As RBMK-type reactors reach the end of their operational lifetime, ensuring structural safety during decommissioning becomes increasingly important. This study evaluates the structural response of an RBMK reactor (High-Power Channel-type Reactor) building floor under a hypothetical accidental drop of a steam drum separator on reinforced concrete floor. Finite element analysis was employed to investigate a horizontal impact scenario in which the drum separator falls onto a rigid floor, generating a peak contact force of 368 MN. The results demonstrate that the impact produces possible localised surface damage zones to the reactor building structures and causes stress to spread into adjacent supporting walls. As heavy equipment fall might cause damage to building structures, assessing its potential consequences is important for evaluating local structural integrity during decommissioning. The findings using simplified FEM models demonstrate the possible damage zones of the steam drum separator room concrete floor of the reactor building under extreme impact loading, while stresses in the reinforcement are below the yield strength of the material, and provide additional information for safety assessment and structural planning during the decommissioning of ageing RBMK facilities.