Hydrogen explosion is one of the most dangerous accident phenomena in both nuclear fusion and fission reactor. In the typical fusion reactor, tritium and deuterium are fuel elements for fusion reaction, and they exist in condensed state around the cryo-pump in the normal operation condition. In the accident situation, increasing heat transfer system temperature can mobilize lots of tritium in the vacuum vessel. And then, coolant can react with high-temperature structures generating hydrogen gas inside vacuum vessel. In addition, air ingress into plasma terminates fusion reaction producing activated dust on the first wall plasma facing surfaces. Through these series of process, mobilized tritium and dust aerosol can explode in the vacuum vessel and damage the containment building integrity, making a sort of source term leakage to environment through various system volumes. In this paper, preliminary safety analysis for hydrogen and dust explosion accident for assumed design of Korean fusion demonstration reactor is conducted. An MELCOR system analysis code is used to simulate this accident and parametric analysis to investigate effect on the accident scale and safety systems like detritiation system such as heating, ventilation, and air conditioning isolation system and pressure suppression system. As a result, aerosol distribution of each control volume and its release route are estimated, and final aerosol release to the environment is compared with release guideline for ITER. The amount of mobilized aerosol in demonstration reactor is evaluated conservatively using ITER normal operation condition.