Thermo-hydraulic design and monitoring of high heat flux components for the ITER neutral beam test facility

The beamline components of the ITER neutral beam test facility are typical nuclear fusion high heat flux components as the local power density can reach 20 MW/m2. Cooling circuits in the most heated elements of these components are provided with twisted tapes promoting transfer of water vapor bubbles from the nucleation surface to the subcooled bulk; these cooling channels are named swirl tubes. A customized finite element software simulating one dimensional single phase flow with two phase heat transfer was produced to analyze the thermohydraulic behavior of the components. Accurate finite element models were developed for direct transfer of information at nodes of coupled degrees of freedom in the fluid and thermal domains. Nonlinear transient thermohydraulic analyses were carried out to simulate thermal cyclic loadings produced by pulses of the particle beam with duration up to one hour with achievement of the steady state condition. Furthermore, breakdown pauses with power switched off for 50 ms and then ramped up in 80 ms was simulated to consider the sudden interruptions occurring for arc discharges at the beam source. The customized finite element code is capable to produce a detailed output file with nodal and element values of the convective heat transfer coefficient, subcooled boiling contribution, bulk temperature and pressure, saturation temperature at given pressure along the cooling channel, channel wall temperature, local onset nucleate boiling temperature. These data and the contour plots can be used to check the model conditions, especially to verify: the exchanged heat flux at the cooling channel inner surface against the critical heat flux for possible local burnout, and the coolant temperature to be compared with the saturation temperature to avoid bulk boiling. The beamline components of the ITER neutral beam test facility will be instrumented with about one thousand sensors including thermocouples and fiber Bragg grating optical sensors for measurement of absolute temperature and accelerations. Thermal sensors mounted on swirl tubes, leading edges, and wall panels will provide signals for monitoring, thermal control, and protection of the components. Thermal sensors installed at the outlet of cooling tubes will be used for calorimetry i.e. for evaluation of the thermal power spreading among actively cooled elements. Fiber Bragg grating accelerometers detecting vibrations produced by vapor bubble collapse will be installed at the cooling outlet of high heat flux elements where heat transfer in subcooled boiling condition is expected in order to detect incipient critical conditions.