Characterizing Flow-Induced Vibrations of Fuel Assemblies for Future Liquid Metal Cooled Nuclear Reactors Using Quasi-Distributed Fibre-Optic Sensors  
[PERSON] 1,2,*  
1 Department of Applied Physics and Photonics (TONA), Vrije Universiteit Brussel, Brussels Photonics (B-PHOT), Pleinlaan 2, 1050 Brussels, Belgium; [EMAIL_ADDRESS] (T.G. ); [EMAIL_ADDRESS] (H.T. ); [EMAIL_ADDRESS] (F.B.) 2 Flanders Make, Oude Distersebebaan 133, 3920 Lommel, Belgium 2  
[PERSON] 3 Belgian Nuclear Research Centre (SCK\(\bullet\)CEN), Boeretang 200, 2400 Mol, Belgium; [EMAIL_ADDRESS] (G.K.); [EMAIL_ADDRESS] (K.V.T.) [PERSON] 3 Belgian Nuclear Research Centre (SCK\(\bullet\)CEN), Boeretang 200, 2400 Mol, Belgium; [EMAIL_ADDRESS] (G.K.); [EMAIL_ADDRESS] (K.V.T.) [PERSON] 1,2 [PERSON] 1,2  
1 Department of Applied Physics and Photonics (TONA), Vrije Universiteit Brussel, Brussels Photonics (B-PHOT), Pleinlaan 2, 1050 Brussels, Belgium; [EMAIL_ADDRESS] (T.G. ); [EMAIL_ADDRESS] (H.T. ); [EMAIL_ADDRESS] (F.B.) 2 Flanders Make, Oude Distersebebaan 133, 3920 Lommel, Belgium 2  
[PERSON] 1,2 [PERSON] 1,2  
1 Department of Applied Physics and Photonics (TONA), Vrije Universiteit Brussel, Brussels Photonics (B-PHOT), Pleinlaan 2, 1050 Brussels, Belgium; [EMAIL_ADDRESS] (T.G. ); [EMAIL_ADDRESS] (H.T. ); [EMAIL_ADDRESS] (F.B.) [PERSON] 1,2 [PERSON] as optical fibre based sensors have contributed to the advancements in vibration measurements in such situations [3; 4; 5; 6]. In this work we report on such a safety study that is part of extensive research activities conducted with the aim to develop and commission MYRRHA, which is a prototype nuclear reactor belonging to the so-called fourth generation [7; 8], and which is currently being designed at the Belgian Nuclear Research Centre (SCK\(\bullet\)CEN). MYRRHA will be an accelerator driven and spallation source based reactor, cooled with a molten lead-bismuth eutectic (LBE) alloy (Figure 1). As in any nuclear reactor, the vibration levels of reactor internals must be kept within certain limits. The research presented in this paper has been undertaken with the objective to provide valuable input for a safety study of the MYRRHA fuel assembly, specifically by studying the flow-induced vibration (Figure 1). In nuclear reactors, flow-induced vibrations are generally the result of the reactor coolant interacting with flexible reactor internals [9; 10; 11]. These internals are typically fuel pins, shielding plates, or heat exchanger tubes. In this paper, we focus on the fuel pins in the fuel assembly. Knowledge and understanding of the fuel pin vibrations is critical to assess the possibility of occurrence of failure modes corresponding to mechanical wear, such as fretting or fatigue. Regardless of where these vibrations take place in the reactor, they are always undesirable and often unexpected. Therefore, a thorough analysis of flow-induced vibration of the fuel pins is a crucial aspect of the design cycle in view of assessing the lifetime of reactor internals and ensuring safe operation of these reactors. Different vibration sensors or techniques are available to carry out such measurements [12; 13; 14; 15]. However, we had to work in liquid LBE, which has a density of more than 10 times that of water, is corrosive and opaque [16]. In addition, to meet the requirements of the nuclear reactor, the LBE is heated to 200-400 \({}^{\circ}\)C, whilst flowing at a speed close to 1.7 m/s in the fuel assembly [7; 8]. This shortens the list of potentially adequate sensors or techniques. Piezoelectric sensors, for example, would suffer from electrode corrosion or require bulky protective packaging. In our application, the spacing between the individual fuel pins is limited to a few millimetres, which leaves little room to install vibration sensors. Furthermore, and in order to obtain reliable vibration readings, the sensors should not affect the flow of LBE. The sensors should therefore have very small dimensions.