Nuclear reactor safety system
The three primary objectives of nuclear reactor safety systems as defined by the U.S. Nuclear Regulatory Commission are to shut down the reactor, maintain it in a shutdown condition and prevent the release of radioactive material.A reactor protection system is designed to immediately terminate the nuclear reaction. By breaking the nuclear chain reaction, the source of heat is eliminated. Other systems can then be used to remove decay heat from the core. All nuclear plants have some form of reactor protection system.Control rods
s are a series of rods that can be quickly inserted into the reactor core to absorb neutrons and rapidly terminate the nuclear reaction. They are typically composed of actinides, lanthanides, transition metals, and boron, in various alloys with structural backing such as steel. In addition to being neutron absorbent, the alloys used also are required to have at least a low coefficient of thermal expansion so that they do not jam under high temperatures, and they have to be self-lubricating metal on metal, because at the temperatures experienced by nuclear reactor cores oil lubrication would foul too quickly.Safety injection / standby liquid control
are able to SCRAM the reactor completely with the help of their control rods. In the case of a loss of coolant accident, the water-loss of the primary cooling system can be compensated with normal water pumped into the cooling circuit. On the other hand, the standby liquid control system consists of a solution containing boric acid, which acts as a neutron poison and rapidly floods the core in case of problems with the stopping of the chain reaction.
Pressurized water reactors also can SCRAM the reactor with the help of their control rods. PWRs also use boric acid to make fine adjustments to reactor power level, or reactivity, using their Chemical and Volume Control System. In the case of LOCA, PWRs have three sources of backup cooling water, high pressure injection, low pressure injection, and core flood tanks. They all use water with a high concentration of boron.Essential service water system
The essential service water system 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 fouled by 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 systems
Emergency core cooling systems are designed to safely shut down a nuclear reactor during accident conditions. The ECCS allows 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
The High Pressure Coolant Injection 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 a threshold. 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.Automatic depressurization system
The Automatic Depressurization System consists of a series of valves which open to vent steam several feet under the surface of a large pool of liquid water in pressure suppression type containments, or directly into the primary containment structure in other types of containments, such as large-dry or ice-condenser 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 the high pressure systems. Some depressurization systems are automatic in function, while others may require operators to manually activate them. In pressurized water reactors with large dry or ice condenser containments, the valves of the system are called Pilot-operated relief valves.Steam relief valve
The Relief valves also known as Pressure Relief Valve or Safety Relief Valve are part of a group of valves that can be used in case of a reactor overpressure or during transition between normal operation mode and shutdown to reduce the pressure in the reactor vessel or in the steam lines. These valves usually discharge steam into the wetwell as mentioned above for the Automatic Depressurization System which discharges also in the wetwell.Low pressure coolant injection system
An LPCI is an emergency system which consists of a pump that injects a coolant into the reactor vessel once it has been depressurized. In some nuclear power plants an LPCI is a mode of operation of a residual heat removal system, also known as an RHR or RHS but is generally called LPCI. It is also not a stand-alone valve or system.Core spray system (only in BWRs)
This system uses spargers within the reactor pressure vessel to spray water directly onto the fuel rods, suppressing the generation of steam. Reactor designs can include core spray in high-pressure and low-pressure modes.Containment spray system
This system consists of a series of pumps and spargers that spray coolant into the upper portion of the primary containment structure. It is designed to condense the steam into liquid within the primary containment structure in order to prevent overpressure and overtemperature, which could lead to leakage, followed by involuntary depressurization.Isolation cooling system
This system is often driven by a steam turbine to provide enough water to safely cool the reactor if the reactor building is isolated from the control and turbine buildings. Steam turbine driven cooling pumps with pneumatic controls can run at mechanically controlled adjustable speeds, without battery power, emergency generator, or off-site electrical power. The Isolation cooling system is a defensive system against a condition known as station blackout. This system is not part of the ECCS and does not have a low coolant accident function. For pressurized water reactors, this system acts in the secondary cooling circuit and is called Turbine driven auxiliary feedwater system. In boiling water reactors, this system acts in the primary cooling circuit, also known as Main Cooling Circuit. This system is called Reactor Core Isolation Cooling System.Emergency electrical systems
Under normal conditions, nuclear power plants receive power from generator. 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 are usually sized such that a single one can provide all the required power for a facility to shut down during an emergency. Facilities have multiple generators for redundancy. Additionally, systems that are required to shut down the reactor have separate electrical sources so that they do not affect shutdown capability.Loss of electrical power can occur suddenly and can damage or undermine equipment. To prevent damage, motor-generators can be tied to flywheels that can provide uninterrupted electrical power to equipment for a brief period. 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 shut down a plant, open certain valves or power control room equipment. They can also be used to start up the EDGs.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 those that are gaseous at the reactor's operating temperature, 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/or concrete structure that contains the reactor vessel. In most reactors it also contains the radioactively contaminated systems. The primary containment system is designed to withstand strong internal pressures resulting from a leak or intentional depressurization of the reactor vessel.