As an advanced Gen III+ plant with passive safety systems, the AP1000® plant is uniquely equipped to handle an extended station blackout (SBO) event similar to what occurred at the Fukushima-Daiichi plants in March of 2011. These passive systems have been designed to maintain core cooling for up to 72 hours following all design basis events without the need for AC power or operator action. These core and containment cooling systems self-actuate such that even DC power is not required for their actuation. The Fukushima-Daiichi event demonstrated the effectiveness and desirability of the AP1000 systems.
The AP1000 plant, like other pressurized water reactors (PWRs), is provided with defense-in-depth active systems, such as auxiliary feed water pumps, to remove decay heat using the steam generators in the event that offsite power is lost. During an SBO the diesel generators powering this active equipment would not be available. In the event of an SBO the safety-grade heat removal function would be accomplished by the passive residual heat removal (PRHR) heat exchanger (HX) located in the in-containment refueling water storage tank (IRWST). The PRHR HX is designed to remove decay heat from the reactor coolant system (RCS) to the water in the IRWST, which increases in temperature and eventually boils. Steam from the IRWST is vented to the containment atmosphere and actuates the passive containment cooling system (PCS), which is used to apply water to the outside of the steel containment vessel and passively remove heat via evaporation to the environment. Steam that is condensed on the inside surface of the containment vessel forms a water film that flows down the containment wall and is returned to the IRWST using a system of water collection gutters and piping. The PCS is sized to remove reactor decay heat for 72 hours without the need for operator action.
Effective operation of the PRHR heat exchanger and PCS to remove decay heat from the reactor core to the environment depends on the ability to maintain water in the IRWST. Condensate that is not collected and returned to the IRWST is lost into the containment sump. There are several possible sources of loss. At the start of IRWST boiling, all containment structures will condense steam until their surface temperature approaches the steam temperature. This process is dependent on the heat capacity of these structures, and all condensation formed on these structures is considered lost. Since the containment wall is cooled by the PCS operation, condensation continues on the inside surface of the containment throughout the event. There are areas on the containment wall where condensate could be lost including the region at the top of the dome where the surface is nearly horizontal, and areas where weld seams and other obstructions could strip off some condensate film. To determine the coping time limits following an extended SBO, it is necessary to characterize these condensate losses.
A Phenomena Identification and Ranking Table (PIRT) process was conducted to determine the important phenomena associated with the return of condensate to the IRWST. This PIRT process identified the need for further experimentation to quantify the losses. This paper describes the PIRT and the experimental facility design used to determine the condensate return losses arising from phenomena identified by the PIRT.