Containment is a key component of the mitigation part of the defence in depth philosophy, since it is the last barrier designed to prevent large radioactive release to the environment.

In advanced heavy water reactor (AHWR), a passive containment cooling system (PCCS) is envisaged which can remove long term heat from containment following loss of coolant accident (LOCA). Immediately following LOCA, steam released is condensed in water pool by vapor suppression system. For subsequent long term cooling, PCCS is provided. PCCS, by definition is able to carry out its function with no reliance on external source of energy.

As mentioned earlier, gravity driven water pool acts as the heat sink for a number of passive heat removal systems including the PCCS. The passive external condensers (PECs) of the PCCS are connected to the pool as shown in Fig. III-3. The containment steam condenses on the outer surface of the tubes of PEC. The water inside the tubes takes up heat from air/vapor mixture and gets heated up. Due to the heating up of water, the natural circulation of water from the pool to PEC and from PEC to pool is established.

One important aspect of PCCS functioning is the potential degradation of heat transfer on PEC outer surface due to the presence of noncondensable gases in the containment. The presence of noncondensable (NC) gases in vapor can greatly inhibit the condensation process. Extensive R&D work is in progress to address this issue. Another aspect of PCCS functioning is the blockage of passive external condenser by noncondensable gas due to the stratification of noncondensable gas/vapor in the containment. In case of AHWR, the noncondensable gas is likely to escape through the central opening provided in the GDWP to the dome region. Experiments are planned to confirm this.