Nuclear Power Plants: When The Backups Fail

September 24, 2013, by Ken Jorgustin

nuclear-power-plant-diesel-generator

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 up by several layers of redundancy, such as diesel generators and battery buffers. But what happens if the grid goes down… how long will the backups work and how long before nuclear power plants begin to melt down?


 
What’s the worst-case scenario?
The event we are looking to avoid is damaging the core by removing the heat by coolant (water) and pumps (electricity). Once you start damaging the core, you are then releasing radioactive material into the coolant and thereby increasing the chances that something travels outside the reactor.

How do you typically cool a reactor?
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, the heat can build up and can eventually damage the radioactive fuel or the reactor.

What happens when a reactor gets disconnected from the grid?
Pushing water past the core means pumps that are generally run by electricity. When the grid goes down, there are emergency diesel generators. You also have a battery system to keep instruments and safety systems running. First you rely on the grid. If the grid is no longer available, you use diesel generators. If there is an issue with the diesels, you have a battery backup. And the batteries usually last long enough for you to get the diesels going.

How much time is there before a meltdown?
It depends on the plant (boiling-water reactor or a pressurized-water reactor). Basically though, broadly, you have many hours to restore power to the system to get normal cooling going. Apparently it’s really not possible to get more specific than “many hours.” But generally, it’s fair to say… less than 24 hours.

Why do nuclear power plants need electricity to be cooled?
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, because 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.

What kinds of events could knock out a diesel generator?
There is always the possibility of just plain old failure. That’s why you have multiple diesels at a plant for redundancy’s sake. It can be the case that the diesel generator itself is running properly but the distribution system between it and the plant is down. 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 the 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 you had to stop, at any point, carrying away that heat (via water, pumps, and electricity), it would start building up again. Emergency cooling systems have to be available for weeks.

 
Assuming the core itself and/or the reactor containment and physical integrity of the nuclear power plant has not been compromised (example: Fukushima damage via massive earthquake,) then it’s fair to say that after a grid-down situation, apparently 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, etc.) 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 nuke plant…

Some information sourced from scientificamerican.com