Analytical and CFD analysis of the transfer and storage of the Fuel Assemblies in MYRRHA

The present thesis addresses the study of the fuel cycle of the MYRRHA reactor, focusing on thermal-hydraulic design of the intermediate storage facility (Fuel Hot Cell). The primary objective is to define a completely passive Decay Heat Removal System (DHRS) capable of ensuring fuel integrity during storage operations.
The analysis identified the "Fast Unloading" strategy as the reference accident scenario, necessary in case of extended maintenance, major problem in the core or for request of the Regulator. In this condition, the entire core (121 Fuel Assemblies) is transferred to the Hot Cell shortly after reactor shutdown, imposing the maximum thermal load on the cooling system.
To manage this critical scenario, a natural circulation cooling system using Nitrogen was proposed and analyzed. A detailed Computational Fluid Dynamics (CFD) analysis was performed on a representative cluster of Fuel Assemblies to investigate the flow phenomenology and heat transfer efficiency.
The numerical results demonstrated the effectiveness of the proposed design, which ensures a stable natural draft, predicting a Peak Cladding Temperature (PCT) of approximately 250 °𝐶, widely below the safety limit of 400 °𝐶.
In conclusion, the study validates the feasibility of a gas-cooled passive system,
demonstrating that the MYRRHA Fuel Hot Cell can safely withstand severe unloading scenarios without relying on active components, thus fulfilling the highest safety standards.