Measurements of gas generation, water content and change in the water distribution in a heater experiment in the underground laboratory Mont Terri

Geologic clay formations are foreseen for disposal of high level radioactive waste. In order to investigate the suitability of the disposal methods as well as the alteration of the host rock with regard to the mineralogy, chemistry, and mechanical behaviour, a heater experiment has been performed in the Opalinus clay of the Mont Terri research laboratory in Switzerland.An electrical heater was installed in a vertical borehole in the clay rock. Heating up to almost 100 °C started in February 2002 and was finished in September 2003.In this paper the results of gas release, pore water distribution, and pore water chemistry in the clay rock as a consequence of heating are presented.Carbon dioxide and hydrogen sulphide are the most prominent gas components released from the Opalinus clay. These components are important for long-term safety aspects as they may interact directly with the waste container, the solidification matrix, or with the waste itself. Additionally, these components will alter the chemical milieu in the test field. They may be generated by chemical reaction (oxidation or decomposition) or by microbiological activities. As the pressure increase in the boreholes did not coincide with the amount of the released gases, it is concluded that the Opalinus clay in the test field is not gastight.The geoelectric measurements on water distribution showed a resistivity decrease during the first months of heating as a result of the temperature increase; afterwards, resistivity remained constant. The in situ tomography measurements as well as pre- and post-heating laboratory tests showed that no desaturation effects occurred in the clay rock during the heating phase.The water extracted from the boreholes indicated a higher content of bicarbonate, which indicates the dissolution of carbonates. The value of pH is almost neutral, slightly lower than expected.