Seals monitoring systems using wireless communications

Document available in extended abstract form only. Wireless monitoring based on electromagnetic waves is a promising application for deep geological nuclear waste repositories. It should allow data transmission without installing wires across the various seals (disposal cell plugs, gallery plugs, shaft plugs). Developments of the wireless system (e.g. transmitter and receiver) are in progress in order to fit the repository requirements. A common research program has been elaborated by RWMC and Andra. The present work aims at developing the wireless monitoring technology to intermediate level waste (ILW) disposal facilities concept. In this concept, ILW packages will be emplaced in disposal cells with concrete liner. After the operational phase, the cells will be backfilled with sealing material. In practice, this work demonstrates the feasibility of adapting and optimizing the wireless transmission system for specific repository cases. After preliminary transmission studies, it was decided to make a representative test in situ of a wireless transmission through the clay from a sealed side to an accessible side of the repository. In order to reduce the attenuation of magnetic flux caused by steel components between the transmitter and the receiver, the receiving antenna is installed in a dedicated borehole (drilled from the accessible side). Two types of reception antennas have been designed. According to its coil orientation, type A antenna measures the electromagnetic wave perpendicular to the borehole axis. On the other hand, type B antenna with a coil set in-line with the tubular casing, measures the electromagnetic wave parallel to the borehole axis. The outside cylinder (pressure tight case) is made of PVC considering the attenuation of electromagnetic flux. According to the direction of electromagnetic flux and position of the boreholes in the final repository design, type A or type B will be chosen. For the 'representative' test, a borehole, TSF1002 has been drilled in the first experimental gallery of Andra's underground laboratory. On the 'sealed' side, e.g. TSF1001 (400 mm of diameter), a battery-powered monitoring system has been installed. This autonomous system includes three pressure sensors and a temperature probe. As shown in figure 2, wireless transmission and measurements have been perfectly working since the installation. This transmission is designed to last more than ten years. In figure 2, blue and brown lines show the pressure variation in the TSF1001 boreholes, between -4 and -7 m, where 3 meters of bentonite are hydrated periodically. Since September 2011, water has been added in this borehole, in order to increase the bentonite pressure (cf. PRT measurements on the time chart). The green clear curve, related to the pore water pressure, indicates that the bentonite is not totally hydrated. When the saturation mechanism will begin, the pore water pressure will increase. Transmission numerical analyses Numerical analyses were carried out by using software for three-dimensional electromagnetic field analyses called JMAG. In order to analyze the electromagnetic phenomena which occurred in the drifts and boreholes, precise modeling of electromagnetic field and variety of materials which will affect the electromagnetic flux is inevitable. On this case, JMAG has an advantage of high capacity of analysis and efficiency of modeling. Figure 3 shows the magnetic density of all direction. A robust and efficient system was deployed for underground seal monitoring. The results of simulations of these wireless transmission match with the measured values. There is a case that the received voltage is higher than the theoretical value because the magnetic flux is amplified by the steel pipes. (authors)