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
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. 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.
In most plants ECCS is composed of the following systems:
- 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 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 the Residual Heat Removal System (RHR or RHS). LPCI is generally not a stand-alone system.
- This system uses spargers within the reactor pressure vessel to directly spray water onto the fuel rods themselves. It suppresses generation of steam, ensuring continued coolant injection, and sprays water directly on the fuel rods themselves in the event of core uncovery. In some reactor types there are both high-pressure modes and low-pressure modes for corespray.
Containment 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 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.