The three primary objectives of nuclear safety systems as defined by the Nuclear Regulatory Commission are to shut down the reactor, maintain it in a shutdown condition, and prevent the release of radioactive material during events and accidents. These objectives are accomplished using a variety of equipment, which is part of different systems, of which each performs specific functions.

Reactor protection system (RPS)

A reactor protection system is composed of systems that are designed to immediately terminate the nuclear reaction. While the reactor is operating, the nuclear reaction continues to produce heat and radiation. By breaking the chain reaction, the source of heat can be eliminated, and other systems can then be used to continue to remove decay heat from the core. All plants have some form of the following reactor protection systems:

Control rods

Control rods are a series of metal rods that can be quickly inserted into the core to absorb neutrons and rapidly terminate the nuclear reaction. See control rods for more information.

Safety injection / standby liquid control

A nuclear reaction can also be stopped by injecting a liquid that absorbs neutrons directly into the core. In boiling water reactors this usually consists of a solution containing boron (such as boric acid), which can be injected to displace the water in the core. A signature of pressurized water reactors is that they use a boron solution in addition to control rods to control the reaction, and so the concentration is simply increased to slow or stop the reaction.

Essential service water system (ESWS)

The essential service water system (ESWS) circulates the water that cools the plant’s heat exchangers and other components before dissipating the heat into the environment. Because this includes cooling the systems that remove decay heat from both the primary system and the spent fuel rod cooling ponds, the ESWS is a safety-critical system. Since the water is frequently drawn from an adjacent river, the sea, or other large body of water, the system can be endangered by large volumes of seaweed, marine organisms, oil pollution, ice and debris. In locations without a large body of water in which to dissipate the heat, water is recirculated via a cooling tower.

The failure of half of the ESWS pumps was one of the factors that endangered safety in the 1999 Blayais Nuclear Power Plant flood, while a total loss occurred during the Fukushima I and Fukushima II nuclear accidents in 2011.

Emergency core cooling system (ECCS)

An emergency core cooling system comprises a series of systems that are designed to safely shut down a nuclear reactor during accident conditions. Under normal conditions, heat is removed from a nuclear reactor by condensing steam after it passes through the turbine. In a boiling water reactor, condensed steam (water) is fed back into the reactor. In a pressurized water reactor, it is fed back through the heat exchanger. In both cases, this keeps the reactor core at a constant temperature. During an accident, the condenser is not used, so alternate methods of cooling are required to prevent damage to the nuclear fuel.

These systems allow the the plant to respond to a variety of accident conditions, and additionally introduce redundancy so that the plant can be shut down even with one or more subsystem failures.

In most plants, ECCS is composed of the following systems:

High pressure coolant injection system (HPCI)

This system consists of a pump or pumps that have sufficient pressure to inject coolant into the reactor vessel while it is pressurized. It is designed to monitor the level of coolant in the reactor vessel and automatically inject coolant when the level drops below certain setpoints. This system is normally the first line of defense for a reactor since it can be used while the reactor vessel is still highly pressurized.

Depressurization system (ADS)

This system consists of a series of valves which open to vent steam several feet under the surface of a large pool of liquid water (known as the wetwell or torus) in pressure suppression type containments, or directly into the primary containment structure, in other types of containments, such as large-dry, ice-condenser, and sub-atmospheric containments. The actuation of these valves depressurizes the reactor vessel and allows lower pressure coolant injection systems to function, which have very large capacities in comparison to high pressure systems. Some depressurization systems are automatic in function but can be inhibited, some are manual and operators may activate if necessary.

Low pressure coolant injection system (LPCI)

This system consists of a pump or pumps which inject additional coolant into the reactor vessel once it has been depressurized.

In some nuclear power plants, LPCI is a mode of operation of a residual heat removal system (RHR or RHS). LPCI is generally not a stand-alone system.

Corespray system

This system uses spargers (special spray nozzles) within the reactor pressure vessel to spray water directly onto the fuel rods, suppressing the generation of steam. Reactor designs can include corespray in high-pressure and low-pressure modes.

