Temperature Resistant Fiber Bragg Gratings for Online Monitoring of Future Sodiumcooled Fast Reactors: Paving the Way to SHM Implantation into the Nuclear Industry

Abstract

Advanced manufacturing techniques using thermal engineering (regeneration process) and ultrafast laser micromachining of silica optical fibers have tackled the challenges of making high temperature resistant Fiber Bragg Gratings (FBGs) sensors. Arrays of temperature and strain wavelength-multiplexed FBGs sensors for the online monitoring of future Nuclear Reactors (Generation IV) are becoming a reality. Both regenerated and femtosecond FBGs have been intensively tested up to 950 °C for temperature mapping and recently tested also up to 400 °C for strain measurements on metallic devices. These sensors are expected to perform continuous monitoring of sodium-cooled nuclear reactors and to improve their safety and pave the way to Structural Health Monitoring (SHM) implantation into the nuclear industry. We present the manufacturing of i) regenerated FBGs using an all-optical process and of ii) femtosecond laser-written point-by-point femtosecond (fs PbP) FBGs directly into free-standing singlemode optical fibers. The presented techniques may not only speed up the manufacturing process but also improve the mechanical reliability of the FBGs. The temperature stability of both kinds of FBGs is assessed through long term ageing up to 950 °C highlighting their temperature resistance. Both are appropriate candidates for online thermometry even in severe environments such as those encountered in future Sodium-cooled Fast Reactor. Strain sensing is under development using fs PbP FBGs to operate beyond 400 °C using a traction machine and standard metallic specimens.