Calorimeter conceptual design for Neutral Beam Injector of DTT – CFD optimisation and thermal stress analysis

A conceptual design of the calorimeter for the Neutral Beam Injector (NBI) of the Divertor Tokamak Test facility (DTT, a new tokamak whose construction is starting in Frascati, Italy) has been developed. The DTT NBI calorimeter features two beam stopping panels made of CuCrZr cooled by pressurized water flowing through deep drilled cooling channels with twisted tape insertions. The proposed design is based primarily on the expected beam power distribution of DTT NBI, thermal-hydraulic factors (maximum temperature of the structure and coolant, pressure drop) and geometrical constraints (beam cross-section, panel inclination angle, available space for the calorimeter, etc.). Main design choices are made on the reduced model, representing a section of the panel with a single cooling channel. Detailed computational fluid dynamics (CFD) simulations of several designs were performed, leading to an optimized design in terms of minimizing the structure and coolant temperatures while respecting the operating conditions, allowed pressure drop in the cooling loop and overall space constraints. The mechanical response of the calorimeter structures, in terms of deformations and stresses, has been also analysed for the optimized design. To obtain realistic results, the whole panel (not only the reduced section of it) with appropriate boundary conditions has been considered. To simulate the temperature distribution over the whole panel, a simplified strategy using only the solid domain was developed. Before applying it on the whole panel, it was verified on the reduced model. The follow-up thermo-mechanical simulation of the whole panel use the heat transfer coefficient on the cooling channel interface that has been previously calculated from the accurate fluid-solid CFD simulation of the reduced model. It has been shown that the thermal loads induced by the beam neutral particles dominate the deformations and stresses in the calorimeter.