## 1 Introduction  
In most pressurized water nuclear reactors (PWR), fuel material consists of small pellets that are inserted in thin metallic tubes. Such a metallic tube is called a cladding, and the element consisting of the cladding filled with fuel pellets is referred to as a fuel rod. In industrial plants, thousands of fuel rods are gathered in the reactor core and are surrounded by axially flowing water, which acts as a coolant fluid. The cladding is supposed to physically separate the fuel material from the surrounding water. However, because of combined effects of corrosion, mechanical fretting, high temperatures, and chemical load of fission product, cladding failures can occur during reactor operation. Under some conditions, the failure can produce a pressure wave that propagates in the surrounding water. Such a phenomenon is more likely to occur during a Reactivity-Initiated Accident, as explained in [1, 2]. The detection, localization, and characterization of cladding failures are of interest for studies about fuel behavior in research reactors as well as for optimizing the operation of industrial power plants. However, this phenomenon occurs inside the reactor core, where there are high temperatures, radioactivity, and little available space. Therefore, instrumentation possibilities for failure detection are limited. In such a situation, acoustic and vibration methods are interesting. Thanks to wave propagation phenomena, it is possible to measure waves generated by a source in a limited access area with distant sensors. Concerning a cladding failure in a reactor core, waves propagating in either the structure or the coolant fluid can be used. Indeed, both types of wave can be related to the cladding failure: right after the cladding failure, a fast increase in pressure of the surrounding fluid occurs, because of a thermal reaction between ejected fuel particles and surrounding fluid and, in case of a waterlogged rod, a release of high-pressure steam that was contained inside the rod. The pressure surge generates waves that propagate in the fluid and therefore can be measured by pressure sensors. An increase in the fluid pressure also induces additional stresses in the structure because of fluid-structure interaction. The induced stresses propagate as elastic waves through the structure (the rod and the outer structure) and can be measured by acoustic emission (AE) sensors mounted on the outer structure (The so-called acoustic emission methods consist in studying high-frequency elastic waves generated by damage mechanisms. Therefore, in the current article, acoustic emission sensors are considered as vibration sensors, as vibration refers to elastic waves in the structure). Another advantage of using fluid pressure waves and elastic waves is the possibility to measure them with simple and robust sensors such as piezoresistive and piezoelectric sensors. Those technologies enable producers to design pressure sensors and AE sensors that can withstand radioactivity and relatively high temperature. They are not resistant enough to be placed in the middle of the core, but they can be mounted close to the rod ends. However, the acoustic signals that would be measured during tests in a reactor would be affected by noise, by unknown events, and by propagation effects (dispersion, reflection and fluid-structure interaction). Thus, in order to properly interpret these signals, identify unknown events, and potentially improve measurements techniques, we need more information about wave propagation and fluid-structure interaction than what measurements can provide. We especially lack information about velocities of the different types of waves, signal distortion due to the propagation through a geometrically complex system, and transmission between fluid and solid media. To improve the understanding of these phenomena, we decided to obtain missing information by calculation methods.