The reactor has double containment system viz., primary and secondary containments. Between the two containments, a negative pressure with reference to atmosphere is maintained to ensure that there is no release of radioactivity to atmosphere under accidental conditions. The primary containment envelops the high enthalpy and low enthalpy zones designated as volume V1 and V2 respectively. The volume V2 is normally ventilated to atmosphere through a ventilation duct.

A scheme for containment isolation for AHWR under accidental condition without any active actuation is conceived. The scheme consists of an isolation water tank comprising of two compartments, one in communication with volume V1 through a vent shaft while the other is in communication with volume V2 via the normal ventilation duct as shown in Fig. III-4. A vertical baffle plate, running from the top of the tank, separates the two compartments. The baffle plate however, does not run through the full height of the tank. The bottom portion of the tank allows the two compartments to be in communication. It may be noted that the volume V2 is normally ventilated to atmosphere through a ‘U’ duct, which has a branched connection to isolation water tank outlet. In the event of volume V1 reaching certain pressure, the water level in other compartment of tank rises to spill the water in to the ‘U’ duct. Thus, isolation of volumes V1 and V2 from atmosphere is ensured by securing a water seal at the base of U duct. It is required that the seal be formed in a minimum possible time, typically of the order of few seconds, to ensure fast isolation.