Dust-air two-phase flow characteristics in a fusion experimental reactor during a loss-of-vacuum-accident (LOVA) event were analyzed numerically by three dimensional simulations using a newly developed thermal-hydraulic analysis code. Physical models on the motion of dust were considered to resolve the dust mobilization conveying by the fluid. Air ingress behavior through a breach at the LOVA event was calculated by using compressible Navier-Stokes equations. It was predicted quantitatively from the results of the present numerical study that the dust mobilization receives strongly the effect of the breach size and the fraction of the mobilized dust is determined by a circulating flow and buoyancy-driven exchange flow which are generated in a vacuum vessel of the fusion experimental reactor after the LOVA event.