AbstractDust mobilization and air ingress characteristics in a fusion reactor during a loss-of-vacuum accident (LOVA) event were analyzed numerically using a newly developed thermal-hydraulic analysis code under the conditions of a compressible flow and three-dimensional cylindrical coordinates. Some physical models on the motion of dust were considered to resolve the dust mobilization characteristics. Air ingress behavior through breaches was calculated by using the compressible Navier–Stokes equations. It was clarified quantitatively from the present numerical results that the predicted average pressures in a vacuum vessel (VV) agree well with the experimental results within an error of 10% for different breach diameters and the dust mobilization depends on the flow configurations of the buoyancy-driven exchange flows through breaches and the dust density, and the rate of dust released from the VV through the breaches to the outside depends on the breach positions on the VV, and furthermore, the present numerical approach is very effective to estimate the thermal-hydraulic phenomena inside the VV after the LOVA event occurs.