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ous reasons, SCK•CEN chose to focus on the Boom Clay as potential host formation. After a few
years (1980), the programme resulted in the start of the construction of the HADES Underground
Research Facility (Fig. 1), demonstrating the feasibility of constructing in this type of clay at a
depth of 223 m below surface. When the newly established Belgian Waste Management Agency
NIRAS/ONDRAF took over the R&D programme, the first experimental results from HADES confirmed
the Boom Clay as the reference formation for carrying out its studies on the long-term management
of HLW. An expert assessment in the late eighties confirmed the NIRAS/ONDRAF conclusions
on the suitability of the clay formation for HLW disposal. The Boom Clay had been found
to have a very low hydraulic conductivity, a plastic character with good self-sealing properties and
a high capacity to fix radionuclides and hence, to delay their migration towards the biosphere.
These encouraging results prompted SCK•CEN and NIRAS/ONDRAF to launch an ambitious demonstration
programme with the name PRACLAY. Its main objectives dealt on one hand with demonstrating
industrial techniques suitable for constructing a real repository, and on the other hand to
operate a dummy disposal gallery through a heater test during at least 10 years. Demonstration of
industrial techniques was an important aspect as the construction of the first part of HADES had an
exploratory nature (feasibility). The (mainly manual) excavation and construction techniques had
further disturbed significantly the host formation. In the perspective of a real repository, demonstration
of applicable mechanised techniques was therefore essential. The construction of the
dummy disposal gallery would then allow simulating all repository operation phases up to the heating.
The first objective has been fulfilled through the construction (Fig. 1) of the second shaft, the
connecting gallery, and the PRACLAY test gallery (including the gallery crossing) in 2007.
Figure 1: Construction history of HADES, with the first part (right) from the 1980's, and (left) the
extension in the frame of the PRACLAY project.
2. Methodology
2.1 Preparing for the large scale test – the ATLAS test
A HADES experimental set-up from 1992, named ATLAS, has been reactivated to get additional
data for thermal modelling purposes. The original set-up had been installed in the frame of the EC
INTERCLAY II project (1990-1994) and consisted of a 8 m-long heater (11 to 19 m deep) with two
parallel observation boreholes – all in the horizontal plane. To get a larger monitored zone and to
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