Emergency Core Cooling System Among Safety Procedures at U.S. Nuclear Power Plants

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Researched by Industrial Info Resources (Sugar Land, Texas)--There are many different procedures at nuclear power plants in case of an emergency shutdown. All systems are monitored, such as controls, alarms, temperature gages, signals. Each component is designed for a specific purpose. The plant is staffed around the clock with supervisors, control room operators, and auxiliary equipment operators monitoring the units. Operators constantly perform safety tests on emergency equipment to ensure each part is functioning properly.

The Emergency Core Cooling Systems (ECCS) of nuclear power plants have one major function--providing water to cool the reactor in the event of a loss of coolant from the reactor cooling system. The cooling is needed to remove decay heat still in the reactor's fuel after the reactor is shut down.

The ECCS in some plants may have a second major function in providing chemicals to the reactor and regulating and controlling the amount of power produced. Under normal conditions, heat is removed from a nuclear reactor by condensing steam after it passes through the turbine. In the boiling water reactor (BWR), condensed steam, which is water, is then transitioned back into the reactor or, in a power water reactor (PWR), back through the heat exchanger, which keeps the reactor core at a constant temperature. If a failure occurs, the condenser is not used, so alternate procedures are required and put in place to prevent damage to the nuclear core.

The Cooling System must continually pump water from a large pool that surrounds the core through a set of towers that actually keep the water at a safe temperature. Otherwise, the water will boil off, the fuel rods will melt, and radioactive materials will escape from the reactor's containment dome, which could lead to a meltdown.

Other systems comprise the ECCS, allowing the nuclear plant to respond to accident conditions and create redundancy, so that the plant can still shut down, even if one or more of the systems fail.

Emergency makeup cooling pumps usually are motor-driven. In some cases, steam-driven pumps are used in the case of boiler water system (high-pressure coolant injection). This system is normally the first line of defense for the reactor, since it can be used while the reactor is still highly pressurized. The Low-Pressure Coolant Injection System is responsible for depressurization.

The Core Spray System suppresses the generation of steam, ensuring continued coolant injection, and sprays directly on fuel rods in the event the core is uncovered.

The Isolation Cooling System is driven by the turbine and provides enough water to safely cool the reactor, if the reactor building is isolated from the control and turbine building. It does not require large amounts of electricity to run; instead, it runs off the plant's batteries. For the motor-driven pumps, power may be received from emergency diesel generators if power is lost from normal power supply.

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