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.
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 hours. The performance evaluation is based on monitoring the signalto-noise ratio of the grating's spectral response as a function of time. We show that accurate detection of the Bragg peak becomes very challenging after 400 hours 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.
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