Large Break Loss of Coolant Accident (LBLOCA) is a design basis postulated event wherein a failure in large diameter pipes of the reactor coolant pressure boundary is postulated. Discharge in excess of the makeup capacity of the pressure control system, in any of the large diameter pipes in the main coolant system will cause voiding in the reactor core and add positive reactivity due to positive void coefficient characteristic of Pressurized Heavy Water Reactors (PHWRs). Sensing escalation of neutronic power beyond the demarcated set level, reactor protection systems get triggered and terminate the power transient. To study the system behavior during LBLOCA scenario and evaluation of Shutdown System performance for the governing case of LBLOCA with respect to power rise in 700 MWe PHWRs, NPCIL in-house developed System Thermal Hydraulic 3-D Neutronic code ATMIKA is used.