ÃÃThe lack of as-built drawings and information for many old nuclear facilities impedes the planning for a decommissioning. Traditional manual walkthroughs subject workers to lengthy exposure to radiological and other hazards. And also there have been a delay in the dismantling schedules and that in an increase in the amount of waste, which have caused a heavy expenditure for the decommissioning cost. In order for a 3D model to represent the real dismantling operation environment, realistic dismantling activities and scenarios were modeled. We present a dismantling digital mock-up system that can show a dismantling process through an animation and enable us to evaluate the major parameters that can directly affect the cost in a static simulation as a preview to the dismantling activities on a hypothetical dismantling environment. We carried out a prototype 3D animation and a simulation system for dismantling the thermal column in Korean Research Reactor-1(KRR-1). The results show that the modeling of a realistic scenario makes it possible to demonstrate the adequacy of the design and can provide operators with a high fidelity under the real working conditions.