• Direct Contact Condensation of steam
• Experimental activity
• Experimental rigs
• ITER
• Unstable regimes
• Vibrations

Direct Contact Condensation (DCC) of steam is an efficient way to remove heat and mass with passive devices and it is an important process encountered in many engineering situations, especially in the nuclear field. All conventional nuclear fission plants take advantage of this process to suppress pressure at atmospheric pressure conditions. On the other hand, since the ITER (International Thermonuclear Experimental Reactor) reactor operates at high vacuum conditions, it has been designed to withstand a maximum pressure of only 0.15 MPa and it requires suppression and therefore steam condensation at sub-atmospheric pressure conditions. This is an absolute novelty in the nuclear field and deserves to be studied in depth given the growing interest in nuclear fusion reactors. Associated with the DCC of steam, pressure oscillations can arise when several thermo-hydraulic parameters vary, which result in vibrations on the structures, the integrity of which can be endangered depending on their intensity and frequencies. In particular, during unstable condensation regimes, it is possible to observe pressure instabilities associated with oscillations which involve the pool and the surrounding structures. This issue is even more significant when the vibrations occur at low frequencies and approach the natural ones of the structures which can go into resonance. This PhD thesis therefore focuses attention on the vibratory aspects associated with the phenomenon of DCC at sub-atmospheric pressure conditions.
In the introduction chapter the relevant safety and protection system called Vacuum Vessel Pressure Suppression System (VVPSS) of the ITER experimental nuclear fusion plant has been described along with the engineering issues associated with its operability. The VVPSS is mainly constituted by 4 Vapour Suppression Tanks (VSTs) measuring 4.7 m in height, with a diameter of 6.2 m and an inner volume of 100 m3 each partially filled with water, 3 with 60 m3 and one with 40 m3. The 3 VSTs containing 60 m3 of water, also called Large LOCA Tanks (LLTs), are designed to manage bigger LOCA (Loss Of Coolant Accident) events, while the one containing 40 m3, also called Small LOCA Tank (SLT), is designed to manage smaller LOCA events. Each tank is crossed by a sparger immersed in water to release the steam together with the potential presence of non-condensable gases. In the second chapter a summary of the state of the art (scientific literature) regarding pressure oscillations that arise during the DCC process has been illustrated. This literature search showed many discrepancies between the study results. The third chapter contains the description of the small-scale test facility where the superheated steam produced by 137 kW steam generator was conveyed in a 4 m3 condensation tank. In this experimental rig two experimental campaigns were carried out during which the vibrations that involved the sparger due to the steam DCC were studied (even in the presence of non-condensable gas). In the same chapter also a calibration procedure and a numerical analysis have been described. The fourth chapter refers to the Large-Scale Test Facility (LSTF) whose design and realization took up most of the doctoral time. This experimental rig quite faithfully replicates an ITER VST and includes a 92 m3 tank in which the steam produced by a 1750 kW steam generator is condensed. This chapter contains in the first part a detailed description of the facility and in the second part an analysis of a particular condensation situation encountered during the experimental tests performed for ITER organization. In this particular condition heavy vibrations of the whole structure were observed. The last chapter is finally dedicated to the results obtained from these studies and to some perspectives for future research activities.