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SELFRAC Résumé de rapport

Project ID: FIKW-CT-2001-00182
Financé au titre de: FP5-EAECTP C
Pays: Belgium

Conclusions on the long-term performance of a nuclear waste repository in respect of the EDZ zone around excavation - Synthesis report

Performance Assessment is concerned with the potential radiological consequences of the disposal system. Hence, the analysis of the evolution of the repository system, of which the EDZ is a part, is an essential part of PA. The EDZ is initiated during the repository construction. Its behaviour is a dynamic problem, dependent on changing conditions that vary from open-drift period, to initial closure period, to the entire heating-cooling cycle of the decaying waste. Other factors concern the even longer-term issues of chemical reactions and biological activities.

The EDZ originates in the redistribution of stress that results from excavation and/or excavation method. Fracturing of the rock cannot be avoided in practice. However it is possible to limit the extents of the fractures by controlling the convergence. During the consolidation phase sealing occurs; the open fractures close progressively. During the exploitation phase, sealing can be slow or even inhibited due to the ventilation of the galleries which may prevent a full saturation of the host-rock. Open fractures favour the oxidation of the host rock. However, this oxidation phenomena has only been observed along fracture planes. Within the clay matrix, no evidences of oxidation were found. After the closure of the disposal galleries, the system will progressively resaturate and suction will disappear.

The backfilled galleries will be saturated with water. Favourable conditions to sealing will be recovered. In case of indurated clay host rock bentonite backfill material generating swelling pressure are considered to accelerate the sealing process. Consequently, it can be assumed that the remaining open fractures will seal and reducing chemical conditions will progressively be restored within the EDZ. Once saturation is attained, the thermal load may cause an increase of stresses because of thermal expansion of solids and pore water and heating-related reduction of strength. This process could further increase the size of the EDZ but in turn, heating may also increase creep rates and thus accelerate the closure of open fractures and reduce the influence of EDZ. Peak temperatures within the EDZ should normally be reached after a few tens of years. A subsequent slow cooling phase will follow. Finally, EDZ and engineered barriers will chemically interact.

Safety assessment calculations for Opalinus Clay and Boom Clay showed little impact of the EDZ on radionuclide release even for very conservative assumptions. The different experiments realised in the frame of SELFRAC show that the effective hydraulic conductivity of the EDZ in the considered rocks will be lower than 10-10m/s within several years as soon as the bentonite backfill of the emplacements drifts become fully saturated and the expected swelling pressure will be build up. The hydraulic parameters of natural fractures in Opalinus Clay indicate that the long-term conductivity of the EDZ is even lower. Consequently, the maximum hydraulic conductivity of the EDZ is expected to be approximately one order of magnitude higher than the value of intact rock, which is in the case of Mt. Terri in the overall range of 2x10-12 to 2x10-14m/s (Heitzmann, 2004).

Moreover the assumption that the presence of an EDZ provides a fast path for the escape of solutes from the canister to the geosphere and biosphere is now recognized to be an oversimplification. It may be true that the EDZ is, at least over a period of time, a zone of relatively high permeability, but whether flow can take advantage of it to transport solute to the accessible environment requires an evaluation of the total flow system. Thus, if the high-permeability zone is surrounded by low-permeability regions, or the hydraulic gradient is sufficiently low, there will be an insufficient supply of flowing water in the EDZ to negatively impact the repository performance.

Consequently EDZ should not be considered as a critical issue for the performance assessment of radioactive waste repositories in argillaceous formations. For the safety cases, however, a profound scientific knowledge on the subject is necessary. The results of the SELFRAC project undoubtedly strengthen the sound scientific background. Nevertheless there are some important remaining questions, mostly related to the response of the EDZ to gasses and chemical and thermal changes through time. The understanding of these phenomena is also necessary to support future safety cases.

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