The AP-600 obtains its emergency cooling from huge water tanks mounted above the reactor. Some of these are pressurized with nitrogen gas, allowing them to inject water even if the reactor remains at high pressure, as it may in some accident scenarios. In most cases, neither electric power nor operator action are needed to start injection. For example, if the pressure in the reactor falls due to a break in the system, the valves connecting some of the tanks to the reactor are automatically pushed open by the fact that the pressure on the tank side is being higher than on the reactor side. Actually, present reactors have similar systems. The "accumulators" mentioned in Chapter 6 operate on the same principle, but with enough water for only about 15 minutes, as compared with several hours in the AP-600.
One of the large tanks above the reactor serves as a place to deposit heat. It is connected to the reactor by two pipes, one leading to the bottom of the reactor vessel and one to the top. As water in the vessel is heated, it automatically rises in the upper pipe and is replaced by cool water from the lower pipe, establishing a natural circulation which transfers heat from the reactor into the water tank. This is analogous to air heated by a radiator in a house rising (because it is lighter) and spreading through the room to transfer the heat from the radiator to all the air in the room.
Water tanks pressurized with nitrogen gas also provide sprays to cool the atmosphere inside the containment and remove some of the volatile radioactive materials from the air in the event of an accident. Again, no pumps are needed.
The steel containment shell is cooled by air circulating between it and the concrete walls, again by gravity-induced convection. In addition there is water draining by gravity onto the containment shell, though the air circulation alone is sufficient to provide the necessary cooling. Air circulation keeps the pressure inside below 40 pounds per square inch in the worst accident scenarios.
Probabilistic risk analyses yield estimates that a core damage accident can be expected only once in 800,000 years of reactor operation, and that there is less than a 1% chance that this will be followed by failure of the containment. This makes the AP-600 a thousand times safer than the current generation of reactors. It is also much simpler, reducing the number of valves by 60%, large pumps by 50%, piping by 60%, heat exchangers by 50%, ducting by 35%, and control cables by 80%. The volume of buildings required to have a very high degree of earthquake resistance is thereby reduced by 60%. It is estimated that the plant can be constructed in 3 to 4 years. All of these factors contribute to reducing the cost.