Passive Core Cooling System - Safety Injection and Reactor Coolant Inventory Control

The PXS uses three sources of water to maintain core cooling:

All of these injection sources are connected directly to two nozzles on the reactor vessel. These connections, which have been used on existing two-loop plants, reduce the possibility of spilling part of the injection flow.

High Pressure Safety Injection with CMTs

Passive reactor coolant makeup is provided to accommodate small leaks following transients or whenever the normal makeup system is unavailable. Two Core Makeup Tanks (CMTs), filled with borated water, are designed to provide this function at any Reactor Coolant System (RCS) pressure using only gravity as a motivating force. These tanks are designed for full RCS pressure and are located above the RCS loop piping. If the water level in the pressurizer reaches a low-low level, the reactor is tripped, the reactor coolant pumps are tripped, and the CMT discharge isolation valves open automatically. The relative elevations of the CMTs and the pressurizer are such that if RCS level continued to decrease, the water in the CMTs would drain into the reactor vessel.

Medium Pressure Safety Injection with Accumulators

As with current pressurized water reactors (PWRs), accumulators are required for large loss-of-coolant accidents (LOCAs) to meet the need for higher initial makeup flows to refill the reactor vessel lower plenum and downcomer following RCS blowdown. The gas pressure forces open check valves that normally isolate the accumulators from the RCS. The accumulators are sized to respond to the complete severance of the largest RCS pipe by rapidly refilling the vessel downcomer and lower plenum. The accumulators continue delivery to assist the CMTs in rapidly reflooding the core.

Low Pressure Reactor Coolant Makeup from the IRWST

Long-term injection water is provided by gravity from the In-containment Refueling Water Storage Tank (IRWST), which is located in the containment just above the RCS loops. Normally, the IRWST is isolated from the RCS by self-actuating check valves. This tank is designed for atmospheric pressure. As a result, the RCS must be depressurized before injection can occur. The AP600 automatically controls depressurization of the RCS to reduce its pressure to about 10 psig, at which point the head of water in the IRWST is sufficient to overcome the small RCS pressure and the pressure loss in the injection lines. The automatic depressurization system (ADS) is made up of four stages of valves to permit a relatively slow, controlled RCS pressure reduction. The first three stages are connected to the pressurizer and discharge through spargers into the IRWST. The fourth stage is connected to a hot leg and discharges through redundant isolation valves to the containment. The ADS stages are actuated by CMT level.