A. Rezayat

Signal-to-Noise Ratio Evaluation of Fibre Bragg Gratings for Dynamic Strain Sensing at Elevated Temperatures in a Liquid Metal Environment

Vibration measurements of the fuel assembly of a nuclear reactor are a very useful tool to determine the health and lifetime of the reactor core. The importance of these measurements is exacerbated in the new generation of heavy liquid metal reactors, where the fuel assembly is exposed to a corrosive molten metal coolant at 300 °C and where the space between the individual fuel pins is limited to a few millimeters. In this paper, we consider fibre Bragg gratings as potential candidates for carrying out fuel pin vibration measurements in such an environment. We describe a dedicated method to integrate fibre Bragg gratings in a fuel pin, and we subject this pin to conditions close to those encountered in a real heavy liquid metal reactor. More specifically, we report on the performance of draw tower gratings used as a vibration sensor when the fuel pins are immersed in heavy liquid metal at 300 °C for up to 700 h. The performance evaluation is based on monitoring the signal-to-noise ratio of the gratings spectral response as a function of time. We show that accurate detection of the Bragg peak becomes very challenging after 400 h of exposure. Additionally, we succeed to extend the useful lifetime with a factor of two by using an appropriate integration of the fiber in the fuel pin and by using an alternate peak detection algorithm.

Vibration Monitoring Using Fiber Optic Sensors in a Lead-Bismuth Eutectic Cooled Nuclear Fuel Assembly

Excessive fuel assembly vibrations in nuclear reactor cores should be avoided in order not to compromise the lifetime of the assembly and in order to prevent the occurrence of safety hazards. This issue is particularly relevant to new reactor designs that use liquid metal coolants, such as, for example, a molten lead-bismuth eutectic. The flow of molten heavy metal around and through the fuel assembly may cause the latter to vibrate and hence suffer degradation as a result of, for example, fretting wear or mechanical fatigue. In this paper, we demonstrate the use of optical fiber sensors to measure the fuel assembly vibration in a lead-bismuth eutectic cooled installation which can be used as input to assess vibration-related safety hazards. We show that the vibration characteristics of the fuel pins in the fuel assembly can be experimentally determined with minimal intrusiveness and with high precision owing to the small dimensions and properties of the sensors. In particular, we were able to record local strain level differences of about 0.2 μϵ allowing us to reliably estimate the vibration amplitudes and modal parameters of the fuel assembly based on optical fiber sensor readings during different stages of the operation of the facility, including the onset of the coolant circulation and steady-state operation.

Detection, Localization and Quantification of Impact Events on a Stiffened Composite Panel with Embedded Fiber Bragg Grating Sensor Networks

Nowadays, it is possible to manufacture smart composite materials with embedded fiber optic sensors. These sensors can be exploited during the composites’ operating life to identify occurring damages such as delaminations. For composite materials adopted in the aviation and wind energy sector, delaminations are most often caused by impacts with external objects. The detection, localization and quantification of such impacts are therefore crucial for the prevention of catastrophic events. In this paper, we demonstrate the feasibility to perform impact identification in smart composite structures with embedded fiber optic sensors. For our analyses, we manufactured a carbon fiber reinforced plate in which we embedded a distributed network of fiber Bragg grating (FBG) sensors. We impacted the plate with a modal hammer and we identified the impacts by processing the FBG data with an improved fast phase correlation (FPC) algorithm in combination with a variable selective least squares (VS-LS) inverse solver approach. A total of 164 impacts distributed on 41 possible impact locations were analyzed. We compared our methodology with the traditional P-Inv based approach. In terms of impact localization, our methodology performed better in 70.7% of the cases. An improvement on the impact time domain reconstruction was achieved in 95.1% of the cases.

Design and Testing of an Active Aeroelastic Test Bench (AATB) for Unsteady Aerodynamic and Aeroelastic Experiments

The Active Aeroelastic Test Bench (AATB) is designed for the study of low subsonic unsteady aerodynamics through forced motion experiments, as well as active aeroelastic experiments. First, the forces generated on a wind tunnel model subjected to an arbitrary oscillation in pitch and plunge are measured during forced motion experiments. Next, by operating the AATB in closed loop, the occurring aerodynamic forces are translated into pitch and plunge displacements through emulated stiffness of the actuators. This allows for the emulation of different linear and nonlinear elastic behaviour of the wind tunnel model. Via this emulated stiffness, it is also possible to simulate the longitudinal rigid body motion of a flexible wind tunnel model, and hence, study the response and the coupling between flexible and rigid modes. The obtained data is used for the validation and development of frequency-domain system identification methods, in order to generate aeroelastic models from noisy measurement data. This paper threats with the design and initial testing of the AATB for a rigid wing with constant section.