Optical Fiber-Based Crack Monitoring on Engineered Barrier of Radioactive Waste Repository

Monitoring on engineered barrier of radioactive waste repository is an issue of safe operation and staged closure. Optical fiber sensor, which is based on back-scattering, is expected to reduce a negative impact on the barrier performance because of multi-channel sensor. In Japanese concept of intermediate depth disposal, a cement wall is expected to work as a low diffusion layer of the engineered barrier system, and thus crack on the wall is critical for the ensure of its function. To monitor the occurrence and development of crack on the surface of the concrete structure, optical fiber is mounted on the surface of full-scale mockup and has been evaluated through a cracking test in underground research laboratory. The authors apply both Brillouin-scattering- and Rayleigh-scattering-based optical fiber sensor to measure strain distribution when crack generates, and then the pros and cons of each sensor are experimentally clarified in terms of crack monitoring. The results indicated that Rayleigh-scattering-based measurement can predict the stress concentration before cracking, and Brillouin-scattering-based measurement can trace the crack opening after cracking. Based on measurement in situ with the use of optical fiber sensor, it can be said that crack monitoring on cementitious diffusion layer is feasible, and then integrating different scattering sensor is proposed for wide coverage of measurement.