​Systems capable of ensuring structural health monitoring (SHM) in harsh environments require special sensors. Fiber Bragg grating (FBG) systems can provide temperature readings inside of complex equipment like nuclear reactors, where the flow of neutrons is intense. However, until recently, there had been little study of FBG sensor behavior when subjected to high temperatures and neutron and gamma radiation. CEA-List, a CEA Tech institute, joined forces with other CEA divisions and with SCK in Belgium to test this promising technology in gamma irradiators and research reactors.

Regenerated* fiber Bragg gratings are made from patterns that are laser-inscribed into the cores of optical fibers and heat-treated (using a "regeneration" process, hence the name) to withstand temperatures up to 900 °C. This type of fiber Bragg grating is particularly well-suited to harsh environments. Here, the researchers exposed sensors to 1 MGy of gamma radiation and temperatures of 250 °C. The temperature measurement error after exposure to radiation was less than 2.7 °C, about the same as for a dose ten times smaller at ambient temperature.

Therefore, high temperatures appear to be beneficial to the behavior of regenerated FBG systems subjected to radiation. New tests with in-line monitoring will now be completed with the goal of identifying the optimal parameters for fabricating FBGs for use in research reactors. Ultimately, these advances could lead to in-core measurement, enabling much more detailed monitoring of reactors during operation.

*Fiber Bragg gratings are like mirrors that reflect particular wavelengths called Bragg wavelengths. Variations in the wavelength reflected can be used to determine the local temperature variation, which can thus be measured