THERMOFLUID-DYNAMICS OF DEMO DIVERTOR CASSETTE

The divertor is a critical in-vessel component of nuclear fusion reactors, being responsible for the fulfilment of certain fundamental functions for the machine: it must be able to handle the power deposited by charged particle and neutron irradiation, ensure the presence of channels through which the fusion ashes can be removed from the Vacuum Vessel (VV), provide plasma-compatible surfaces, and shield the VV and magnets from nuclear loads. The heat load that can be tolerated by the divertor under normal and off-normal operating conditions is a pivotal parameter when dimensioning a fusion power plant since exceptionally high heat fluxes can be observed in some regions of the divertor, in the order of some tens of MW/m2. It is therefore clear that the proper functioning of this component in steady-state or long pulse conditions is inextricably linked to the correct design of its cooling circuit, which is required to prevent structural and functional materials to operate outside their operative limits, avoid unduly high pressure drops, and operate at the highest possible temperature to ensure the maximum achievable thermodynamic cycle efficiency while complying with all the applicable constraints. In particular, for the case of the European DEMO power plant, during the Pre-Conceptual Design (PCD) phase which ended in 2020, attention was focused on the study of a “Double-Circuit” divertor concept, in which two independent cooling circuits served by two different Primary Heat Transfer Systems (PHTS) were used to cool the divertor Plasma-Facing Components (PFC) and the Cassette Body (CB). The possibility to adopt a single cooling circuit to serve both PFCs and CB, suggested during the divertor final design Review Meeting of the PCD phase, led to the definition of a new divertor concept, namely the ”Single-Circuit Cooling Option”. This novel divertor was originally conceived to allow for a simpler balance of plant design, as it would require a single PHTS, and to ease remote maintenance, as only one inlet and one outlet pipe should be cut and reweld for each divertor cassette during replacement operations. The research activities carried out during the Ph.D. aimed to identify the strengths and possible shortcomings of this divertor design solution. To this purpose, attention was at first focussed on the development, validation and application of a dedicated numerical tool able to perform quick parametric analyses of the divertor cooling circuit, allowing for a first screening of coolant operating conditions and cooling circuit layouts that do not comply with most of the applicable thermal and thermal-hydraulic requirements and constraints. Therefore, with the final aim to perform a detailed thermofluid-dynamic assessment with the tools of Computational Fluid Dynamics (CFD), a simulation technique based on an equivalent porous medium concept was developed. With this technique it is possible to reduce the computational costs required to simulate the coolant flow inside swirl tape-equipped cooling channels, being the most computationally demanding components of the entire divertor cooling circuit, without compromising the quality of the results. Finally, the complete 3D-CFD thermofluid-dynamic simulation of the entire single-circuit cooling option divertor cassette was performed, confirming the outcomes of parametric analyses and highlighting the occurrence of additional criticalities in terms of cooling performance and thermal hot spots. In this dissertation are reported the methodologies developed and their validation, together with the models, assumptions, and outcomes of this research campaign.