Passive containment cooling water storage tank

PCCWST Passive Containment Cooling Water Storage Tank... 

The passive containment cooling water storage tank which is incorporated into the shield building structure above the containment... 

Additionally, a passive containment cooling ancillary water storage tank and two recirculation pumps are provided for onsite storage of additional passive containment cooling system cooling water, to transfer the inventory to the passive containment cooling water storage tank, and to provide a back-up supply to the fire protection system seismic standpipe system. 

Provision of water to the passive containment cooling water storage tank beyond 72 hours into the accident sequence. 

One shortcoming of the system is that after 72 hours the Passive Containment Cooling Water Storage Tank (PCCWST) on top of the Containment Shield Building needs to be refilled. This requires active pumping. A seismically qualified tank and diesel driven pumps are to be added to the Auxiliary Building, to provide a supply of water to the PCCWST adequate for 7 days post accident (see Section 8.4.3.9 for more details). 

Three parallel supply lines allow water flow from the passive containment cooling water storage tank to the containment shell. Diversity is provided in the actuation by using motor-operated valves for one path. 

Emergency core cooling system consists of two trains. Each train meets the single failure principle. System includes high and low pressure sub-system. High-pressxue subsystem includes passive (hydro-accumulators) and active pumps and water storage tanks) features for water injection in reactor. Low-pressure sub-system ensures returning a condense accumulated in containment, into the reactor by recirculation pumps. 

To achieve this, the containment building is made of steel to provide efficient heat transfer fi-om within to outside containment. During normal operation, heat is removed from the containment vessel by continuous natural circulation of air. During an accident, however, more heat removal is required and air-cooling is supplemented by evaporation of water, provided by the passive containment cooling system water storage tank. 

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