Nuclear Power Plant Backup Generators: What Happens If They Fail?
Nuclear power plants continue to need electricity even after the reactor shuts down. Pumps, valves, instruments, controls, and other safety equipment must keep working so the plant can remove heat from the reactor and protect the fuel.
If outside grid power is lost, emergency diesel generators are designed to start automatically and supply selected safety systems. Batteries provide another layer of protection for vital controls and instruments. U.S. plants also have portable generators, pumps, and related equipment for longer emergencies.
But what exactly do nuclear power plant backup generators power, how long can they operate, and what could happen if several layers of backup power failed?
What Do Nuclear Power Plant Backup Generators Power?
Emergency diesel generators do not power the entire nuclear plant. They supply selected electrical buses and safety equipment needed to shut down and cool the reactor.
The exact equipment varies by reactor design, but it may include cooling-system pumps or their supporting equipment, motor-operated valves, control-room instruments, battery chargers, ventilation, emergency lighting, and other systems needed to protect the reactor and spent fuel.
Their purpose is not to keep producing electricity for customers. Their purpose is to give plant operators the power needed to safely control and cool the facility.
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Avoiding The Worst-case Scenario
The danger is losing the ability to remove heat from the reactor fuel. If cooling cannot be maintained, temperatures can rise enough to damage the fuel.
Severe fuel damage can release radioactive material into the reactor coolant and containment. Whether radioactive material reaches the environment depends on the extent of the damage and whether the plant’s containment and other protective barriers remain effective.
If cooling and water inventory cannot be restored, fuel damage may progress into a partial or extensive core melt. A core melt does not automatically mean that large amounts of radioactive material will escape the plant, because the reactor vessel and containment provide additional barriers.
How Nuclear Reactors Are Cooled
The basic goal is straightforward: keep water available and carry heat away from the reactor fuel.
Control rods can stop the sustained fission chain reaction quickly. However, radioactive decay inside the fuel continues producing heat after shutdown. This is called decay heat.
If that decay heat is not removed, temperatures can continue rising and eventually damage the fuel.
What happens when a nuclear reactor gets disconnected from the grid?
Many cooling and support systems require electrically powered pumps, valves, controls, and instruments. If offsite grid power is lost, emergency diesel generators are designed to start automatically and energize selected safety-related electrical buses.
If the emergency diesel generators also fail, batteries can temporarily support vital controls, instruments, communications, and selected equipment. However, batteries are not normally capable of powering all the large pumps and motors used by the plant.
Why Nuclear Power Plants Require Power From The Grid
During normal operation, plant equipment may receive electricity from the plant’s main generator or from the outside grid, depending on the plant design and operating condition. If the reactor and main generator shut down, the outside grid normally becomes an important source of power for cooling and other plant systems.
How Long Can Nuclear Plant Backup Power Last?
There is no single answer that applies to every nuclear plant or every emergency.
Emergency diesel generators can continue operating as long as they have fuel and their cooling, lubrication, electrical distribution, and control systems remain functional. Many U.S. plant designs maintain enough onsite diesel fuel for approximately seven days, although the exact capacity and requirements differ by facility.
Batteries provide a shorter bridge. At many plants, designated station-blackout battery loads have historically been supported for roughly four to eight hours. Batteries mainly operate controls, instruments, communications, and selected equipment rather than the plant’s largest pumps.
A station blackout occurs when a nuclear power plant loses its outside grid power and its onsite emergency AC power sources, such as the emergency diesel generators.
Station batteries may continue powering essential controls, instruments, communications, and selected equipment for a limited period. However, batteries alone generally cannot operate all the large AC-powered pumps and motors that might be needed during a prolonged emergency.
U.S. nuclear plants now also maintain portable FLEX equipment, including generators, battery chargers, pumps, hoses, and connections that can be used during an extended loss of electrical power. Additional equipment and supplies may be brought in from offsite.
Following the Fukushima accident, U.S. nuclear plants added another layer of emergency protection known as FLEX, referring to Diverse and Flexible Coping Strategies.
FLEX equipment can include portable generators, battery chargers, pumps, hoses, cables, and connection points. This equipment can provide power and water needed to maintain or restore reactor cooling, containment functions, and spent-fuel-pool cooling if the grid and permanently installed backup systems become unavailable. Additional equipment and supplies can also be brought in from offsite response centers.
In simple terms, FLEX provides a backup for the plant’s normal backup systems.
Because reactor designs and accident conditions differ, it is not accurate to say that every plant would melt down after a certain number of hours, weeks, or days. The important question is whether operators can continue removing decay heat and maintaining enough water around the fuel.
What kinds of events could knock out the backup diesel generators?
Nuclear plants normally have multiple emergency power sources so that the failure of one generator does not remove all backup AC power. However, a severe event could potentially damage several layers of equipment or the electrical distribution system connecting that equipment to the plant.
At Fukushima Daiichi, the earthquake caused the reactors to shut down and disrupted offsite grid power. Emergency generators initially started, but the tsunami flooded most of the backup generators and important electrical equipment. The resulting prolonged loss of power and cooling contributed to severe fuel damage in three reactors.
There’s another possibility whereby the diesel generator itself may be running properly but the distribution system between it and the plant becomes damaged in some way.
In a worst case scenario, it could be conceivable that the diesel fuel itself could run out if the distribution and supply systems that provide the fuel become interrupted.
Sources
- U.S. Nuclear Regulatory Commission, Station Blackout
- U.S. Nuclear Regulatory Commission, Diverse and Flexible Coping Strategies
- International Atomic Energy Agency, Fukushima Daiichi Nuclear Accident