Fiber Bragg Gratings (FBGs) can be used as sensors for strain, temperature and pressure measurements. For this purpose, the ability to determine the Bragg peak wavelength with adequate wavelength ...resolution and accuracy is essential. However, conventional peak detection techniques, such as the maximum detection algorithm, can yield inaccurate and imprecise results, especially when the Signal to Noise Ratio (SNR) and the wavelength resolution are poor. Other techniques, such as the cross-correlation demodulation algorithm are more precise and accurate but require a considerable higher computational effort. To overcome these problems, we developed a novel fast phase correlation (FPC) peak detection algorithm, which computes the wavelength shift in the reflected spectrum of a FBG sensor. This paper analyzes the performance of the FPC algorithm for different values of the SNR and wavelength resolution. Using simulations and experiments, we compared the FPC with the maximum detection and cross-correlation algorithms. The FPC method demonstrated a detection precision and accuracy comparable with those of cross-correlation demodulation and considerably higher than those obtained with the maximum detection technique. Additionally, FPC showed to be about 50 times faster than the cross-correlation. It is therefore a promising tool for future implementation in real-time systems or in embedded hardware intended for FBG sensor interrogation.
•We describe an analysis technique to evaluate nuclear fuel pins.•We test a single fuel pin mockup subjected to turbulent axial flow.•Our analysis is based on operational modal analysis (OMA).•The ...accuracy and precision of our method is higher compared to traditional methods.•We demonstrate the possible onset of a fluid-elastic instability.
Flow-induced vibration of nuclear reactor fuel pins can result in mechanical noise and lead to failure of the reactor's fuel assembly. This problem can be exacerbated in the new generation of liquid heavy metal fast reactors that use a much denser and more viscous coolant in the reactor core. An investigation of the flow-induced vibration in these particular conditions is therefore essential. In this paper, we describe an analysis technique to evaluate flow-induced vibration of nuclear reactor fuel pins subjected to a turbulent axial flow of heavy metal. We deal with a single fuel pin mockup designed for the lead–bismuth eutectic (LBE) cooled MYRRHA reactor which is subjected to similar flow conditions as in the reactor core. Our analysis is based on operational modal analysis (OMA) techniques. We show that the accuracy and precision of our OMA technique is higher compared to traditional methods and that it allows evaluating the evolution of modal parameters in operational conditions. We also demonstrate the possible onset of a fluid-elastic instability by tracking the modal parameters with increasing flow velocity.
The fiber Bragg gratings (FBGs) inside photonic crystal fibers (PCFs) having high fiber birefringence, such as in PCF with butterfly shape microstructure, are characterized by two Bragg peaks. The ...spectral distance between these two peaks is not affected by a change of temperature or by a longitudinally applied strain, but this spectral separation evolves when the sensor is subjected to a transversal strain. This makes such FBGs inscribed in highly birefringent PCFs very interesting for structural health monitoring of civil structures, as well as for operation in harsh environments such as the ones associated with the nuclear industry. In this paper, we monitor the radiation-induced Bragg wavelength shift (RI-BWS) of the punctual sensor and the radiation-induced attenuation in the fiber used for its inscription up to a dose of 1.5 MGy(SiO 2 ). Even if we observe a RI-BWS for both Bragg peaks, the spectral distance between them only slightly evolves, of less than 10 pm. These gratings are then very promising sensors for the structural health monitoring of nuclear facilities.
We present fiber Bragg grating based transversal load sensing with a highly birefringent microstructured optical fiber. For the bare fiber, the change of the Bragg peak separation under a transverse ...line load was simulated with a finite-element model and experimentally verified. We also show that microstructured optical fibers with fiber Bragg gratings can be successfully embedded in a carbon fiber reinforced composite material. The linear dependence of the Bragg peak separation to a transversal stress in the composite sample was measured to be 15.3 pm/MPa.
•We evaluate six different vibration sensing techniques.•We compare the SNR and R2 of the response signal of the sensors.•The amplitudes were up to 60m at a frequency of 8.5Hz.•The optical techniques ...and the MEMS-type accelerometer prove superior.•Considering the constraints, we believe that the FBG are our best option.
