High-temperature ultrasonic Doppler velocimetry for Lead-Lithium flows

Nuclear fusion reactors are a promising candidate of the future power source. Magnetic confinement fusion (MCF) blankets adopt high-temperature liquid metals (LM) such as the lead-lithium eutectic alloy (PbLi), as a coolant/tritium breeder. Developments of the LM-MCF blankets require comprehensive understanding of the high-temperature LM flows under the influence of the plasma-confining magnetic field. A high-temperature ultrasonic Doppler velocimetry (HT-UDV) has been developed as a flow diagnostic technique to acquire local velocity profiles of the opaque, high-temperature LM flow. This paper describes HT-UDV technique that has been successfully applied to measure velocity profiles of PbLi flows. The impact of tracer particles is investigated to determine requirements for HT-UDV measurement of PbLi flows. The HT-UDV system is tested on a PbLi flow driven by a rotating-disk in an inert atmosphere. We find that a sufficient amount of particles contained in the molten PbLi are required to successfully measure PbLi velocity profiles by HT-UDV. An X-ray diffraction analysis is performed to identify those particles in PbLi, and indicates that those particles were made of the lead mono-oxide (PbO). Since the specific densities of PbLi and PbO are close to each other, the PbO particles are expected to be well-dispersed in the bulk of molten PbLi. We conclude that the excellent dispersion of PbO particles enables the HT-UDV to obtain reliable velocity profiles for operation times of around 12 h.