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Backfill and material behaviour in underground salt repositories, phase II


The principal scientific objective of the project was to extend the basis for optimising the repository design and construction and for predicting the long-term performance of the following barriers in a repository for radioactive waste in rock salt: (i) the host rock including the Excavation Disturbed Zone (EDZ) around emplacement rooms, (ii) the backfill (crushed salt), and (iii) the materials used for constructing durable waste containers. The work was divided into in situ studies, laboratory investigations, modelling studies, and desk studies. In situ investigations were carried out in the Asse salt mine subsequent to completion of the large-scale TSDE (Thermal Simulation of Drift Emplacement) in which two simulated emplacement drifts had been electrically heated to between 170 and 200°C by disposal cask mock-ups over more than eight years. In one drift, backfill, two heaters, and measuring instruments were recovered. Local conditions in backfill and surrounding rock were examined and samples of native salt and consolidated backfill were studied in laboratories. Instruments were retrieved for analysis of their performance during the experiment execution and for recalibration. Extensive geophysical studies characterized the development of the EDZ aroundexcavations. Laboratory tests measured properties of specimens extracted from the backfill in the dismantled drift as well as cores from the surrounding rock. Physical, mechanical, and hydrological experiments quantified parameter values requisite for material models and described attributes of backfill and rock salt at advanced stages of compaction and deformation. An extensive array of 280 candidate container material samples was recovered and examined to evaluate their state of corrosion that was minimal after more than ten years of exposure in the TSDE experiment. A suite of modelling studies analysed the thermomechanical evolution of the TSDE and evaluated the predictive capabilities available to address complex modelling demands associated with long-term repository predictions. These computational studies helped to refine models and codes based on forensic investigations in the dismantled experimental drift and from laboratory tests. Advancements recognized in a breadth of applications improved comparisons between computational results and in situ experimental results from the TSDE. Post-test investigations served to reconcile lingering differences between experiments and calculations witnessed at the conclusion of the BAMBUS-I project. The primary results encompass comprehensive computational approaches that demonstrate successful prediction of temperature profiles, stress distributions, displacements, and the attendant properties of compacted backfill, intact rock salt and the EDZ. In a desk study the boundary conditions for the retrievability of highly active waste and spent fuel were investigated. The consequences of the accessibility of the waste during the retrievability period were assessed. In addition, the technical and scientific results of the TSDE project (1985 to 2002) were reviewed and documented. For use in future repository design and construction, an easily accessible data acquisition system was developed.