Monitoring structures in harsh environments requires appropriate sensors. Bragg gratings are used to provide temperature maps in complex installations such as nuclear reactors, which are subject to intense neutron fluxes. Their behavior under the combined effect of high temperatures and gamma and neutron radiation has been little studied until now. CEA-List is collaborating with other CEA departments and with SCK in Belgium to test this promising technology in gamma irradiators and research reactors.
Consisting of patterns laser-etched on optical fibers and heat-treated in a so-called "regeneration" process to withstand temperatures of up to 900°C, regenerated Bragg gratings* are a class of Bragg gratings particularly suited to harsh environments. Researchers have shown that with sensors subjected to a gamma radiation dose of 1 MGy and a temperature of 250°C, the temperature measurement error following irradiation is less than 2.7°C, i.e. of the same order of magnitude as for a dose ten times lower at room temperature.
High temperatures therefore appear to have a beneficial effect on the radiation behavior of regenerated Bragg gratings. Future tests with on-line monitoring will enable us to identify the optimum parameters for manufacturing these gratings for use in research reactors. With the prospect of much closer monitoring of these reactors through measurement. in core.
*Bragg gratings act as mirrors, reflecting light of a specific wavelength called the Bragg wavelength. Any variation in the wavelength reflected by the grating can be traced back to the local variation in temperature, which can then be measured..