## 1 Introduction
We focus here on the case of the experimental devices used in research reactors for the study of a single rod rather than the industrial systems case containing several rods. Some analytical methods may be able to approximate pressure and stress evolution in a very simplified model of the device, such as the methods developed for studies on water hammer ([3,4]). Nevertheless, because of the complex geometries of devices used in research reactors, specific and accurate calculations were necessary to get results that could be compared with experimental data from real tests in a reactor. Thus, a numerical model of a typical experimental device was built to run finite element computations. A numerical approach enables a relatively accurate model of the structure (in terms of geometry and material composition). However, modeling the failure phenomenon and its direct effects requires a lot of information that are not available in the literature. Moreover, characteristics of such a phenomenon are complex and deeply depend on experimental conditions and the initial state of the rod (such as its internal composition, burn-up, initial leakage, cladding oxidation, etc. ), as explained in [1,2]. Therefore, it is impossible to precisely estimate general failure characteristics and build a numerical model of the phenomenon that would be representative of every possible failure. Hence, the objective of the numerical approach presented in this paper is to obtain information about wave propagation and the associated fluid-structure interaction, without an exhaustive and realistic simulation of the failure phenomenon itself. Although propagation phenomena depend quantitatively on the system properties, the global phenomenological interpretation that is presented here is valid for every system consisting in a metal structure and a liquid-filled annular channel. The current paper deals with preliminary calculations that are necessary to better understand the evolution of pressure profile in the fluid and stress profile in the structure along the part of the device containing the tested fuel rod. Only the results related to pressure in the fluid are presented, because the interpretation of structural wave signals depends too much on the sensor being used, whose response highly affects the measured signal. While the response of commercial pressure sensors can be assumed to be flat up to 25% of their resonant frequency (usually given by the producer) and the output of such sensors can reliably be considered as the actual pressure (which is a simple scalar value), the interpretation of AE sensors' signals leads to some issues. Firstly, most of the AE sensors are intended to be used at their resonant frequency and have consequently a non-flat response. Secondly, the actual physical value measured by the sensor (displacement, velocity, acceleration) and the directivity is seldom known. Estimating the response of AE sensors is a recurrent problem that has been studied several times (for instance, [5; 6; 7]). The results depend on the model of the sensor, and there is currently no simple solution to accurately estimate an AE sensor response. As a consequence, the interpretation of the raw physical values provided by the numerical simulation is suited for signals measured by any model of fluid pressure sensors, but it might be unsuited for AE sensors signals. The first part of the paper consists of the description of a typical experimental test device (materials, geometry, flow parameters). The second part describes the numerical model and the methodology used for the computation. The results are introduced, analyzed, and discussed in the third part. As a result of non-disclosure obligation, physical values cannot be explicitly shown. Thus, length and pressure values are given in arbitrary units, which are defined in the following section. The arbitrary unit of length, referred to as A.U.L., is defined as the length of the main section of the device (Figure 1). Its order of magnitude is one meter.