Nuclear safety systems
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This article covers the technical aspects of active nuclear safety systems, for a general approach to nuclear safety see nuclear safety.
The three primary objectives of Nuclear Safety Systems as defined by the Nuclear Regulatory Commission are to shutdown the reactor, maintain it in a shutdown condition and prevent the release of radioactive material during events and accidents [1]. These objectives are accomplished using a variety of equipment which is broken up into systems each of which perform specific functions.
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[edit] Reactor Protection System (RPS)
The Reactor Protection System is composed of systems which 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:
[edit] Control Rods
Control rods are a series of metal rods which can be quickly inserted into the core to absorb neutrons and rapidly terminate the nuclear reaction. See Control rods for more information.
[edit] Safety Injection/Standby Liquid Control
A nuclear reaction can also be stopped by injecting a liquid which 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 to control the reaction in addition to control rods and so the concentration is simply increased to slow or stop the reaction.
[edit] Emergency Core Cooling System
The Emergency Core Cooling System (ECCS) comprises a series of systems which 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. Then in a BWR, condensed steam (water) is then fed back into the reactor or, in a PWR, back through the heat exchanger; which 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.
In most plants ECCS is composed of the following systems:
[edit] High Pressure Coolant Injection System (HPCI)
This system consists of a pump or pumps which 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 pressurized.
[edit] Depressurization System
This system consists of a series of valves which open to vent steam into a primary containment structure which depressurizes the reactor vessel and allows lower pressure coolant injection systems to function.
[edit] 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, the LPCI is a mode of operation of the Residual Heat Removal System (RHR or RHS). LPCI is generally not a stand-alone system.
[edit] Internal/Core Spray System
This system consists of a series of pumps and spargers (special spray nozzles) which spray coolant into the primary containment structure. It is designed to condense the steam in the primary containment structure to prevent it from rupturing.
These systems allow the plant to respond to a variety of accident conditions and at the same time creates redundancy so that the plant can still be shutdown even if one or more of the systems fails to function.
[edit] 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.
[edit] 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.
[edit] Motor Generator Flywheels
Losses of electrical power can occur suddenly which 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.
[edit] 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.
[edit] Containment Systems
Containment systems are designed to prevent the release of radioactive material into the environment.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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:
[edit] 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.
[edit] 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.
[edit] References
- American National Standard, ANSI N18.2, “Nuclear Safety Criteria for the Design of Stationary Pressurized Water Reactor Plants,” August 1973.
- IEEE 279, “Criteria for Protection Systems for Nuclear Power Generating Stations.”
