Temperature Resistant Fiber Bragg Gratings for On-Line and Structural Health Monitoring of the Next-Generation of Nuclear Reactors

Abstract

The harsh environment associated with the next generation of nuclear reactors is a great challenge facing all new sensing technologies to be deployed for on-line monitoring purposes and for the implantation of SHM methods. Sensors able to resist sustained periods at very high temperatures continuously as is the case within sodium-cooled fast reactors require specific developments and evaluations. Among the diversity of optical fiber sensing technologies, temperature resistant fiber Bragg gratings are increasingly being considered for the instrumentation of future nuclear power plants, especially for components exposed to high temperature and high radiation levels. Research programs are supporting the developments of optical fiber sensors under mixed high temperature and radiative environments leading to significant increase in term of maturity. This paper details the development of temperature-resistant wavelength-multiplexed fiber Bragg gratings for temperature and strain measurements and their characterization for on-line monitoring into the liquid sodium used as a coolant for the next generation of fast reactors.

Overview of high temperature fibre Bragg gratings and potential improvement using highly doped aluminosilicate glass optical fibres

“…In contrast to thermal stabilization of Type I and Type In FBGs, the high temperatures involved in the regeneration leads to the complete relaxation of stresses in the fibre. Regenerated gratings have, over the years, proven to be a viable sensing technology in a range of areas, for example in the profiling of high temperature manufacturing equipment [1], dual pressure/ temperature sensing for gas turbines [44], sodium cooled nuclear reactors [20], high temperature air flow meters for internal combustion engines [45] and train engine temperature regulation [3]. Regenerated gratings have also been utilised to make the first accurate measurements of fibre viscosity [46].…”

“…One of them is the necessity to find coatings, packaging and attachment procedures that can withstand such high temperature regimes. This includes the development of specialty coatings (silicone based coatings like polyimide up to 350°C (e.g., [13,14]) in continuous, metallic coatings like Al [15], Cu [16,17], Mo-Cu to 800°C [18], Au to 750°C [19], and packaging in metallic capillaries such as Inconel 600 [20] or steel to 1000°C [19]. An approach to attach these FBG sensors, especially to metallic substrates, is to bond them using high temperature resistant adhesives.…”

Fiber-Optic Multipoint Sensor System with Low Drift for the Long-Term Monitoring of High-Temperature Distributions in Chemical Reactors

“…So far, there are only a few publications on the drifts of RFBGs at high temperatures that are based on time scales of one year or more. In [13], Laffont et al reported on the long-term annealing of four packaged RFBG sensors at temperatures ranging from 760 • C to 890 • C. After 9000 h of isothermal annealing, the RFBG elements experienced wavelength shifts larger than 500 pm, corresponding to temperature drift rates of more than 30 K/a [13]. This is considerably higher than the maximum drift rate of 2.3 K/a found in the present study, but this may be attributed to the significantly higher operating temperatures in [13].…”

“…The disadvantages of type II FBGs arise from their polarization sensitivity [10,11] and strong cladding mode coupling [5,10], which can limit their multiplexing capabilities. Type II-PbP gratings show significant wavelength drift when exposed to high temperatures [12,13]. For type II-PM FBGs, a stabilization of the wavelength drift after an annealing procedure of 100 h at 1000 • C has been reported [14].…”

“…When compared to type II FBGs, RFBGs typically exhibit lower values of grating reflectivity. However, the high-temperature annealing process that is necessary to form RFBGs leads to a reduction of their tensile strength, but it has the advantageous consequence that this type of high-temperature resistant grating shows low drift rates of the Bragg wavelengths when compared to type II-PbP FBGs [12,13]. The issue of the mechanical robustness of the fiber-optic sensor system has to be addressed with appropriate sensor packaging and with specific measures during the annealing process.…”