Thermal-hydraulic study of the DEMO divertor cassette body cooling circuit equipped with a liner and two reflector plates

In the framework of the Work Package DIV 1 – “Divertor Cassette Design and Integration” of the EUROfusion action, a research campaign has been jointly carried out by University of Palermo and ENEA to investigate the steady-state thermal-hydraulic performances of the DEMO divertor cassette cooling system. The research activity has been focussed onto the most recent design of the Cassette Body (CB) cooling circuit, consistent with the DEMO baseline 2017 and equipped with a liner and two Reflector Plates (RPs), whose main functions are to protect the underlying vacuum pump hole from the radiation arising from plasma and shield the PFCs inlet distributors, respectively. The research campaign has been carried out following a theoretical-computational approach based on the finite volume method and adopting the commercial Computational Fluid-Dynamic (CFD) code ANSYS-CFX. The CB thermal-hydraulic performances have been assessed in terms of coolant and structure temperature, coolant overall total pressure drop and flow velocity distribution, mainly in order to check coolant aptitude to provide a uniform and effective cooling to CB, liner and RPs structures. Moreover, the margin against coolant saturation has been evaluated in order check whether any risk of its bulk vaporisation is prevented. The outcomes of the study have shown some criticalities, mainly in terms of coolant bulk vaporisation occurrence at the corner of the Inner Vertical Target (IVT) and uneven coolant flow distribution among RPs plasma-facing channels, that have suggested some design variations whose effectiveness has been numerically assessed. In particular, the solution proposed to contain and reduce the critical region of the IVT corner has been predicted to be particularly effective, while further studies are needed to improve coolant flow distribution among the RPs plasma-facing channels. Models, loads and boundary conditions assumed for the analyses are herewith reported and critically discussed, together with the main results obtained.