Numerical analysis of supersonic jet flow and dust transport induced by air ingress in a fusion reactor

Air ingress into the vacuum vessel (VV) in the case of loss of vacuum accident (LOVA) may lead to the radioactive dust re-suspension, migration even explosion and thus pose a great threat to the safe operation of future fusion reactors. Therefore, it is crucial to understand the flow characteristic and radioactive dust transport behavior induced by LOVA. However, few studies have well identified the highly under-expanded jet flow characteristic in the LOVA scenario, i.e. the Mach disk is failed to be captured in the existing studies. In this study, a CFD-DPM one-way coupled multiphase approach is established by using computational fluid dynamics code ANSYS FLUENT and applied to the LOVA analysis for the first seconds. The results shows that air ingress into the VV behaviors like a very highly free under-expanded jet at the initial stage, in which the Mach disk has been formed at ~ 6 ms. The flow field changes dramatically at the position of the Mach disk. The jet core before the Mach disk has the maximum velocity with ~8 Mach and the corresponding lowest static pressure (~100 Pa) and temperature (few tens of K) there. The friction velocities in the lower part of the VV, which is of the greatest concern because of the deposited dust there, is generally larger than 15 m/s near the inlet region. The crude prediction of particle trajectories shows that dust would be rolled up and then follow the air motion. This study will provide a basis for the further safety analysis and accident prevention related to dust transport and dust explosion of the future fusion reactors.