In the AHWR, natural convection is the mode of coolant circulation to remove heat from the reactor core under both normal and shutdown conditions. Figure VI-3 shows the main heat transport (MHT) system and the passive decay heat removal system of the AHWR. A two phase steam water mixture generated in the core flows through the tail pipes to the steam drum, where steam gets separated from water. The separated water flows down through the downcomers to the reactor inlet header (RIH). From the header it flows back to the core through inlet feeders.

During a shutdown, core decay heat is removed by isolation condensers (ICs) submerged in a 6000 m3 capacity GDWP. Passive valves are provided downstream from the ICs. These valves operate on steam drum pressure and establish an interaction between steam drums and the ICs in hot shutdown conditions. The steam, brought to the ICs by natural convection, condenses inside the IC pipes immersed in the GDWP. The condensate is then returned to the core by gravity.

The ICs are designed to bring MHT temperature down from 558 K to 423 K. The water inventory in GDWP is adequate to cool the core for more than three days without any operator intervention and without boiling of the GDWP water.

During normal shutdown, when the main condenser is available, decay heat is removed by natural convection in the main heat transport circuit and heat is transferred to the ultimate heat sink through the main condenser. The IC system removes heat when the main condenser is not available. In the case of unavailability of both the IC and the main condenser, decay heat can be removed by an active system making use of MHT purification coolers.