The recent Fukushima accident has raised strong concern and apprehensions about the safety of reactors in case of a prolonged Station Black Out (SBO) continuing for several days. In view of this, a detailed study was performed simulating this condition in Advanced Heavy Water Reactor. In this study, a novel concept of moderator cooling by passive means has been introduced in the reactor design. The Passive Moderator Cooling System (PMCS) consists of a shell and tube heat exchanger designed to remove 2 MW heat from the moderator inside Calandria. The heat exchanger is located at a suitable elevation from the Calandria of the reactor, such that the hot moderator rises due to buoyancy into the heat exchanger and upon cooling from shell side  water returns  to Calandria forming a natural circulation loop. The shell side of  the heat exchanger is also  a natural circulation loop connected to an overhead large water reservoir, namely the GDWP.  The objective of the PMCS is to remove the heat from the moderator in case of  an SBO and maintaining its temperature below the permissible  safe limit (100^◦C)  for at least 7 days. The paper first describes the concept of the PMCS. The concept has been assessed considering a prolonged SBO for at least 7 days, through an integrated analysis performed using the code RELAP5/MOD3.2 Considering all the major components of the reactor. The analysis shows that the PMCS is able to maintain the  moderator temperature below boiling conditions for 7 days.