sealing" (or "self-healing") is investigated within NF-PRO through in situ experiments and laboratory studies on rock samples. Experimental and modelling work is also carried out to investigate the migration of gases (mainly H2 resulting from the iron canister/liner alteration) from the near- to the far-field through the EDZ.
Finally, NF-PRO will integrate results from studies on detailed processes in the EDZ in more comprehensive assessments allowing for the evaluation of the general impact of the EDZ on the overall performance of the near-field system.