Total instantaneous blockage (TIB) at the inlet of a sub assembly (SA) of sodium cooled Fast reactors is a beyond design basis event that is a theoretical envelope of all small blockages. The extent of core damage propagation is an important input for the thermal design of the core catcher, which is provided for the collection and cooling of molten core. This study aims to conduct a coupled thermal-hydraulicreactor physics analysis of TIB in a medium sized Sodium cooled Fast Reactor (SFR). Towards this, thermal model is developed by solving heat transfer equations. A reactor Physics model is developed, which solves point kinetics equations numerically. The developed model can predict the temperature distributions of the fuel, clad, coolant and hexcan during accident conditions upto the start of hexcan melting. Various reactivity feedbacks and power changes during the accident are calculated. Time of occurrence of sodium boiling, clad melting, and fuel melting are obtained. It is found that TIB is detected earlier than that predicted by previous study without using coupled analysis.