The most effective means of describing the function of each of these passive safety systems is to relate their operation in response to a small break loss of coolant accident (SBLOCA). The first phase of a SBLOCA is the subcooled blowdown phase. During this phase, high pressure subcooled liquid is venting from the break under choked flow conditions. The primary system pressure and primary liquid inventory will be decreasing. When low pressure or low liquid level is sensed in the pressurizer, a safety signal is issued resulting in the following automatic actions:

• Scram reactor

• Open the PRHR inlet and outlet valves

• Open the CMT outlet valves

• Isolate Steam Generators (feedwater and main steam)

• Trip reactor coolant pumps (coastdown).

Natural circulation is established in the PRHR loop and the CMT loops. Boiling occurs on the PRHR tubes and hot water begins to fill the top of the CMTs. If the plant continues to depressurize, eventually the primary system reaches the saturation pressure corresponding to the hot leg temperature. Depending on the break size, the system pressure will reach a plateau during which the loop will experience a period of two-phase natural circulation.

If primary coolant inventory continues to decrease, eventually the CMTs will begin to drain. At a predetermined CMT level, the ADS-1 valves will open followed by the ADS 2-3 valves. System pressure will drop very quickly as a result of the ADS 1-3 venting steam into the IRWST. The primary system pressure soon drops below the accumulator tank pressure; and significant quantities of cold borated water are injected from the accumulator into the reactor vessel.

If the CMT liquid level continues to decrease, the ADS-4 actuation setpoint will be reached. The ADS-4 valves open, dropping the primary system pressure below the head pressure of the IRWST liquid. The IRWST drains by gravity into the reactor vessel, out the break and ADS 4 valves into the containment sump. Eventually the IRWST and containment sump liquid levels equalize and the sump valves are opened, establishing long term sump recirculation cooling.

Steam vented through the ADS-4 valves condense on the inside surfaces of the containment vessel. The containment vessel is externally cooled by air and water as needed. The condensate inside the containment is returned to the containment sump and IRWST, where it is available for sump recirculation.