From monitoring to prediction: fiber optic–based detection of damage precursors in radioactive waste packages

The long-term safety of deep geological repositories relies on the integrity and performance of engineered and geological barrier systems under coupled thermo–hydro–mechanical–chemical (THMC) processes. Among these barriers, engineered waste packages play a critical role in ensuring the isolation and protection of radioactive waste during the early phases of repository evolution. Reliable experimental methodologies to detect their mechanical behaviour, damage mechanisms, and early-stage degradation processes are therefore essential for safety assessment and performance prediction.

Within the framework of the PALLAS experimental project, this study investigates the use of distributed fiber optic sensing (DFOS) for real-time monitoring of deformation and damage in a full-scale mock-up of a mortar radioactive waste package. Fiber optic sensors were embedded directly within the package to continuously measure strain variations during controlled mechanical loading and chemical degradation experiments, including accelerated sulfate attack representative of aggressive geochemical conditions. The experimental program aimed to reproduce relevant repository-induced stress and chemically driven alteration processes affecting engineered barriers.

The results demonstrate that DFOS enables continuous, high-resolution monitoring of deformation, crack initiation, propagation, and localization within the waste package. During chemical degradation, the technique allows for identification and spatial mapping of fracture development, as well as the estimation of crack width and length. Under mechanical loading, strain evolution not only captures fracture onset and growth but also reveals early strain anomalies that act as precursors to damage, enabling the identification of zones prone to future cracking prior to any visible manifestation. This predictive capability represents a significant step toward anticipating barrier degradation.

Overall, these findings illustrate the interest of distributed fiber optic sensing technologies for the monitoring of engineered barrier systems. The results demonstrate the capability of the approach to measure strain evolution and to identify deformation signatures associated with different coupled processes under controlled experimental conditions. While the experimental configuration relies on non-representative waste packages and requires sensor installation within the structure, the study provides valuable insights into the applicability and limitations of such monitoring techniques for future developments aimed at waste package and concrete structure monitoring.