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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(SiO2). 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. [ABSTRACT FROM AUTHOR]

This paper reports the characterization of two scintillating materials in powder form, Gadox and YVO embedded in a light-activated resin, used in a probe developed for oncological brachytherapy in-vivo dosimetry. The materials were characterized in terms of internal absorption, scintillation decay time, and light yield. The measurement of the optical characteristics highlighted a significant internal absorption at the scintillation light wavelength, with values of 6.5 dB/mm for Gadox and 14.1 dB/mm for YVO. Measurements of the characteristics scintillation time and of the light yield were performed with a novel method based on single photon counting, profiting from the long decay time of the materials under study. Measurements have been complemented by a two-step simulation with Geant4 to study the energy deposition followed by a ZEMAX OpticStudio® ray tracing to estimate the light collection efficiency. The decay time for scintillating materials were measured to be τ G a d o x = (458 ± 3 ± 3) μ s and τ Y V O = (451 ± 8 ± 3) μ s and the estimated values of the light yield are (7.1 ± 0.5) × 104 photon/MeV for Gadox and (4.8 ± 0.5) × 104 photon/MeV for YVO. [ABSTRACT FROM AUTHOR]

In-core temperature measurements are crucial to assess the condition of nuclear reactor components. The sensors that measure temperature must respond adequately in order, for example, to actuate safety systems that will mitigate the consequences of an undesired temperature excursion and to prevent component failure. This issue is exacerbated in new reactor designs that use liquid metals, such as for example a molten lead-bismuth eutectic, as coolant. Unlike water cooled reactors that need to operate at high pressure to raise the boiling point of water, liquid metal cooled reactors can operate at high temperatures whilst keeping the pressure at lower levels. In this paper we demonstrate the use of optical fibre sensors to measure the temperature distribution in a lead-bismuth eutectic cooled installation and we derive functional input e.g. the temperature control system or other systems that rely on accurate temperature actuation. This first-of-a-kind experiment demonstrates the potential of optical fibre based instrumentation in these environments. We focus on measuring the surface temperature of the individual fuel rods in the fuel assembly, but the technique can also be applied to other components or sections of the installation. We show that these surface temperatures can be experimentally measured with limited intervention on the fuel pin owing to the small geometry and fundamental properties of the optical fibres. The unique properties of the fibre sensors allowed acquiring the surface temperatures with a resolution of 30 mK. With these sensors, we assess the condition of the test section containing the fuel assembly during different steps of the operation of the facility, including the heating and verification of the vacuum of the loop as well as the filling and draining of the LBE loop. We also identify a simulated electrical shut-down and heating circuit failure in the primed installation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]