Multi-scale thermal-hydraulic modelling for the Primary Heat Transfer System of a tokamak

The EU DEMO reactor is currently in its pre-conceptual design phase by the EUROfusion Consortium members; it aims to be the first tokamak fusion reactor to demonstrate the capability to produce net electrical energy from fusion reactions. To this aim, it must prove tritium self-sufficiency, and so it will be the first tokamak to include a Breeding Blanket (BB), to breed tritium exploiting lithium and the neutrons coming from the fusion reactions. Moreover, to prove feasibility of fusion electricity, the EU DEMO reactor will also be the first to include the power conversion chain, converting the heat coming from fusion reactions into electrical energy, through a Primary Heat Transfer System, which removes the heat deposited in the components close to the plasma and delivering it to the Power Conversion System, that, in the end, produces electricity. Within this framework, a new computational tool is developed, supported by the EUROfusion Programme Management Unit. This code, called the GEneral Tokamak THErmal-hydraulic Model (GETTHEM), aims at fast, system-level, transient thermal-hydraulic modelling of the EU DEMO Primary Heat Transfer System and Balance-of-Plant (BoP), including all the in-vessel and ex-vessel cooling components, and it is the first system-level code of this type explicitly developed for fusion applications. The thermal-hydraulic models of the in-vessel components are developed, starting from the BB, as it is the most thermally loaded component and, consequently, the most important for the BoP. The GETTHEM development currently focuses on two out of the four BB concepts studied in the EU, namely the Helium-Cooled Pebble Bed (HCPB) and the Water-Cooled Lithium-Lead (WCLL) BB concepts. Considering that the EU DEMO is still in pre-conceptual design, the code focuses on execution speed, while maintaining an acceptable accuracy, typically modelling the different components as 0D/1D interconnected objects. GETTHEM is applied to analyse the coolant distribution in the HCPB BB, as well as the maximum temperature reached under normal operating condition in the structural material of both BB concepts, which must stay below 550 °C as a safety requirement. The model is capable to highlight if and where the coolant distribution in the HCPB BB should be optimized in order to avoid an overheating of the structures, allowing at the same time to reduce the compression power needed to circulate the coolant. It also can show if in some regions of the BB, for both coolant options, more detailed analyses are needed, as the current design, tailored on the equatorial BB region, somehow penalizes the regions far from the equatorial plane. Moreover, a separate module of the code is developed, aiming, through suitable simplifications, at fast modelling of accidental transients such as in-vessel Loss-Of-Coolant Accidents (LOCAs). Such module of the code is applied to the parametric analysis of an in-vessel LOCA for HCPB and WCLL, exploiting the code speed to rapidly screen the effect, for instance, of different break sizes, contributing to the proper sizing of the Vacuum Vessel Pressure Suppression System.