This article covers the technical aspects of active nuclear safety systems. For a general approach to nuclear safety, see nuclear safety

Nuclear safety

Nuclear safety covers the actions taken to prevent nuclear and radiation accidents or to limit their consequences. This covers nuclear power plants as well as all other nuclear facilities, the transportation of nuclear materials, and the use and storage of nuclear materials for medical, power,...

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The three primary objectives of nuclear reactor safety systems as defined by the Nuclear Regulatory Commission

Nuclear Regulatory Commission

The Nuclear Regulatory Commission is an independent agency of the United States government that was established by the Energy Reorganization Act of 1974 from the United States Atomic Energy Commission, and was first opened January 19, 1975...

 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

Nuclear chain reaction

A nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The specific nuclear reaction may be the fission of heavy isotopes or the fusion of light isotopes...

, the source of heat can be eliminated, and other systems can then be used to continue to remove decay heat

Decay heat

Decay heat is the heat released as a result of radioactive decay. This is when the radiation interacts with materials: the energy of the alpha, beta or gamma radiation is converted into the thermal movement of atoms.-Natural occurrence:...

 from the core. All plants have some form of the following reactor protection systems:

Control rods

Control rod

Control rod

A control rod is a rod made of chemical elements capable of absorbing many neutrons without fissioning themselves. They are used in nuclear reactors to control the rate of fission of uranium and plutonium...

s 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

Boric acid

Boric acid, also called hydrogen borate or boracic acid or orthoboric acid or acidum boricum, is a weak acid of boron often used as an antiseptic, insecticide, flame retardant, as a neutron absorber, and as a precursor of other chemical compounds. It exists in the form of colorless crystals or a...

), 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

Decay heat

Decay heat is the heat released as a result of radioactive decay. This is when the radiation interacts with materials: the energy of the alpha, beta or gamma radiation is converted into the thermal movement of atoms.-Natural occurrence:...

 from both the primary system and the spent fuel rod

Nuclear fuel

Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...

 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

Cooling tower

Cooling towers are heat removal devices used to transfer process waste heat to the atmosphere. Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or in the case of closed circuit dry cooling towers rely...

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The failure of half of the ESWS pumps was one of the factors that endangered safety in the 1999 Blayais Nuclear Power Plant flood

1999 Blayais Nuclear Power Plant flood

The 1999 Blayais Nuclear Power Plant flood was a flood that took place on the evening of December 27, 1999. It was caused when a combination of the tide and high winds led to the sea walls of the Blayais Nuclear Power Plant in France being overwhelmed...

, 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

Boiling water reactor

The boiling water reactor is a type of light water nuclear reactor used for the generation of electrical power. It is the second most common type of electricity-generating nuclear reactor after the pressurized water reactor , also a type of light water nuclear reactor...

, condensed steam (water) is fed back into the reactor. In a pressurized water reactor

Pressurized water reactor

Pressurized water reactors constitute a large majority of all western nuclear power plants and are one of three types of light water reactor , the other types being boiling water reactors and supercritical water reactors...

, 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

Nuclear fuel

Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...

.

These systems allow 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 normally runs off the plant batteries, rather than the diesel generators. The Isolation cooling system can also be manually run without battery power. 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

Battery (electricity)

An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...

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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

Krypton

Krypton is a chemical element with the symbol Kr and atomic number 36. It is a member of Group 18 and Period 4 elements. A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionally distilling liquified air, and is often used with other...

, xenon

Xenon

Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...

 and iodine

Iodine

Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....

. 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

Neutron capture

Neutron capture is a kind of nuclear reaction in which an atomic nucleus collides with one or more neutrons and they merge to form a heavier nucleus. Since neutrons have no electric charge they can enter a nucleus more easily than positively charged protons, which are repelled...

 cross sections.

Reactor vessel

The reactor vessel

Reactor vessel

In a nuclear power plant, the reactor vessel is a pressure vessel containing the Nuclear reactor coolant and reactor core.Not all power reactors have a reactor vessel. Power reactors are generally classified by the type of coolant rather than by the configuration of the reactor vessel used to...

 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

Containment building

A containment building, in its most common usage, is a steel or reinforced concrete structure enclosing a nuclear reactor. It is designed, in any emergency, to contain the escape of radiation to a maximum pressure in the range of 60 to 200 psi...

 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

China Syndrome

The term China syndrome describes a nuclear reactor operations accident characterized by the severe meltdown of the core components of the reactor, which then burn through the containment vessel and the housing building, then notionally through the crust and body of the Earth until reaching...

. The Chernobyl

Chernobyl

Chernobyl or Chornobyl is an abandoned city in northern Ukraine, in Kiev Oblast, near the border with Belarus. The city had been the administrative centre of the Chernobyl Raion since 1932....

 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

Corium (nuclear reactor)

Corium, also called fuel containing material or lava-like fuel containing material , is a lava-like molten mixture of portions of nuclear reactor core, formed during a nuclear meltdown, the most severe class of a nuclear reactor accident...

 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.

Standby gas treatment

A Standby Gas Treatment (SBGT) system is part of the secondary containment system of a nuclear power plant. When called upon to operate, the SBGT system filters and pumps air from secondary containment to the environment and maintains a negative pressure within the secondary containment in order to limit the release of radioactive material.

Each SBGT train generally consists of a mist eliminator/roughing filter; an electric heater; a prefilter; two absolute (HEPA

HEPA

High-Efficiency Particulate Air or HEPA is a type of air filter. Filters that are awarded the HEPA accolade are used in various locations, whether in medical facilities, automotive vehicles, airplanes, home filters, or wherever very pure air is sought. The filter must satisfy certain standards of...

) filters; an activated charcoal filter; an exhaust fan; and associated valves, ductwork, dampers, instrumentation, and controls. The signals that trip the SBGT system are plant-specific; however, automatic trips are generally associated with the electric heaters and a high temperature condition in the charcoal filters.

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.

See also

• Boiling water reactor safety systems
  Boiling water reactor safety systems
  Boiling water reactor safety systems are nuclear safety systems constructed within boiling water reactors in order to prevent or mitigate environmental and health hazards in the event of accident or natural disaster....
  
  
• Nuclear accidents in the United States
  Nuclear accidents in the United States
  According to a 2010 survey of energy accidents, there have been at least 56 accidents near nuclear reactors in the United States . The most serious of these was the Three Mile Island accident in 1979...
  
  
• Nuclear safety in the U.S.
• Passive nuclear safety
  Passive nuclear safety
  Passive nuclear safety is a safety feature of a nuclear reactor that does not require operator actions or electronic feedback in order to shut down safely in the event of a particular type of emergency...