ESV Euridice GIE

Safety (sub) functions concerned (if relevant)

In a HLW repository, the presence of canisters in the storage galleries, as a thermal source, will provoke an increase of the temperature in the surrounding host rock. This temperature variation will directly affect the hydro-mechanical properties of this rock. To study the long-term safety of a storage site, it is necessary to assess the influence of this thermal loading, on the host rock characteristics. Since Boom Clay is a potential host formation for HLW storage, two instrumented demonstration tests have been elaborated in the HADES URL.

The aim of the experiment is to study the THM behaviour of the Boom Clay formation around a vertical lineic thermal source representing a stacking of HLW canisters. The experimental scale was chosen as close as possible to the real scale. Globally, it was necessary to measure the THM response of the clay host rock to this thermal loading. A derivated objective was to measure the amplitude of expected independent or coupled processes in order to identify a bottom line of the observed phenomena and to propose a relevant conceptual model for the near-field behaviour.

The external diameter of the thermal probes is equal to 30 cm (to be compared to the 43 cm diameter of the COGEMA canisters) and the length of the heated part is 2 metres. The "CACTUS" programme is composed of two in situ tests. The first test (CACTUS 1) served as preliminary approach. Each test is composed, in addition to a thermal probe placed in a large diameter central borehole, of a peripheral instrumentation located in six small diameter boreholes. The peripheral instrumentation has been installed first, in order to record stabilized the hydro-mechanical parameters prior to the drilling of the central borehole. The physionomy of the instrumentation is identical for both tests.

Modelling work of the THM behaviour of the Boom Clay

C. Trentesaux, "Modelling of thermo-hydro-mechanical behaviour of the Boom Clay 'Cactus test'", Final report, EUR 17558, 1997.

Before the excavation, it seems that the mechanical influence zone of the access gallery on the clay host rock has an extension that is larger than seven times its radius. During the excavation of the first central borehole, that lasted two days, a brutal response is observed: a drop of the interstitial pressure around the excavation, a drop of radial stress and an increase of the orthoradial stress. An increase of the water content is observed, while the density does not vary a lot. During the excavation of the second central borehole (CACTUS 2), an increase of the radial and orthoradial stresses are first observed during the drilling prior a decrease in the following days. In both cases, the deviatoric stress, defined by the difference between radial and orthoradial stresses has increased during the excavation. According to analytical calculations, the deviatoric stress is proportional to the excavation radius, and inversely proportional to the square of the distance from the borehole axis.

After the excavation, the interstitial pressures and the total stress components tend to their original values. In CACTUS 2, the equilibrium of the interstitial pressures was not reached yet when the heating phase was started. A re-consolidation of the clay host rock is observed after the excavation.

During the heating phase, the temperature measurements are coherent. The temperature field has an axisymetric configuration around the probe. The temperature rapidly decreases with the distance from the heat source. After one year of heating, the temperature increase is equal to 130-140°C on the probe, while it does not go beyond 20°C at a distance of 1.5m. The thermal analysis is made independently from the hydro-mechanical analysis, for the principal heat transfer process is conduction. The experimental measurements are successfully modelled by a numerical simulation with the following parameters:
Boom Clay thermal conductivity: 1.7 W.m-1.K-1
Boom Clay thermal diffusivity: 6 10-7 m2.s-1

Three phases are observed for the THM parameters evolution in a given point of the host rock:

Initial phase: the heating phase is started, but due to the thermal inertia, no temperature variation is observed at this point;
Transient phase: the temperature rapidly increases;
Near equilibrium phase: the temperature slowly and steadily increases near the probe.

A modification of the water content during the first heating phase of CACTUS 1;
Displacements in the massif of low amplitude;
A slight increase of the density.

All the measurements could be used for the prediction of the hydro-mechanical response of a saturated clay host rock under the effect of a thermal loading. The results of the two tests coincide.

The observed THM couplings are different in nature:

On a short-term basis, the steep increase in temperature in the clay host rock gives rise to local interstitial overpressures These overpressures dissipate slowly due to the low permeability of the medium. The stress variations result from the thermal dilatation of the hot zone in the host rock and from the evolution of the interstitial overpressures
On a long-term basis, in a permanent regime, the hydraulic field has slightly changed while the compression state due to the heating is still noticed.