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
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
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 50MWe (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 NSSS module design, unlike a conventional PWR design, eliminates the external
piping necessary to connect the steam generators and pressurizer to the reactor
- The NSSS module is designed to operate efficiently at full power conditions using
natural circulation as the means of providing core coolant flow, eliminating the
need for, and risk associated with, reactor coolant pumps.
- The absence of penetrations below the top of the reactor core.
- No insulation on the reactor pressure vessel.
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
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
Learn more about NuScale's spent fuel storage to protect the environment