Thermal-hydraulic and thermo-mechanical analysis and design optimization of the neutralizer for the Neutral Beam Injection system of the Divertor Tokamak Test (DTT) facility

Nuclear fusion promises to be a sustainable, reliable and safe energy source. In Italy, the DTT S.c.a.r.l. is actively engaged on this front with a project, proposed by ENEA and supported by EUROfusion, which has as its goal the construction, at the ENEA research center in Frascati, of a tokamak fusion experiment, aimed at testing possible solutions for the divertor, one if not the most critical component of ITER (under construction in Cadarache, France) and of the subsequent European DEMO. The main objective of the divertor is the mitigation of the power exhaust issue, due to the enormous thermal heat flux carried to the walls by the plasma particles. A key role inside the tokamak is played by the high-energy neutral beam injection system (NBI), necessary to heat the plasma to an operating temperature of about 100 million degrees. The purpose of this work is the analysis and the design of one of the key components of the NBI: the neutralizer. This component has the task of neutralizing the D+ ion beam, accelerated towards the vacuum chamber of the tokamak, in order to allow the desired penetration of the beam to the central part of the plasma. The thermal loads due to the interaction between the deuterium beam and the walls (in CuCrZr) of the neutralizer make it necessary to have an ad-hoc refrigeration system for the neutralizer. The relatively low cost and the simplicity of supply and management have suggested the use of pressurized demineralized water as the most suitable heat transfer fluid. Starting from the conceptual model of the neutralizer provided by the DTT S.c.a.r.l., the initially foreseen geometric configuration is first studied, in the reference operating conditions indicated in the technical specifications. Computational Thermal Fluid Dynamics (CtFD) analyses are carried out using the StarCCM+ commercial code. The thermal load due to electrons and deuterium ions is considered; however, since the load due to the stray magnetic field was not considered so far, as it is not known for the time being, a safety margin was assumed from the boiling condition, likely resulting in a rather conservative design. The results of the analysis highlight the incompatibility of the proposed configuration in the operating conditions with the requirements imposed on the necessary flow rates and pressure losses. On the other hand, there is a large margin of operation from the thermal point of view, so that a series of optimizations is proposed, related on the one hand to the fluid dynamics of the collectors, on the other to the removal of some turbulence promoters and the reduction of the circulating flow. The optimized solution, from the thermal-hydraulic point of view, is finally subjected to a further verification: using as input the temperature distribution accurately calculated in the CtFD study, it is possible to compute the expected deformations and the thermomechanical stresses, providing possible hints for further optimization of the component in the future engineering design phase.