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PReliminary Demonstration Test for Disposal of High Level Radioactive Waste in CLAY (CERBERUS)


PRACLAY aims at demonstrating the constructive and operational soundness of the Belgian Disposal Facility for High Activity Vitrified Waste in the Boom clay layer, 230 m under the surface in North-Belgium, with the following four steps:

a) excavate a mini-tunnel similar, except in length, to the contemplated disposal galleries, using the industrial techniques that would have to be dealt with for the full-size repository.
b) erect and support a large connecting chamber of the type needed at the intersection between main and disposal galleries.
c) install and operate a dummy gallery geometrically identical, except in length, to an actual disposal gallery.
d) monitor the thermal and mechanical behaviour of the clay layer, gallery concrete lining, filling material and metal shroud surrounding the dummy waste.
This project aims to demonstrate the constructive and operational soundness of vitrified waste disposal facilities, to investigate the near field effects produced by a combined heater and radiation source emplaced in the Boom clay, and to monitor the short and long term mechanical behaviour of tunnelled structures in Boom clay.

To date, the project has focussed on the design, selection and justification of the demonstration experiment main features, the gathering of technical information needed for selection and installation of the monitoring and heating equipment, and a comparative assessment of the available technologies for excavating the repository galleries and erecting their lining.

The CERBERUS test (control experiment with radiation of the Belgian repository for underground storage) is aimed at simulating the near field effects in an argillaceous environment of a Cogema high level radiactive waste (HLW) canister after 50 years cooling time. Various validation exercises of computer codes such as the program DOSEGEO (computation of dose rate), the program SOURCE (computation of pore water pressure around drilling and digging works) and the program TEMPPRES (computation of the thermohydromechanical behaviour of the Boom clay) are utilized.

The measurements of the dose rate have been performed by means of an ionisation chamber, Calcium sulphide: dysprosium, lithium fluoride and perspex dosimeters were also used. The results are compared to those obtained with the DOSEGEO model.
In the hydrological field, it appears that the pore water pressure are very sensitive to any temperature variation, and raising the pore water pressure can generate cracks.
The first simulation using the TEMPPRES model and the best available parameters for the behaviour of the Boom clay at high temperature was performed. It indicates that the general trends of the pore water pressure evolution can be reproduced, but the maximum value occurs later and the pore water pressure is generally overestimated in the simulation.

Follow up and monitoring of the long term behaviour of the concrete lined part, also called Test Drift, of the HADES underground research facility, of its terminal shotcreted front and of the surrounding clay mass.

Measurements around the Test Drift show a slow stabilization.

Comparison between pressure and deformation on the different measuring sections show a good agreement of the convergence and confinement curves.

Comparison between direct measurements of the lining pressure (by way of pressur
e cells) and calculated ones (from load cells measurements) show that pressure cells measurements are systemically lower than those of the load cells, but that the pressure build up is similar.

Measurements of the displacements in the clay mass above the Test Drift (settlingmeter and inclinometer devices installed before construction) were compared to those, at the same distance of the gallery axis, of the extensometer installed during the construction: the delayed convergence (the displacement after installation of the lining, measured by the second device) is only a small part of the total one. This is confirmed by the comparison between the clay wall displacement (inferred from the first type of devices) and the lining diametrical convergence.
Work programme:

- Carry the engineering work needed for detailed design of the experiment
- Review the multiple experience gathered in the Boom clay with the needed monitoring devices
- Erect a large connecting chamber
- Establish the detailed instrumentation programme
- Excavate the mini-gallery and install the equipment (dummy waste, instrumentation, etc)
- Operate and monitor the experiment not covered in the present contract
- Proceed with interpretation of results period

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