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Containment Isolation

Containment isolation is provided to prevent or limit the escape of fission products that may result from postulated accidents. In the event of an accident, the containment isolation provisions are designed so that fluid lines penetrating the containment boundary are isolated. The containment isolation system consists of the piping, valves and actuators that isolate the containment.

Containment isolation is improved in the AP1000 because:

• The number of normally open penetrations is reduced by 50 percent, thanks to the simpler passive safety systems
• Penetrations that are normally open and at risk are fail safe - they fail in the closed position
• There is no recirculation of irradiated water outside of containment for design-basis accidents
• The steel containment is a high integrity steel pressure vessel, rather than a concrete vessel

The function of the AP1000 passive containment cooling system (PCS) is to prevent the containment vessel from overheating and exceeding the design pressure, which could result in a breach of the containment and the loss of the final barrier to radioactive release.

Passive Containment Cooling System (PCS)

The PCS consists of the following components:

• Air inlet and exhaust paths that are incorporated in the shield building structure
• An air baffle that is located between the steel containment vessel and the concrete shield building
• A passive containment cooling water storage tank that is incorporated in the shield building structure above the containment
• A water distribution system
• An ancillary water storage tank and two recirculation pumps for onsite storage of additional
• PCS cooling water, heating to avoid freezing, and for maintaining proper water chemistry

Natural Circulation

The PCS is able to effectively cool the containment following an accident such that the design pressure is not exceeded and the pressure is rapidly reduced. The steel containment vessel itself provides the heat transfer surface that allows heat to be removed from inside the containment and rejected to the atmosphere. Heat is removed from the containment vessel by a natural circulation flow of air through the annulus formed by the outer shield building and the steel containment vessel it houses. Outside air is pulled in through openings near the top of the shield building and pulled down, around the baffle and then flows upward out of the shield building. The flow of air is driven by the chimney effect of air heated by the containment vessel rising and finally exhausting up through the central opening in the shield building roof.

Water Evaporation

If needed, the air cooling can be supplemented by water evaporation on the outside of the containment shell. The water is drained by gravity from a tank located on top of the containment shield building.

Three normally closed, fail-open valves will open automatically to initiate the water flow if a high containment- pressure threshold is reached. The water flows from the top, outside, domed surface of the steel containment shell and down the side walls allowing heat to be transferred and removed from the containment by evaporation. The water tank has sufficient capacity for three days of operation, after which time the tank could be refilled, most likely from the ancillary water storage tank. If the water is not replenished after three days, the containment pressure will increase, but the peak pressure is calculated to reach only 90 percent of design pressure. After three days, air cooling alone is sufficient to remove decay heat.

In-Vessel Retention of Core Damage

The AP1000 is designed to mitigate a postulated severe accident such as core melt. In this event, the

AP1000 operator can act to flood the reactor cavity- the space immediately surrounding the reactor vessel with water from the in-containment refueling water storage tank (IRWST), submerging the lower portion of the reactor vessel. An insulating structure that surrounds the reactor vessel provides the pathway for water cooling to reach the vessel; flow around the bottom vessel head and up the vessel-insulation wall annulus; and to vent resulting steam from cooling the vessel from the reactor cavity. The cooling is sufficient to prevent molten core debris in the lower head from melting the steel vessel wall and spilling into the containment. Retaining the debris in the reactor vessel protects the containment integrity by simply avoiding the uncertainties associated with ex-vessel severe accident phenomena, such as ex-vessel steam explosion and core-concrete interaction with the molten core material.

Main Control Room Emergency Habitability System

The main control room can be isolated in case of high airborne radiation levels. The main control room

(MCR) emergency habitability system is comprised of a set of compressed air tanks connected to a main and an alternate air delivery line. Components common to both lines include a manual isolation valve, a pressure-regulating valve, and a flow metering orifice. This system is designed to provide the ventilation and pressurization needed to maintain a habitable environment for up to 11 people in the MCR for 72 hours following any design-basis accident.