Natural circulation concept is used in the AC600/1000 passive safety system design. The system is actuated by gravity, natural circulation or pressurized gas. Following accident, AC600/1000 is able to maintain core cooling and containment integrity without operator’s intervention. This is an important safety requirement in the AC600/1000 design.

Emergency residual heat removal system (ERHRS) is used to remove decay heat from reactor core following accident. Secondary side of SQ emergency feedwater tank and air cooler establish a natural circulation cycle. Air coolers are located in a chimney. The heat transfer area is about 750m for each air cooler. Emergency feedwater tank volume is 25m for each SG cycle.

Passive safety injection system of AC600/1000 (PSIS) is mainly used to mitigate the consequence of LOCA. The core cooling water is provided through use of the following four

water sources: core make-up water tank (volume is 2 or 3 x 40m ), accumulator (volume is 2 or 3 x 40m ), refueling water storage tank and containment sump. Low pressure safety injection subsystem uses two or three active pumps, design flow rate of each which is 142 kg/s.

The containment of AC600/1000 is a two-shell structure. Between inner steel shell and outer reinforced cylindrical concrete shell, there is a baffle to form an annular wind duct. Containment top water storage tank capacity can meet the requirement of 72 hours for steel shell cooling after large LOCA. In the top of containment, there is a cooling water distribution to make the heat removal more efficient and quick in the early phase of an accident. For long term cooling, peak pressure of the containment is not larger than 90% of containment design pressure. For severe accident, containment pressure is below failure pressure. Containment spray system is eliminated in the AC600/1000 design.