The proposed fuel handling scheme of the future Indian fast reactors (FBR 1 and 2) envisages the use of a dedicated flask called subassembly transfer flask (STF) for handling of subassemblies out of the reactor vessel. The STF transfers the fuel subassembly (SA) between fuel transfer port (FTP-I) on roof slab and fuel transfer position 2 (FTP-2) on RCB floor and vice versa. This machine consists of a flask mounted over a self-propelled carriage, which moves between the fuel transfer port of reactor vessel at one end and fuel transfer port of inert gas filled SA transfer carriage cell (STC cell) at another end. STF transfer/receives transfer pot holding irradiated/FSA (Fresh Subassembly) to/from the transfer carriage through the transfer ports. STF traverse between the two ports on rails and raises/lowers the transfer pot by wire ropes, Multibody dynamics approach is used to study the dynamic behavior of the STF under an earthquake loading. The present work is a detailed MBD investigation of the performance of STF under seismic events to evaluate the forces experienced by the different components of the machine and to study the motion of SA inside STF during a seismic excitation. Rigid body dynamic behaviour is considered for the docked STF at FTP loaded with SA, but the locking pins are modelled as flexible elements. Nonlinear contact behaviour between STF and rails during seismic excitation is observed and the functionality effectiveness of locking mechanism provided for the machine is verified through MBD investigation using RecurDyn solver for analysis.