Passive Safety Systems
The NuScale Power Module™ 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. Although not credited, the reactor pool provides an additional means of fission product retention beyond that of the fuel, fuel cladding, reactor pressure vessel, and the containment for certain events. The containment vessel is designed to withstand the environment of the reactor pool as well as the high pressure and temperature of any design basis accident.
The containment vessel pressure is maintained at a vacuum under normal operating conditions, providing for reduced moisture that could contribute to component corrosion and impact the reliability of instrumentation and other systems within the containment vessel. The vacuum essentially eliminates convection heat transfer removing 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 the available oxygen, which is beneficial for severe accident combustible gas control.
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) Module
The module is composed of a reactor core, a pressurizer, and two steam generators integrated within the reactor pressure vessel and housed in a compact steel containment vessel.
Since the module is located entirely inside the containment vessel (which is immersed in water), there is greater flexibility to design for seismic forces. The likelihood of releasing a large amount of radioactive materials into the environment is already extremely low since each 50 MWe (gross) NuScale Power Module houses approximately 5% of the fuel inventory of a conventional 1,000 MWe nuclear reactor.
Important safety features of the NSSS module 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. Each train is capable of removing 100 percent of the decay heat load to cool the reactor coolant system. Each train has a passive condenser submerged in the reactor pool. The condensers are maintained with sufficient water inventory for stable operation.
Emergency Core Cooling System (ECCS)
The ECCS consists of two independent reactor vent valves and two independent reactor recirculation valves. The ECCS provides a means of decay heat removal in the event of a loss of coolant accident or a loss of the main feed water flow in conjunction with the loss of both trains of the DHR system.
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 and maintains them at acceptably low levels 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. Since the containment vessel is evacuated to a low absolute pressure during normal operation, only a small amount of non-condensable gas will be present inside the containment vessel.
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.