<p>The passive decay heat removal system (PDHRS) is a first-of-a-kind (FOAK) system that has been introduced in 700 MWe Indian pressurized heavy water reactors (PHWRs) for the decay heat removal function during the station blackout (SBO) scenario to enhance reactor safety. It ensures the continued operation of core cooling in thermosyphon mode via steam generators (SG) to PDHRS. To assess the heat-removal capability of PDHRS, performance tests were conducted at different project sites of 700 MWe Indian PHWRs during phase-A commissioning. These tests indicate the heat removal capability of PDHRS. Integrated SBO analysis for 700 MWe Indian PHWRs was performed using RELAP5/MOD3.4. Analysis indicates that after pressuriser isolation, the primary heat transport (PHT) pressure drops sharply, and voids begin to appear, which further aggravate as cooling continues. Also, internal leaks in the PHT system contribute to higher void formation. Voids in the PHT system, up to a certain extent, the coolant flow in the thermosyphoning mode is enhanced due to a higher density difference. But as the voids increase above certain limits (i.e., when voids penetrate to the cold leg of SG U tubes), then coolant flow oscillation, even flow reversal, was observed during SBO analysis. To keep the PHT system solid, it was proposed to actuate the emergency core cooling system (ECCS) so that the PHT system would remain in a sub-cooled state throughout the SBO duration. A PDHRS performance test in a 700 MWe Indian PHWR was performed to demonstrate the feasibility of ECCS actuation and riding over on the PHT system for meeting the shrinkage requirement due to PHT cooldown and system leakage. This study also covers the simulation of the PDHRS test conducted in a 700 MWe Indian PHWR with ECCS riding on the PHT system using thermal-hydraulic computer code RELAP5/MOD3.4. Analytical results predicted through code are compared with actual test results and found to be in good agreement with test results.</p>