## 2 Description of the System  
The studied system is based on the typical geometry of experimental devices used in French research reactors for the study of a single rod's behavior (for instance, the REPNa devices in the CABRI reactor [8], GRIFFONOS and ISABELLE devices in the OSIRIS  
Figure 1: Simplified drawing of the test device (fuel rod in red, other structural parts in gray, and fluid in blue). Dimensions are relative to the length of the main section, \(L\)\(\sim\)1 m. The most external tube and the gaseous gap are not represented. reactor [9], the forthcoming ADELINE device in the RJH [10]). Such a device contains a fluid channel where the tested rod sample is placed. The device is inserted in the reactor (usually a pool reactor) that generates a neutron flux representative of the one met by a rod in an industrial reactor. The channel containing the tested rod is connected to an independent water loop that recreates typical thermohydraulic conditions of industrial pressurized water reactors (water at 280 \({}^{\circ}\)C and 155 bar, flowing at 3.4 m \(\cdot\) s\({}^{-1}\)). The typical characteristics of such a device are as follows:  
* An overall length of several meters;
* A central section of about one meter, containing a fuel rod sample of about 60 cm;
* A structure mostly made of stainless steel and, in the central section, Zircaloy (because of its neutron-transparency property);
* Various instrumentation downstream and upstream from the central section. In the current study, only the central section and its inlet and outlet sections are considered. Figure 1 shows a simplified drawing of this central section. Typically, this part can be described as an annular structure with several coaxial layers. From the outside to the inside, these layers are an external tube, an annular gap filled with gas, a second tube made of Zircaloy (called the \"channel tube\" in Figure 1), an annular water channel, and the tested fuel rod. The gaseous gap is intended to mechanically and acoustically separate the inner channel from outside events. In this paper, we focus on what happens inside the test device only and neglect the effects of the external layer. Thanks to the gaseous gap, these are realistic simplifications (it was verified in a real device that the sensors are almost insensitive to outside events). Thus, we consider only the part of the device including the channel tube, the water channel, and the fuel rod. Only that part is presented in Figure 1, which shows a simplified drawing of the studied part of the test device. In the following paragraphs, we refer to the channel tube as the \"outer structure\". From a longitudinal point of view, the section upstream from the rod is called the \"inlet section\", the section downstream is called the \"outlet section\", and the section containing the rod is called the \"main section\". The main section is separated from the inlet and outlet sections by short transition sections. In these transition sections, the channel cross-section is reduced to enable a mechanical connection between the outer structure and the rod and extensions assembly. There, the channel cross-section is not annular but consists of several holes. For the sake of understanding, an example of such a transition cross-section is shown in Figure 2.  
Several measurement points are considered in the system. Two of them, referred to as P1 and P2, are located in the inlet and outlet sections and can be considered as realistic measurement points, as it is actually possible to place sensors at equivalent positions in a real device (see Figure 1). Therefore, they show the actual possibilities that can be expected from pressure measurements in a real reactor. The other points, defined in Section 4, are located in the central section, or very close to it. They are used to get information for the understanding of the studied phenomena, but they are not related to available sensor positions in the real device.