## 4 Summary and Conclusions  
In this paper we presented the experimental investigation of the flow-induced vibrations in a realistic mock-up of the fuel assembly of MYRRHA. The mechanical integrity of the fuel assembly may be impacted as a result of, for example, fretting wear or mechanical fatigue. We conducted the experimental analysis on a full-scale mock-up fuel assembly in a closed-loop LBE experimental test facility at a temperature of 200 \({}^{\circ}\)C, and at LBE flow rates that are representative for MYRRHA. The analysis relies on quasi-distributed vibration measurements that were conducted on the central fuel pin in the fuel assembly using densely multiplexed FBGs that were surface mounted in a groove in the fuel pin cladding. We have designed and built an adequate test facility to mimic the environment in MYRRHA, and developed a mounting method that allowed integrating the FBGs strain sensors in the fuel assembly with minimal intrusiveness. We validated the latter by inspecting the reflection spectra of the 84 integrated FBGs during different stages of installation and showed that nearly all  
Figure 8: The histogram of the vibration (in terms of strain) gives a more complete view of the fuel pin vibration. Average strain amplitude exceeding 2 \(\mu\epsilon\) rarely occurs at MYRRHA nominal flow speeds. Figure 7: The trend of the rms vibration amplitude (in terms of strain) measured with the different FBGs located on the central fuel pin in the two perpendicular directions with respect to the support rails. of the sensors retained excellent spectral characteristics, even when installed in the heated and filled test facility. The instrumented fuel assembly allowed for characterizing the vibration of the fuel assembly with a sufficiently high precision. In particular, the precise measurements allowed us to reliably estimate the vibration amplitudes of the fuel assembly. In addition, we studied the fluctuation of the vibrations with flow speed. We observed that the measured vibration amplitudes are very small owing to the proper design of the fuel pins and test facility. For LBE coolant flow velocities, as anticipated in MYRRHA (equal to 1.72 m/s), we observed an average vibration amplitude just below 0.7 \(\mu\epsilon\) for the direction parallel with the support rails and just over 0.5 \(\mu\epsilon\) for the direction perpendicular to the support rails. Note that owing to the non-irradiated experimental fuel assembly described in this paper, spatial tolerances are expected to be at their largest. Irradiation induced swelling of the fuel pins would lead to closing gaps and thereby possibly reducing the flow-induced vibration. In addition, we could assess the strain along the length of the fuel pin using the mounted quasi-distributed FBG sensors. We found that the trend of the vibration amplitude along the fuel pin slightly differs depending on the direction of movement. The trend essentially resembles the second bending mode of a classical cantilevered beam along the support rails, whilst in the perpendicular direction, the trend follows the third bending mode. We believe that the difference can be attributed to the different fuel pin support conditions in both directions. We also provided a more complete description of the vibration by evaluating the distribution of the measured strains, which should adhere to a Gaussian distribution given the nature of the excitation forces. We found that the average strain excursions exceeding 2 \(\mu\epsilon\) occur less than 0.1% of the time and that the kurtosis of the distribution was nearly equal to 3, illustrating the Gaussian nature of the vibration. To close, owing to the unique properties of FBG based sensors, we were able to obtain fuel pin vibration characteristics that are important to assess structural degradation resulting from fretting wear or mechanical fatigue, and that provide important feedback to the designers of the MYRRHA fuel assembly.