Experimental analysis of flow-induced vibrations and application to the fuel rod bundle of the MYRRHA reactor

Mechanical damage due to excessive vibration caused by a coolant flow is known to limit the life of nuclear reactor components. It is therefore important to understand and characterize these flow-induced vibrations and damage mechanisms in order to prevent safety issues and potential accidents by adequately designing the reactor components in order to mitigate against the occurrence of excessive vibrations and resulting damage. In this doctoral thesis we focus on the experimental investigation of flow-induced vibrations in the fuel assembly of MYRRHA - The Multi-purpose hYbrid Research Reactor for High-tech Applications, which result from the circulation of the liquid lead-bismuth eutectic (LBE) mixture that is used as a coolant in that particular case. Measuring vibration requires the use of adequate sensors or measurement techniques. The design of MYRRHA comes along with a number of constraints and specific requirements for the instrumentation that would allow measuring these vibrations such as for example very limited space for installing sensors and no visual access to the fuel assembly due to the opaque coolant. To select the most adequate sensor or measurement technique that complies with the MYRRHA setting, we benchmarked the performance of several possible sensors and techniques for measuring fuel pin vibrations. We demonstrated that optical fibre Bragg grating (FBG) based transducers are the most adequate sensors for measuring the fuel pin vibrations. Knowing that the FBGs are a-priori fit to the task, their eventual selection required investigating whether they can reliably withstand the reactor environment. We have therefore assessed and improved their lifetime to comply with the MYRRHA requirements and by doing so we qualified the FBG based sensors as fuel pin vibration sensors in the LBE environment. Following the selection of the sensors, we had to ensure that the vibration measurement data returned by these sensors could be processed to enable a detailed characterization of the vibrations in fuel assembly. We therefore selected and applied the most adequate modal analysis techniques that allows accurately evaluating the dynamic behaviour of the fuel pins in the fuel assembly based on vibration measurements. We successfully applied a new methodology based on a least-squares complex frequency modal estimator to numerous clustered fuel pin configurations in operational conditions. We evidenced that this modal estimator allowed experimentally characterizing the flow-induced vibration of a fuel assembly with unprecedented precision and accuracy. In the final step we combined the results achieved by tackling the two previous challenges and we demonstrated the true merits of our vibration measurement procedure in a prototype test facility that mimics the flow conditions that will eventually be encountered in MYRRHA. By demonstrating the feasibility of carrying out such high quality vibration measurements with techniques that have never been applied earlier in an actual prototype test facility for a novel type of nuclear reactor, we have generated inspiration for a new generation of nuclear reactor instrumentation and condition monitoring systems.