Proof of Concept for Temperature and Strain Measurements With Fiber Bragg Gratings Embedded in Supercontainers Designed for Nuclear Waste Storage

The Belgian Agency for Radioactive Waste and Enriched Fissile Materials (ONDRAF/NIRAS) is developing a conceptual design for a supercontainer made of concrete and carbon steel for the post-conditioning of high-level nuclear waste prior to disposal in a geological repository site. This supercontainer should be instrumented to follow the temperature and strain evolution over time during and after the fabrication process. This paper shows that it is possible to embed bare fiber Bragg gratings in the supercontainer concrete to monitor
in situ
temperature and strain. The reason for choosing bare fibers, despite their known fragile nature, was to optimize the contact between the fibers and the concrete. Due to the simultaneous temperature and strain sensitivity of the fiber Bragg gratings, some sensors were also installed in a protective shielding consisting of plastic or aluminum to isolate them from effects of stress. This makes it also possible to extract both temperature and strain from the measured Bragg wavelength shifts. Moreover, the protective shielding also provides mechanical protection to avoid fiber breakage during concrete casting. The preliminary results of temperature closely agree with measurements obtained from thermocouples located near the fibers for validation purposes. Finally, the use of shielding in some of the fibers allowed to estimate strain levels in the concrete.