Understanding the mechanical interactions between the coolant and the core structure in nuclear reactors helps to determine the lifetime, health or optimal design of the reactor core. The flow of the coolant produces vibrations in the reactor core containing the fuel assemblies that consists of a matrix of fuel pins. We report on an evaluation of the performance of different vibration measurement techniques considered for measuring the flow induced vibrations on a fuel pin mock-up. These techniques include a laser Doppler vibrometer (LDV), a grid method (GRID), fiber Bragg grating sensors (FBGs), electrical strain gages and two types of accelerometers. In this paper we first show the practical aspects of the validation experiments before proceeding with the influence of the techniques on the pin dynamics. Finally we compare the signal-to-noise ratio (SNR) and the level of determination of the response signal of the sensors for low amplitudes and low frequencies. We conclude that for our setup the optical techniques and MEMS-type accelerometer prove to offer superior performance. Considering the space constraints, we believe that the fiber Bragg gratings are the best candidate for vibration monitoring in nuclear reactor core mock-ups.
An experimental study of the mechanical reliability of microstructured optical fibers (MOFs) is reported. Tensile tests were carried on five types of MOFs and two reference fibers, and the tensile ...strengths were analyzed using Weibull statistics. Optical microscopy of the surfaces of rupture allowed identifying the critical flaws and determining the failure mechanisms. First, it appears that the MOFs have lower tensile strengths than standard optical fibers. Second, the mechanical strength of MOFs was found to be related to the dimensions and morphology of the microstructure. Finally, fractographic examinations confirmed that MOFs can fail from defects located in the vicinity of the air holes, in contrast to standard optical fibers for which cracks always start propagating from defects located on the outer silica surface.
We numerically studied the focusing properties of a special type of graded index photonic crystal media, i.e., the so-called photonic crystal Mikaelian lenses (PCMLs). We considered PCMLs with a ...varying air hole diameter and with a varying air hole pitch in hexagonal and rectangular lattices, and we compared their frequency response and polarization dependence. Our eventual objective is to design microstructured optical fibers equipped with a PCML in their cladding region adapted to allow for more efficient femtosecond laser grating inscription. Therefore, we have also evaluated the possibility of using such lenses as transverse focusing elements in microstructured optical fibers.
We report on the sensing characteristics of rocking filters fabricated in two microstructured fibers with enhanced polarimetric sensitivity to hydrostatic pressure. The filter fabricated in the first ...fiber shows a very high sensitivity to pressure ranging from 16.2 to 43.4 nm/MPa, depending on the resonance order and features an extremely low cross-sensitivity between pressure and temperature 28 ÷ 89 × 10(3) K/MPa. The filter fabricated in the second fiber has an extreme sensitivity to pressure ranging from -72.6 to -177 nm/MPa, but a less favorable cross-sensitivity between pressure and temperature of 1.05 ÷ 3.50 × 10(3) K/MPa. These characteristics allow using the rocking filters for pressure measurements with mbar resolution.
We have studied the influence of the fiber coating on the radiation sensitivity of fiber Bragg gratings (FBGs). These FBGs are draw tower gratings written before applying the coating in a fiber with ...a photosensitive 18-mol.% GeO 2 -doped core. Using a Co 60 source we irradiated polyimide, acrylate, and ormocer coated gratings as well as mechanically stripped ormocer coated FBGs up to a 40-kGy dose. This total dose level corresponds to a near-earth space radiation environment mission. We show that the coating type must be taken into account for correctly interpreting ionizing radiation effects on FBGs.
We have studied the effect of gamma-radiation on the small signal gain and on the noise figure of several EDFAs with similar characteristics and based on special highly Er-doped fibers. Those fibers ...were fabricated using direct nanoparticle deposition technology, with different Er 3+ doping levels. The results show that the use of this technology allows improving the EDFA's radiation tolerance by increasing the concentration of Er 3+ ions provided this is not accompanied with higher concentrations of other dopants, which influence the radiation sensitivity of the fiber.
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