Computational Thermal-Fluid Dynamics Analyses of Borated Water Distribution in the Vacuum Vessel of the Divertor Tokamak Test Facility

The Divertor Tokamak Test (DTT) facility is a superconducting tokamak being built at the ENEA Frascati to foster the development of advanced divertor solutions for the EU DEMO. The DTT Vacuum Vessel (VV) will be procured in three multi-sectors, in turn divided into one or more sectors; due to manufacturing and integration constraints, the different sectors cannot share the same design: some of them will be identically repeated (“Regular” sectors), whereas the remaining will all be one of a kind (“Special”). In DTT the VV plays also the role of neutron shield for the superconducting magnets, and therefore it will be actively maintained at the operating temperature of 60°C by borated water in forced flow to counteract the thermal loads (pulsed heating from the plasma and static radiative cooling from the thermal shield). The water will flow in the free space between the two shells composing the VV; given the complexity of the geometry, a careful hydraulic design is mandatory, to avoid local stagnation points which may cause either overheating or freezing. An additional requirement is the possibility to fully drain the entire VV from the bottom to perform baking with nitrogen at ≈ 200°C. The water will enter the sectors in parallel by 9 inlets at the bottom and leave them by 9 outlets (staggered by 20°) at the top. This identifies 18 separate hydraulic paths, covering 20° toroidal portions and therefore reflecting the differences between the “Regular” and “Special” sectors; this asks for a proper balance of the mass flow repartition among them. This work presents the full set of Computational Thermal-Fluid Dynamics (CtFD) analyses of the DTT VV to address the issues above. All the different hydraulic paths are separately analysed with the Star-CCM+ software, with a SST k − ω turbulence closure, proving the effectiveness of their design. In addition, the coolant mass flow rate distribution among the different paths is assessed. To conclude, results from CtFD analyses are exploited to approximate the hydraulic characteristic of each sector and to develop a system-level model of the full VV in Modelica language. The overall VV pressure drop of Δp = 2800 [Pa], the outlet mixing temperature of Tout,mix = 333.06 [K] and a mass flow rate distribution close to the homogeneous condition (≈ 2.22 [kg s−1] in each sector) confirms that no issues are found in the current VV design from the thermal-hydraulic point of view.