An analytical study was performed to understand the physical phenomena after an ingress-of-coolant event (ICE) in a fusion reactor. A simulation code was developed to identify two-phase flow behavior in a fusion reactor vacuum vessel (VV). It consisted of mass, momentum, and energy equations of the liquid and gas. Numerical simulations were carried out to obtain quantitative predictions. Distributions of pressure, temperature, velocity and void fraction in the VV were obtained and an impingement-jet behavior of water under vacuum conditions were clarified numerically.