Passive Safety Systems
The NuScale Power ModuleTM and power plant design incorporates several simple, redundant, and independent safety features, as described below.
The major safety functions of the containment vessel are to contain the release of radioactivity following postulated accidents, protect the reactor pressure vessel and its contents from external hazards, and to provide an interfacing medium (reactor vessel to water, to containment vessel, to the pool) for decay heat removal following an accident or normal reactor shutdown.
The containment vessel is submerged in the reactor pool, which provides a passive heat sink for the containment heat removal under LOCA conditions.
The containment vessel pressure is maintained at a vacuum to eliminate convection heat transfer and the need for direct-contact reactor pressure vessel insulation. The vacuum also enhances steam condensation rates that would occur during an accident with ECCS actuation and limits non-condensable gases.
Following an actuation of the ECCS, heat removal through the containment vessel rapidly reduces the containment pressure and temperature and maintains them at less than design conditions for extended periods of time. Steam is condensed on the inside surface of the containment vessel, which is passively cooled by conduction and convection of heat to the reactor pool water.
Nuclear Steam Supply System (NSSS)
The NSSS is composed of a reactor core, a pressurizer, and two steam generators integrated within the reactor pressure vessel and housed in the steel containment vessel.
Since the NSSS is located entirely inside the containment vessel (which is immersed in water), there is greater flexibility to design for seismic forces.
Important safety features of the NSSS include:
Decay Heat Removal System (DHRS)
The DHRS provides secondary side reactor cooling for non-Loss of Coolant Accident (LOCA) events when normal feed water is not available. The system is a closed loop, two-phase natural circulation cooling system. Redundant trains of decay heat removal equipment are provided, one attached to each steam generator loop.
Emergency Core Cooling System (ECCS)
The ECCS consists of three independent reactor vent valves and two independent reactor recirculation valves. Successful actuation of the ECCS requires two of the three reactor vent valves and one of the two reactor recirculation valves to open.
Following a LOCA or other condition resulting in an actuation of the ECCS, heat removal through the containment vessel rapidly reduces the containment pressure and temperature. Steam is condensed on the inside surface of the containment vessel, which is passively cooled by conduction and convection of heat to the reactor pool water.
Chemical and Volume Control System (CVCS)
The primary functions of the CVCS are to purify reactor coolant, adjust the boron concentration in the reactor coolant, and supply spray flow to the pressurizer. Equipment within the CVCS also allows for chemical addition to the reactor coolant, and heats the reactor coolant during startup. The CVCS contains safety-related valves credited with stopping inadvertent boron dilution and inadvertent addition of reactor coolant inventory.