Performance Comparison of Helium and sCO₂ as Coolants for Modular Finger-Type Divertors

Several studies at the Georgia Institute of Technology have evaluated the thermal and fluids performance of the helium-cooled modular divertor with multiple jets (HEMJ) over the past decade. This finger-type divertor was studied both experimentally at nearly prototypical conditions and numerically at fully prototypical operating conditions using experimentally validated simulations. Recently, supercritical carbon dioxide (sCO₂) has been studied as the primary coolant in power cycles and other applications in various systems, in part because CO₂ achieves the high densities typical of supercritical fluids at relatively low temperatures and pressures, with a critical point of (7.38 MPa, 31°C). This density makes it possible to realize very compact and efficient sCO₂ power cycles. The feasibility of sCO₂ as a coolant for plasma-facing components, specifically the divertor, was therefore evaluated as part of the Fusion Energy System Studies design study activities. This work compares the thermal-fluid performance of helium and sCO₂ in the HEMJ divertor geometry using numerical simulations at prototypical conditions: inlet temperatures T_i = 600°C to 700°C, pressures p ≈ 10 MPa, and steady-state incident heat fluxes on the tile q″ < 17 MW/m². The performance is quantified here as the maximum heat flux that can be accommodated by the plasma-facing tile, the pumping power fraction, defined as the ratio of the coolant pumping power to the incident thermal power, and the operating stress limits based on ASME pressure vessel criteria. As expected, helium requires lower mass flow rates and pumping power fractions within imposed maximum temperature limits for the HEMJ pressure boundary. However, it also appears that neither helium nor sCO₂ can remove 10 MW/m² of incident heat flux while meeting ASME pressure vessel criteria. Finally, the numerical modeling reveals that sCO₂ may remove slightly higher incident heat fluxes than helium due to the imposed stress limits due to the sCO₂ coolant resulting in smaller local temperature gradients, albeit at a considerably higher pumping power fraction.