## 2 The Experimental Platform
### Instrumentation of Fuel Pin and Validation  
In order to conduct reliable measurements on the surface of the individual fuel pins in the fuel assembly, the optical fibres with FBGs need to be adequately mounted. To do so, we followed the procedure as described in detail in [5]. We inserted the optical fibres in a groove that was machined in the fuel pin surface. Owing to the small dimensions of the optical fibre (diameter of only 200 \(\upmu\)m), we  
Figure 3: Concept drawing and photograph of the fuel assembly inlet. could limit the size of the groove to approximately 300 \(\upmu\)m whilst fully embedding the optical fibres in the fuel pin wall. To avoid issues with respect to the interpretation of the vibration data, we opted to manufacture the groove parallel with the fuel pin axis. Once inserted in the grooves, the optical fibres were fixed and protected with a high temperature ceramic alumina adhesive (Resbond 989) that can sustain temperatures well beyond 1000 \({}^{\circ}\)C. The mounting method has also been graphically represented in Figure 4 (left). The FBGs are commercially available UV-written draw tower gratings (DTGs), which are known to feature excellent strength and fatigue characteristics [4; 23; 24]. The former property is especially important during thermal cycles (and shocks) where we encountered corresponding strains as high as 0.5-1% due to thermal expansion. The higher yield strength of DTGs is also particularly important during the installation of the fuel pins in the complex experiments: as they feature a higher strength than the conventional re-coated FBGs, they are less likely to fail during installation and use. An important issue when mounting FBGs in this manner is to avoid distortion of the spectrum resulting from non-uniform strain being applied to the FBG. In addition, the mounting of the FBG in a groove with an adhesive that typically shrinks during curing can induce birefringence. In order to avoid such issues we inserted the FBGs in a small glass capillary (see also Figure 4 (left) and we used a dedicated wavelength shift determination technique, i.e., the phase correlation method, as described in more detail in [25; 26; 27] as well. We instrumented the central fuel pin with 4 optical fibres using the integration scheme as explained above, as graphically represented in Figure 4 (right). We mounted the fibres on 4 sides of the fuel pin so that 2 fibres are essentially sensitive to strain corresponding to bending moments parallel with the support rails, and the 2 other fibres to bending moments perpendicular to the support rails. Each fibre contained 21 FBGs, of which 20 FBGs were distributed uniformly over the fuel pin length. The last FBG, used as a reference, was not inserted into a groove on the fuel pin, but located inside of a stainless steel capillary (as explained in Section 2.4) close to the egress point of the fibre on the fuel pin. In total, 84 FBGs were introduced in the central fuel pin. We chose the Bragg wavelengths or resonance wavelengths of the FBGs so as to fit within the spectral bandwidth of the optical source used in the FBG interrogator (detailed in Section 2.4), and so as to feature a sufficiently large separation of the Bragg wavelengths \(\lambda_{\mathrm{B}}\) (of at least 3 nm) to accommodate for spectral distortion [5; 17] whilst maximizing the number of FBGs in the fuel assembly. Figure 4: **(Left)** Scheme of an optical fibre with draw tower gratings (DTGs) embedded in a small groove on the surface of the pin. (**Right**) The location of the instrumented fuel pin with 4 fibres (represented by the red dots) containing in total 84 FBGs. To validate the mounting procedure, we inspected several properties of the 84 reflection spectra of the FBGs at different phases of the mounting procedure. We recorded the spectra with a laser swept interrogation system with a spectral resolution of 8 pm, and we evaluated 3 main properties. First, we determined how many of the 84 FBGs could still be interrogated in a dependable manner after mounting.