Architecture and management of a geological repository - Andra

• Water contentThe water content of a material can be monitored locally by using a TDR probe (Time DomainReflectometry or electromagnetic reflectometry). It exploits the difference in permittivity betweenrock and water, and allows the water content to be calculated from the permittivity of the porousmedium. This type of probe must be calibrated specifically for the material concerned (back-fill,swelling clay) or the rock to be monitored. It is only used in accessible zones, in order to allow regularmaintenance.The distribution of water content, for example in the near-field of a drift, can also be monitored byelectric tomography, by installing a set of electrodes in the walls and in bore-holes. Severalexperiments carried out by Gesellschaft für Anlagen- und Reaktorsicherheit (GRS), especially in theunderground laboratories of Äspö and Mont Terri, have demonstrated the technical performance ofsuch a system. They demonstrated in particular the capacity to monitor the extent of the desaturatedzone in the walls of the structure.• TemperatureTemperature measurement is the mostreliable and well-proven of all thedifferent types of measurement. Thetemperature is monitored on allstructures, either to contribute to theanalysis of other types of measurementsaffected by thermal variations (in roadFigure 10.3.5 Vibrating wire thermometretunnels depending on the season and thetraffic for example), or to check theexpected thermal variations or to monitor a dimensioning criterion. The most reliable means ofmeasuring the temperature at a specific point is by using a vibrating wire sensor (Figure 10.3.5). Thistype of sensor is particularly robust and has the benefit of a large body of long-term experiencefeedback, in particular in tunnels (The Chamoise tunnel and the Channel tunnel).Moreover, for around ten years,distributedtemperaturemeasurements on long profilesections have been carried outusing fiber optic systems [104](Figure 10.3.6). In view of themore recent use of fiber optic, it isrecommended to combine the twotechniques, operating redundantly.The point measurement sensors andthe fiber optic sensors can beburied in concrete, placed in a boreholeor in backfill, or left in theatmosphere inside the structures.PVCDiameter 10 mmFigure 10.3.6 Fibre optic to take distributed temperaturemeasurements• Concentrations of toxic gases, corrosion, contaminationKevlarFiberProtectionPhysical and chemical sensors used to measure pH or concentrations of hydrogen or oxygen are fittedat a number of industrial sites. They nevertheless have a limited lifetime and require regular recalibration.They can therefore only be used in accessible zones.For example, hydrogen detectors based upon electrochemical or semi-conductor technology arecurrently available. Humidity, temperature, radiological atmospheres or even the length of the cablecan all interfere with their operation and they require regular re-calibration.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM397/495