Architecture and management of a geological repository - Andra

• Absence of a significant impact on the safety functions of structures and on thefavourable properties of the geological mediumThe monitoring equipment used must not affect long-term safety. On the one hand, this concerns anyirreversible changes caused by monitoring equipment during operation of the repository and, on theother, the consequences of abandoning some of the equipment in the structures after they have beenclosed.First and foremost, the layer of undisturbed host rock between the repository and the adjacent layersmust not be disturbed by the presence of instruments. The measurements made in the various Callovo-Oxfordian horizons shall thus either be non intrusive, or situated in the near field of the shafts andunderground structures.The risk of a chemical impact must also be taken into consideration. It shall be overcome by selectingmeasuring instruments made from appropriate materials: The materials inserted (metals, silicas etc.)and any of their corrosion products are similar to those also used in the construction of the structures.Special care shall be taken to ensure that no organic materials areintroduced into the repository, forexample in the form of hydraulic fluids present in pressure cells. The risk of impact on the evolutionof the excavation damaged zone on cell walls depends on any voids that monitoring equipment mightcause in the long term after their complete degradation. However, the volumes occupied by measuringinstruments are small compared to the structure construction and package emplacement clearances.For the same reasons, the presence of monitoring equipment inserted into a swelling clay buffer or aseal does not compromise the swelling and space-filling capacity of the clay they are made of.Finally, and above all, the use of signal transmission equipment requires particular attention in order toavoid any downgrading of the hydraulic behaviour of the geological environment and seals. In fact,the presence of cables or fibre optics could create short-circuits for water flow and the transfer ofradioactive nuclides. Three possible approaches are considered in this respect: not to pass cables oncea seal is being constructed, tolerate cables through seals during a transitional period, then restore theseal, or by-pass the seal during a transitional period, then seal the lateral borehole used for thispurpose.Avoiding passing cables requires either that measurements upstream of the seal are abandoned as soonas it is constructed, or the use of wireless transmission through the geological medium or through theseal. The latter option represents an interesting option for development. It is based, in particular, onexperience feedback from the oil industry and is used in the Meuse/Haute-Marne undergroundlaboratory (EPG probe). Furthermore, the feasibility of wireless transmission in the specificenvironment of disposal cells is the subject of a research and development programme betweenANDRA and the Japanese Radioactive Waste Management Funding and Research Centre (RWMC).In particular, the question of operational life expectancy must be examined.The temporary passing of cables could also be envisaged when constructing seals consisting of precompactedswelling clay bricks. During final closing, the seals could be renewed by recovering thecables, for example by over-coring, followed by sealing the excavation thus made.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM391/495