Flash-Boiling Characterization During Ingress of Coolant Event for Dust Issue in ITER

Dust resuspension inside the vacuum vessel (VV) is one of the safety issues of the fusion reactor ITER. Plasma interaction with the plasma facing components (PFC) leads to their erosion, generating dust. One of the accident scenarios leading to dust resuspension is the ingress of coolant event (ICE) where a leak of the coolant pipes inside the VV conducts to injection and flash atomization of the cooling water. The metallic dust, produced by the erosion, is then oxidized by water throughout an exothermic reaction that produces hydrogen leading to a loss of confinement risk due to hydrogen and dust combustion. The steam flow, produced by the flash atomization of the liquid leaking from the breach, is considered to be the main source of dust resuspension. Therefore, experimentations about the two-phase flow generated by the flashing liquid jet are important to identify the main physical phenomena involved in the aerosol particles resuspension for ITER-like conditions that impose in particular low pressure level. Flash-boiling experiments were conducted under primary vacuum conditions. We studied the behavior and the structure of the flow resulting from superheated water injection into low pressure environment. Using shadowgraphy and particle image velocity (PIV), qualitative information and quantitative measurements on the two-phase flow that develops for different superheat conditions were gathered. The measured spray lateral spreading and droplets velocity are shown to increase with the superheat level. The use of a transparent nozzle also confirmed the strong coupling between the external structure of the atomized spray and the two-phase flow that develops upstream of the coolant circuit breach.