## 2 The Experimental Platform
### Egress of Optical Fibres and Interrogation  
The Bragg gratings are measured in reflection and therefore the optical fibres require only a single end-connection to the data acquisition equipment (consisting of a light source and a detector) located out of the test facility. To that end, we guided the fibres from the fuel pin through small 1 mm outer diameter stainless steel tubes, which were then loosely inserted into 4 mm outer diameter tubes, which served as feedthrough to the loop exterior. So as not to affect (or as little as possible) the movement of the fuel pins because of instrumentation, we designed the set-up such that the clearance of this insertion is nearly double the allowed fuel pin movement from the fixation at the rails. We represented this alignment set-up in Figure 5. The 4 mm diameter tubes are then guided through a series of grids to maintain their relative position and continue up to the outside of the LBE loop at the top flange where a welded Swagelok connection keeps the tubes in place and seals the feedthrough. To simplify the installation of the grids and flanges, each tube had a different length. Above the top flange, we sealed the egress of the optical fibres out of the steel tubes with a UV-curable glue (NOA61), which exhibits excellent adhesion properties to steel and silica. We tested this sealing method, for 1 mm inner diameters and 4 mm outer diameter, up to 10 bar of overpressure and in a vacuum. We observed that even during a rapid increase from room temperature up to 650 \({}^{\circ}\)C at more than 5 \({}^{\circ}\)C/min, no measurable leakage was detected using a commercial He-gas based leak tester. During the commissioning of the test facility with the instrumented fuel assembly installed, we also pressure tested the set-up while heated up to 4 bar, which is well above the expected operational pressure.  
We acquired the spectra of the FBGs using a commercially available FBG-Scan 808D [28], which is a high-speed FBG sensor interrogation unit. The interrogation of the FBG in this device is based on a spectrometer and involves a diffraction grating and light sensor array. Since no moving components or tuning of optical parameters such as the light source wavelength are required, very stable measurements can be conducted with this device. The latter was required for the vibration measurements since the expected wavelength fluctuation was small (as illustrated in [20; 29]) and signal degradation due to the environment is expected to occur (as explained in more detail in [5; 17]). As explained earlier, we accommodated for this degradation in the interrogation by using a more intricate phase-correlation based wavelength shift detection technique to analyse the acquired FBG spectra, and to calculate \(\lambda_{\text{B}}\)[25; 26; 27]. This approach allowed for achieving the best trade-off between  
Figure 5: Illustration of the alignment set-up designed to avoid affecting the vibration due to the egress of the optical fibre and its guide capillary. Note that for sake of readability, the figure is not to scale.  
signal-to-noise ratio and calculation speed even if the FBG spectra become distorted. Owing to the use of the phase-correlation based approach and to the temperature controlled environment (explained in Section 2.1), we were able to record Bragg wavelength shifts as small as 0.25 pm, corresponding to a local strain level difference of about 0.2 \(\upmu\varepsilon\). We recorded the reflection spectra of the FBGs with an acquisition rate of up to 5 kHz which is well beyond the highest expected observable frequency of vibration [29]. To the best of our knowledge, no other (quasi)-distributed measurement technique can attain these sampling speeds.