On the Development of a Self-Sealing Experimental Setup for Natural Claystones Considered as Potential Host Rocks for Radioactive Waste Disposal: Application to the Sandy Opalinus Clay

ABSTRACT A new experimental device allowing the follow-up of the self-sealing of damaged claystone has been developed. This device consists of a constant volume cell with a displacement transducer tracking the closure of the artificially created diametric fracture. Two additional sensors were installed to obtain additional information: a total pressure sensor monitoring the radial swelling pressure and a mini-pressure sensor inserted within the diametric fracture that measure the pressure changes during the sealing process. To simulate in situ conditions, a series of three small-scale mock-up tests were performed on artificially fractured Opalinus Clay samples from the lower sandy facies of the Mont Terri site. The solutions used were synthetic water with the same salt concentration and pH as the natural neutral pore water, a sodium nitrate (NaNO3) saline solution, and an alkaline solution. Results showed that the end frictions, identified from the evolution of the radial swelling pressure, together with the mini-pressure sensor, control the fracture closing. The mini-pressure sensor that was installed in the first test immediately fell out of service because of hydration, and the radial swelling pressure was affected by the rigid-rigid contact between the sample and the total pressure sensor. This series of tests revealed the key points related to the designed cell for further improvement. In the second design, a cavity to house the mini-pressure sensor was engraved on the facet of one of the two half-samples, and the preparation of this sensor was improved to ensure its water tightness. A cylindrical wedge was fabricated to avoid the rigid-rigid contact. A second series of tests with the same three solutions and the same material were then conducted. This series has been running for 270 days and its results are more satisfactory.