The 104 U.S. nuclear power plants are some of the most sophisticated and complex energy systems ever designed. However any complex system, no matter how well it is designed and engineered, cannot be deemed failure-proof.
The emergency power supplies of a nuclear power plant are built with several layers of redundancy such as diesel generators and battery buffers.
But what happens if the power grid goes down? How long will the backups work and how long before nuclear power plants begin to melt down?
Avoiding The Worst-case Scenario
The event we are looking to avoid is removing the coolant systems effectiveness of removing heat from the fuel rods, leading to core damage.
When the core becomes damaged, there may be a release of radioactive material into the coolant – thereby increasing the chances that something travels outside the reactor.
Worse yet, a complete loss of coolant ultimately leading to a ‘meltdown’ and radioactive release into the environment.
How Nuclear Reactors Are Cooled
It’s actually a simple approach: Push water past the nuclear core and carry the heat somewhere else.
The chain reaction that actually runs the reactor can be shut off quickly, however what’s left over in the core, the radioactive material, will continue to give off heat for a long time.
Unless you have a mechanism to remove that heat, it WILL continue to build up and WILL eventually damage the radioactive fuel and the reactor.
What happens when a nuclear reactor gets disconnected from the grid?
Pushing water past the core involves pumps which run by electricity. When the electrical power grid goes down there are emergency diesel generators that automatically kick in. There are also battery systems to keep instruments and safety systems running.
The plant relies on the power grid. If the grid is no longer available, the plant switches to diesel generators. If there is an issue with the diesel generators, there is a battery backup.
Why Nuclear Power Plants Require Power From The Grid
Nuclear reactors produce much more electricity than they need to run their systems. As a basic design feature though, plants are not literally self-powering. That’s by design.
You don’t want to end up in a situation where a problem at the plant cuts off its own power source. Therefore, the primary means of power for a plant in order for it to run is electricity from the grid.
How Long Until Nuclear Reactor Damage?
It depends on the plant (boiling-water reactor vs. pressurized-water reactor). Basically, broadly speaking, there are many hours available to restore power to the system and restore cooling. Apparently it’s really not possible to get more specific than “many hours” (according to what I’ve read).
But generally, it’s fair to say less than 24 hours to restore cooling.
What kinds of events could knock out the backup diesel generators?
There is always the possibility of just plain old failure. That’s why there are multiple generators at a plant for redundancy’s sake.
The Fukushima meltdown occurred because an earthquake and tsunami damaged and flooded the diesel generator systems.
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.
How long will emergency backup systems last?
NRC regulatory requirements for emergency power supplies is that they be available on the order of a month.
Heat is still being generated even after successfully shutting down a reactor. If coolant systems stopped carrying away that heat (via water, pumps, and electricity) it would start building up again. Emergency cooling systems have to be available for weeks after a shutdown.
WEEKS, MAYBE 1 MONTH
Assuming the core itself and the reactor containment physical integrity and the backup power systems of a nuclear power plant has not been compromised, then it might be fair to say that after a grid-down situation an operational plant could stay cool and survive without melting down for several weeks, perhaps a month without resupply.
If we are to ever face a major cataclysmic grid-down (EMP, Solar-flare Carrington Event, other..) then the odds greatly increase that most all distribution supply chains will grind to a halt very quickly, which could in turn create a major problem for refueling diesel generators to keep cooling pumps running. So… after 2 to 4-weeks, you’d best be far away from any nuclear power plant…
(Some information sourced from ScientificAmerican.com)