Containment spray system

This system consists of a series of pumps and spargers which spray coolant into the primary containment structure. It is designed to condense the steam into liquid water within the primary containment structure to prevent overpressure, which could lead to involuntary depressurization.

Isolation cooling system

This system is often driven by a steam turbine, and is used to provide enough water to safely cool the reactor if the reactor building is isolated from the control and turbine buildings. As it does not require large amounts of electricity to run, and runs off the plant batteries, rather than the diesel generators, it is a defensive system against a condition known as station blackout.

Emergency electrical systems

Under normal conditions, nuclear power plants receive power from off-site. However, during an accident a plant may lose access to this power supply and thus may be required to generate its own power to supply its emergency systems. These electrical systems usually consist of diesel generators and batteries.

Diesel generators

Diesel generators are employed to power the site during emergency situations. They usually are sized such that a single one can provide all the required power for a facility to shutdown during an emergency situation which allows facilities to have multiple generators for redundancy. Additionally, systems which are not required to shutdown the reactor have separate electrical sources (often their own generators) so that they do not affect shutdown capability.

Motor generator flywheels

Loss of electrical power can occur suddenly, and it can damage or undermine equipment. To prevent damage, motor-generators can be tied to flywheels which can provide uninterrupted electrical power to equipment for a brief period of time. Often they are used to provide electrical power until the plant electrical supply can be switched to the batteries and/or diesel generators.

Batteries

Batteries often form the final redundant backup electrical system and are also capable of providing sufficient electrical power to shutdown a plant. The DC power generated by batteries can be converted to AC power to run AC devices such as motors using an electrical inverter.

Containment systems

Containment systems are designed to prevent the release of radioactive material into the environment.

Fuel cladding

The fuel cladding is the first layer of protection around the nuclear fuel and is designed to protect the fuel from corrosion that would spread fuel material throughout the reactor coolant circuit. In most reactors it takes the form of a sealed metallic or ceramic layer. It also serves to trap fission products, especially ones that are gaseous at the temperatures reached within the reactor, such as krypton, xenon and iodine. Cladding does not constitute shielding, and must be developed such that it absorbs as little radiation as possible. For this reason, materials such as magnesium and zirconium are used for their low neutron capture cross sections.

Reactor vessel

The reactor vessel is the first layer of shielding around the nuclear fuel and usually is designed to trap most of the radiation released during a nuclear reaction. The reactor vessel is also designed to withstand high pressures.

Primary containment

The primary containment system usually consists of a large metal and concrete structure (often cylindrical or bulb shaped) which contains the reactor vessel. In most reactors it also contains all of the radioactive contaminated systems. The primary containment system is designed to withstand strong internal pressures resulting from a leak or intentional depressurization of the reactor vessel.

Secondary containment

Some plants have a secondary containment system which encompasses the primary system. This is very common in BWRs because most of the steam systems, including the turbine, contain radioactive materials.

Core catching

In case of a full melt-down, the fuel would most likely end up on the concrete floor of the primary containment building. Concrete can withstand very much heat, so the thick flat concrete floor in the primary containment will often be sufficient protection against the so-called China Syndrome. The Chernobyl plant didn't have a containment building, but the core was eventually stopped by the concrete foundation. Due to concerns that the core would melt its way through the concrete, a "core catching device" was invented, and a mine was quickly dug under the plant with the intention to install such a device. The device contains a quantity of metal which would melt, diluting the corium and increasing its heat conductivity; the diluted metallic mass could then be cooled by water circulating in the floor. Today, all new Russian-designed reactors are equipped with core-catchers in the bottom of the containment building.

Non-containable events

Nuclear events outside of the primary containment building will not be contained. Any accident involving the spent fuel pool, which is outside of the primary containment, will not be contained.

Ventilation and radiation protection

In case of a radioactive release, most plants have a system designed to remove radiation from the air to reduce the effects of the radiation release on the employees and public. This system usually consists of the following:

Containment ventilation

This system is designed to remove radiation and steam from primary containment in the event that the depressurization system was used to vent steam into primary containment.

Control room ventilation

This system is designed to ensure that the operators who are required to operate the plant are protected in the event of a radioactive release. This system often consists of activated charcoal filters which remove radioactive isotopes from the air.