Modelling of the Vacuum Pumping system and integration in the Fuel Cycle of a nuclear fusion power plant

Nuclear fusion represents a promising solution for sustainable and large-scale energy production, with tritium playing a central role as a key fuel. Efficient tritium management, including its breeding, recovery, and recycling, is essential to ensure the viability of a fusion power plant, and is governed by the design and performance of the fuel cycle system. Existing models of the nuclear fusion fuel cycle often rely on simplified assumptions, such as the use of residence time to describe the behaviour of gas retention. While computationally efficient, these models lack a detailed description of the physical processes governing the system and offer limited insight into the operation and limitations of key components. A detailed modelling of the exhaust gas pumping system can instead provide critical information regarding tritium burning efficiency, fuel cycle dynamics, and the estimation of the reactor’s startup tritium inventory. In this thesis, a detailed model of the vacuum pumping system was developed, including both cryopumps and vapor diffusion pumps. The work builds upon a fuel cycle model developed at the MIT Plasma Science and Fusion Center (PSFC) and Politecncico di Torino, extending it with a more detailed and physically accurate description of exhaust processes. One of the major improvements introduced was the inclusion of surface saturation effects: condensation and absorption limitations were implemented to reflect the finite capacity of cryogenic surfaces. The pumping speed was characterized dynamically as a function of surface loading, allowing the model to capture the physical behaviour of the pumps over time more realistically. This time-dependent degradation of pumping performance is essential to understand when evaluating the operational efficiency and regeneration requirements of the system. Furthermore, the model takes into account safety-critical aspects such as the risk of hydrogen accumulation and explosion inside the pumping chamber, a particularly important consideration in hydrogen-handling environments. The result is a simulation framework that significantly enhances state-of-the-art modelling approaches, providing deeper physical insight and predictive capability for the design and optimization of first-generation fusion power plant fuel cycles.