Architecture and management of a geological repository - Andra

The deformation measurements carried out on the sleeve indicate the zones that are compressed, inresponse to loads due to a combination of ground pressure and thermal expansion of the sleeve.Temperature monitoring allows accurate interpretation of the data.Also, the monitoring of temperatures on the sleeve allows us to check compliance with the thermalcriteria (T < 90 °C in contact with the argilite).The monitoring equipment installed on the upper part of the sleeve and at the head of the cell issubject to relatively high temperatures. It is also subject to a dose rate which may contribute to thedeterioration of the electronic circuits. The dose rate is approximately 2 Sv/h on the side surface of aC2 disposal package (envelope for the other families of C package) [107]. The sensors tend to absorbthe dose rate received, and this leads to a gradual loss of reliability. It will therefore be necessary toquantify the reliability of these sensors subjected to radiation in the long term.However, the monitoring equipment fitted in the bore-holes are not subject to high level constraintsafter the packages are put in place. The temperatures remain relatively low for around ten years andthe dose rates emanating from the cell are absorbed by the rock. The experience feedback shows thataround 90% of sensors remain operational for a period of several decades. They provide redundancyas regards the losses expected in the sensors integral with the cell's components.In an analogous manner to the B control cells, the monitoring system as set up during construction fora C control cell can therefore in theory be operated without maintenance for several decades duringthis stage. It should nevertheless be noted that after an indeterminate period, there is a risk of losses ofthe sensors integral with the sleeve, whilst sensors integral with the metal plug can be replaced asrequired.10.3.8.3 The evolution of the C control cell monitoring system after the cell is sealedInstalling the closure plug does not modify the environmental conditions in the cell to a great extent,neither does it have implications for the operation of the bore-hole monitoring equipment.It does however modify the measurement transmission conditions of sensors integral with the cell.Firstly, unless a customised system has been provided for during the installation, wired transmissionon the outer surface of the sleeve is cut at the metal plug, as soon as the temporary sleeve is removed.Secondly, the transmission of measurements from the head of the cell (attached to the metal plug)must be adapted for the presence of the cell plug.It is for example possible at the time of construction, to fit the control cell with a wired transmissionsystem redirected through a lateral bore-hole near the metal plug. It would not be disturbed by theremoval of the temporary sleeve, nor by the fitting of the cell plug. This option makes it possible intheory to extend the monitoring of the sleeve and the cell atmosphere up to the next stage, that is tosay the closure of the operating unit. Before carrying out the closure, it is nevertheless necessary to cutthe wired equipment and to seal the lateral bore-hole, in order to restore the hydraulic propertiescompatible with long term safety.As in the case of B cells, it is also possible, during a transitional phase which may be as long asseveral decades, to install a wireless transmission system. A common RWMC/ANDRA researchproject focuses in particular on the transmission from the useful part of a cell, through a swelling clayplug supported by a concrete plug, to the drift. In the first stage, the transmission system could beintegrated into the metal plug, as illustrated in Figure 10.3.17.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM411/495