Passive containment coolers (PCCs) are used to provide post-accident primary containment cooling in a passive mode, as well as to limit post-accident primary containment pressure. The PCCs are located below the GDWP and are connected to the GDWP inventory, see Fig. VI-5. During a LOCA, condensation of steam and cooling of hot air are achieved via cooling provided by natural convection of GDWP water through the PCC tubes. This design feature ensures long term containment cooling after an accident.

Passive containment isolation system

The reactor has a double containment, i. e., incorporates primary and secondary containment. Between the two containments, a negative pressure in relation to the atmosphere is maintained to ensure that there is no release of radioactivity to the atmosphere. The primary containment envelops the high enthalpy and the low enthalpy zones designated as volume V1 and volume V2, respectively. Volume V2 is normally ventilated to the atmosphere through a ventilation duct, as shown in Fig. VI-6.

There is a very remote possibility of a release of radioactivity along with steam into the containment under accidental conditions. Under such accidental conditions, it is of paramount importance to isolate the containment from the atmosphere within a minimum possible time. The AHWR incorporates a scheme of containment isolation requiring no actuation by active means. This passive scheme is based on isolation of the containment atmosphere by establishing a liquid U-seal in the ventilation duct. A theoretical model is formulated to determine the time required for the formation of such a liquid seal.

TO ECC HEADER

ADVANCED ACCUMULATOR WITH FFCD

The scheme consists of an isolation water tank comprising the two compartments, one having a connection with volume V1 through a vent shaft, and the other having a connection with volume V2 via the normal ventilation duct, as shown in Fig. VI-6. 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 communicate. It should be noted that volume V2 is normally ventilated to the atmosphere through a ‘U’ duct, which has a branched connection to the isolation water tank outlet. In the event of volume V1 reaching a certain preset pressure, the water level in another compartment of

CORE

-]—A—|—I—I—x— ►-Ч

Rd liquid

RD POISON

FIG. VI-7. Passive shutdown by MHT high pressure (RD is for rupture disc).

the tank rises to spill the water into the ‘U’ duct. Thus, the isolation of volumes V1 and V2 from the atmosphere is ensured by securing a water seal at the base of the U duct. The seal must form in a minimum possible time, typically in the order of a few seconds, to ensure that the isolation is effective. Tests are to be conducted to identify degrading factors which could adversely affect the performance of this system. A probable degrading factor could be incomplete venting of air from the U tube.