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Bentonite barriers are very important for disposal concepts, especially in granite, and its long term
evolution and final state need to be predicted in order to give credit to them as radionuclide transport
barriers. Water uptake leads to the swelling of the bentonite, sealing the gaps between the bentonite
blocks, and its extrusion into the small fractures intersected by the disposal drift. After the
initial THM transient the bentonite becomes a relatively homogeneous fully saturated barrier with
very low permeability that ensures that radionuclide transport through it is controlled by diffusion.
In addition the swelling pressure of the saturated bentonite ensures that the canister remains in position
without sinking. The swelling pressure also ensures that the buffer has a self-sealing capability.
For a safety case the transient THM phase is of interest in order to obtain good estimates of the
thermal evolution of the near and far field and the hydration of the bentonite barrier. It also defines
the conditions in the EBS for the long-term evolution when canister may fail and radionuclide
transport may occur.
A key purpose of the buffer is to serve as a diffusive barrier between the canister and the groundwater
in the rock. An important performance requirement on the buffer material is to not cause any
harm to the other barriers. Gas build-up from corrosion of canister iron could potentially affect the
buffer performance in four ways:
1. Permanent pathways in the buffer could form at gas break-through. This could potentially lead
to a loss of the diffusive barrier.
2. If the buffer does not let the gas through, the pressure could lead to mechanical damage of the
other barriers. The main concern is damages to the near field rock and the buffer itself.
3. The gas could dehydrate the buffer.
4. A gas phase could push water with radionuclides through the buffer along gas-generated pathways.
The expected evolution of a HLW/ SF repository in salt is the achievement of a tight inclusion of all
wastes within a time period of about a century. Therefore, the normal evolution scenario will not
lead to any release at all. Nevertheless, it cannot be totally excluded that some brine penetrates to
the wastes in an early phase, either through an improper seal or coming from an undetected brine
inclusion in the salt rock. Such disturbed evolution scenarios, unlikely though they may be, are the
subject of most PA calculations for repositories in rock salt.
The purpose of the RTD C3 in the NF-Pro project was to improve the degree of understanding of
the thermo-hydro-mechanical (THM(C)) processes described above in an integrated way to enhance
the predictive capability of the existing models, especially in relation to their link with safety functions
and the assessment of long-term safety.
The main focus of the work was to improve the handling these processes in long term performance
assessment, but issues related to the manufacturing of the EBS has also been addressed .
3. Treatment in performance assessment
The safety assessment of a repository system relies on the safety functions performed by the barriers
of the system, both in near and in the far field. As there is no practical way to control the evolution
of the repository system after closure, it is necessary to construct and to operate the repository
system in a way that ensure that the evolution of the system does not impair the performance of the
required safety functions. This means that it is necessary to have a sufficient understanding of the
processes which control the evolution of the system, to be able to anticipate that the variables of
each barrier are and remain in the ranges for which they can perform their assigned safety functions,
for the relevant time frames [2].
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