Numerical analysis of a water Loss-Of-Coolant-Accident inside the EU DEMO Vacuum Vessel

Nuclear fusion is one of the few technologies with the potential to answer the global need for a reliable, sustainable, carbon-free, electricity source. Europe efforts to advance nuclear fusion research are coordinated by the EUROfusion Consortium, that with the "European Research Roadmap to the realization of Fusion Energy", plans to develop a technology capable of feeding fusion power into the grid by the 2050, using a demonstration fusion power plant: the DEMO. The EUROfusion Roadmap makes it clear that safety is an essential element even in the current pre-conceptual design phase, so this thesis focuses on one of the most representatives "Design Basis Accidents" scenarios: an in-vessel Loss of Coolant Accident, that has the potential to cause substantial damage to the components inside the vacuum chamber. A LOCA occurs when an unpredicted event causes the failure of the first wall (FW) components, exposing the cooling system, and initiating the release of the coolant inside the vacuum vessel (VV). In the case of a water-cooled breeding blanket (WCLL-BB), the release of high-pressure water inside the low-pressure environment generates a supersonic flashing jet, whose behaviour needs to be addressed. The first section of this work concentrates on the physical study of the flashing phenomenon, analysing the thermodynamical and mechanical processes that characterise the phase change mechanism, and the evolution of the jet after the release, followed by a brief review of the main theoretical models presented in literature. The next step was to examine the predictive capabilities of a CFD model in simulating the multiphase transient that forms in the first milliseconds after the release; this research was carried out in two stages. First, using STAR CCM+, a 2D model was generated to simulate the multiphase phenomenon, confronting the various approaches the code proposes, and selecting the most suitable for the case. The code was modelled on the basis of an opportune test case chosen from the literature, and was then validated through the comparison with the numerical and experimental results of the case. The code was then modified to match the parameters of the EU DEMO LOCA scenario, and a more case-relevant simulation was carried out. The code demonstrated the capability of simulating such a violent phenomenon, defining the strategy and the models to be adopted in a future full 3D analysis.