Architecture and management of a geological repository - Andra

10.3.1.1 Overview of the principal motivationsThere are three main reasons for conducting observation and surveillance of the repository: (i) to assistin reversible repository management, (ii) to contribute to the mastering of operational and long-termsafety (iii) to supply additional information through geological reconnaissance.The term “observation” corresponds to the first objective (to assist in reversible management). Theterm “surveillance” corresponds to the second objective (contributing to the mastering of safety). Itshould be noted that the measurements made for suveillance are essentially the same as those thatassist in the reversible management of the repository. The reason for this distinction is purely to dowith the manner in which the measurements are used.10.3.1.2 Assist in reversible repository managementObserving the repository makes it possible to monitor the principal phenomena associated with eachsuccessive stage of the disposal process, strengthen our understanding of the disposal process and thuscontribute to its reversible management. It in fact enables the evolution of the structure and geologicalformation to be characterised during the operating phase, based on a known initial state. Theinformation that it provides represents an important element in decision-making. As an example,observation informs the operator of the temperature reached in the cells, the state of corrosion of metalcomponents or the level of stresses placed on the linings by the geological formation. These elementscontribute to (i) the assessment of the relevance of moving on to the next stage, (ii) the assessment ofthe feasibility of retrieving the packages, (iii) preparing any changes to the design of the nextstructures to be built, particularly in terms of geotechnical dimensioning.10.3.1.3 Contributing to operational and long-term safetyRepository surveillance is aimed at detecting changes that might affect operational safety or long-termfunctions. It makes it possible to increase knowledge of models and parameters involved in theassessment of long-term performance.For operational safety, it aims to prevent the development of dangerous situations in order to providepersonnel and environmental protection. As an example, it provides information about the ambientconditions in the structures (gas, dust, temperature), control of non-dissemination of radioactivity aswell as mechanical integrity and dimensional stability of the structures. However, this conventionalaspect of the operation of nuclear installations will not be discussed in detail in this chapter.For long-term safety, Basic Safety Rule N° III.2.f ([2] - Annexe 1) recommends that the repository beinstrumented 140 . Generally speaking, it is a question of confirming the existence of conditionsconducive to long-term safety. In order to do this, the measurements made are aimed at ensuring thatany short or long-term changes to the structures and adjacent argilite remain within a range compatiblewith subsequent long-term changes, as predicted in the models. Monitoring thus provides informationabout the respect of temperature limits, the short-term state of the damaged argilite zone around thestructures and its medium term evolution, the corrosion of type C (or spent fuel) waste disposalpackages and the detection of any chemical interactions between materials (concretes, argilite, sealswelling clays). The data thus acquired makes it possible to confirm the initial data used in the longtermevolution simulations.Under no circumstances is it intended here to base the assessment of a repository’s safety on itsmonitoring. However, in accordance with the practices in existing nuclear installations, theinformation gleaned from monitoring would make it possible to update the safety reference base,introducing the possibility of carrying out periodic reassessments.140 “In view of the extended period covered by the operation of the repository and the disturbances induced during this period, it appearsessential to provide appropriate instrumentation for monitoring changes in the parameters associated with the site and structures. Thisinstrumentation should be installed as soon as possible, so as to provide monitoring of the structures and site, not only during but alsobefore the repository operating period.” (Excerpt)DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM385/495