Architecture and management of a geological repository - Andra

10.3.4.1 Lifetime of monitoring equipment and environmental conditionsThe equipment used to monitor the structures must therefore be able to operate reliably over longperiods of time (at the century scale).Moreover, as soon as the package emplacement phase begins, the disposal cells will no longer beaccessible. Consequently, it must be possible to take remote measurements and it will be difficult toreplace a defective sensor during or after the filling of the cells. The same applies to back-filling orsealing of drifts. The distribution and number of measuring instruments in these structures musttherefore make allowance for certain sensor failure rates, so as to be able to have sufficient dataavailable for as long as possible in order to monitor the evolution of the structures.For the most part, the cells represent harsh environmental conditions, in terms of radioactivity andtemperature, under which the monitoring equipment will have to operate correctly. The sensors musttherefore be able to operate correctly at a temperature approaching 100 °C in type C (and spent fuelCU) cells, and up to 60 °C in the liners of cells containing low exothermic type B waste.The operation of the sensors, particularly their electronic components may also be disturbed by theeffects of radiation. Depending on the equipment available, the sensors selected will be as invulnerableas possible to the effects of radiation and placed in the cell in a manner that minimises these effects.Other environmental conditions are not specific to radioactive waste disposal but related more to thegreat depth of the structures and to the small quantities of water contained in the rock (such conditionscan be encountered in deep road tunnels). In particular, precautions must be taken when fitting sensorsdesigned to measure the high interstitial pressures around structures.The conditions in the cells, however, limit losses of sensors due to corrosion, which are frequent inroad or rail tunnels. In fact, the concrete liner and on rock walls of type B disposal cells are highly desaturatedand the ventilated air in them is relatively dry. In type C cells, corrosion occurs mainly underanoxic conditions and, thus, very slowly.10.3.4.2 Discretion of monitoring equipmentThe “discretion” of a monitoring device is its ability to limit any disturbance it might cause in themonitored object to an insignificant level. This discretion usually means that the measurement is notsignificantly influenced by the presence of the instruments. In the repository context, it also means thatthe use of monitoring equipment shall not have a significant impact on (i) the operating conditions, (ii)the mechanical strength of the structures, and (iii) long-term performance (particularly the hydraulicproperties of the seals and rock). “Discretion” is therefore a major constraint to be considered in thedevelopment of a repository observation and surveillance programme.• Absence of significant impact on the operating conditions of the structuresThe implementation of monitoring shall not interfere with the operation of the repository. Thus, anytransmission cables used must not impede the placement of packages in cells. In order to achieve this,they may be integrated into the liner or secured to the excavated wall of the drift or the cell. Moreover,care must be taken when using a monitoring system to avoid producing sparks, particularly in cellsliable to produce hydrogen. Any power sources, for instance batteries fueling wireless transmission,are prone to this risk.• Absence of a significant impact on the mechanical strength of the structuresAmong other things, the use of monitoring instruments is motivated to supply data to supportreversible management of the repository. This must not be achieved at the cost of a reduction in thereversibility. For example, the integration of monitoring devices into the structures must not lead tomore rapid deterioration of their mechanical strength or their dimensional stability. Indeed, theintegration of sensors into concrete, frequently practised in civil engineering, can be used withnegligible impact on the structure, given the small size of the objects inserted.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM390/495