PRACLAY experiment in line with scientific and technical expectations

Newsletter No 3

Since November 2014, EIG EURIDICE has been conducting a large-scale heating experiment in the PRACLAY gallery of the underground laboratory HADES in Mol. The goal of this experiment is to study the impact of heat, coming from high-level radioactive waste, on Boom Clay. Until today, a little more than three years after its start in 2014, the complex experimental set-up has proved to be technically very robust. The heating system, with its control and visualization functions, is fully operational; the seal structure, closing off the heated and pressurised part the gallery, works as planned; and the concrete gallery wall has remained stable. No abnormal evolutions have been observed scientifically and the observations confirm and substantiate the existing knowledge of the clay behaviour. The experiment shows that the favourable properties of the clay to confine high-level radioactive waste do not change significantly and are preserved during heating. This intermediate conclusion also demonstrates the importance of this experiment for the ONDRAF/NIRAS’ research programme on geological disposal of radioactive waste.

Second report

On 19 August 2015, a little less than 10 months after the switch-on of the heating system, the intended temperature of 80°C at the interface between the concrete gallery and the Boom Clay was reached. This was the start of the ten-year stable heating phase at 80°C.

The main measuring results of the starting phase and the first two years of the stable heating phase have been compiled in a second technical-scientific report titled “The PRACLAY Heater test after two years of the stationary phase”. In this report, EURIDICE makes a first evaluation of the objectives of the experiment by comparing the observations with the prior model calculations. The report can be downloaded here. It is only available in English. A printed copy can be requested at

Cover PRACLAY rapport

Temperature evolution in the clay

In order to study the changes in clay as a result of heating, the researchers installed measuring instruments in a network of boreholes around the PRACLAY gallery. The chart below represents the evolution of temperature since the start of the experiment on 3 November 2014. The displayed temperature measurements are those in a vertical borehole (yellow line on the drawing), leaving from the heated part of the gallery. The temperature measurements confirm the existing models.

In the following years, the heat will further spread through the clay and temperature changes will also be observed at a greater distance from the heated zone. This way, scientist can further refine their model calculations  and knowledge of the clay behaviour.

Schedule PRACLAY

Figure - The points on the dashed lines show the temperature (T) of the clay in function of the distance to the gallery, measured in a 20 meter long vertical borehole (yellow line on the drawing). At the start, the natural temperature in the clay was 17°C (blue curve). On 19 August 2015, the intended temperature of 80°C where the gallery meets the clay was reached (red curve). Since then, the temperature where it meets the clay has been kept constant. In the clay, further away from the gallery, the temperature continues to rise with time. The orange curve represents the situation at the end of 2017.


Would you like to know more about the goals and set-up of the PRACLAY experiment? Make sure to read our first newsletter.