Eli5: Why do nuclear plants need external power? Couldn’t they just siphon off some of the power the plant makes to accomplish the same tasks (like an alternator running the electrical components in a car)?
In: 2
Yes, and they do that. However if you shut off the reactor, you can’t use its power output. The core, fresh out of neutron bombardment, still generates enough decay heat to melt itself into a puddle on the bottom on the reactor vessel though (this occurence is known to lead to many cups of coffee being consumed and frequent swearing).
So, you need to get electricity from *somewhere* to run the cooling pumps. Usually you just grab power from the main network, but just in case there is an outage, nuclear power plants have several diesel generators ready to do that, just make sure you don’t run out of fuel as well before the grid gets unfucked.
They are designed to generate massive amounts of power, not very small amounts like their systems use. If you would like to use the alternator analogy, it would be like an alternator being so massive that the engine has to run at full throttle to make power. Anything less and it doesn’t make the power needed to run other things. Since the grid is designed to be stable, they don’t build any steam redundancy into the power generation system. Those are just more components to fail/leak in an already hazardous environment.
Yes, unless something goes wrong. The big nuclear disaster we all know about (Chernobyl and also Fukushima) was caused because they didn’t have enough power to make the cooling system work properly.
Most of the time this would be not a problem, but when it is, it is critical. It is like you don’t need enough life jackets and lift boats on the titanic… until you do, and then you really really need them.
Some designs of nuclear reactors require a lot of time (and energy) to spin up and down, including most early generation, commercial reactors.
Not having power during either the bringup or shutdown steps (which can be measured in days) leads to… very very very bad outcomes.
Reactors can be designed to not need this. A good chunk of passively safe reactors will do this. (Passively safe simply means human monitoring isn’t needed during failure cases, but that happens to correspond well with reactors that can shut down without external inputs).
If a power source is required to shut down, then you have to make damn sure there is one available at all times. Even if during an orderly shutdown you may be able to use the reactors own power, not all shutdowns are orderly, and you really don’t want to be in a position you can’t shut down a malfunctioning reactor because that reactor is the only thing providing power needed to bring the reaction to a stop…
most power plants are not actually capable of “black start” operations, and those that are usually have dedicated systems just to do that. hydro is the notable exception, and that is among the reasons the hoover damn was built. it can (and does) feed power into other power stations to bootstrap them when they lose power.
An airliner has 4 separate electric generators and an emergency battery. It’s called redundancy. Basically you calculate the worse scenario, design the plane in a way that work for that scenario, and add an extra backup system, just in case the worse scenario happens while you are one backup system short.
To guarantee you have the power to control and cool the plant in the worse scenario, you want to have at least 3 sources of electricity. One is the plant itself so the plant works even if the grid fails. One is the grid so if the plant fails the power is still there. One is a backup generator, generally driven by a diesel engine, that powers the plant if the plant AND the grid are failed.
Then each of these devices is not 100% reliable or may be down for maintenance.
So generally, you expect to have each of the 3 power sources actually subdivided in multiple sub system and redundant parts; meaning multiples ways the plant can power itself, multiple connections to the grid, and multiple generators. Last, the plant may have several overlapping/redundant electric circuits to guarantee that the available power from the 3 sources can be sent around the plant even with a damaged electric system.
You find the same concept in most man made vehicles/plants. Your car has a brake, an emergency brake, engine brake, can be steered off-road to slow down. That’s just a car and gives already 4 ways to slow down.
Yes, they can and do.
But if there’s a failure they still need power. Need to power valves, control systems, cooling water etc. They also need power just to start up.
For obvious reasons you don’t want things to stop working in a nuclear plant if there’s a failure inside, so they need backup power.
Even the alternator needs the battery and starter motor to give the initial crank to start the engine
Latest Answers