How do control rods in nuclear reactors work?
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Radioactive materials are unstable. They have too many tiny particles shoved together in the nucleus of the atom, and they won’t last forever. *At some point*, a chunk of atom will break free and shoot out like a bullet. This is radiation.
Different types of radioactive material give off radiation at different rates. Some of it (like Uranium 238) is so stable that it takes billions of years to break down. Some other types break down in fractions of a second. The thing is, if your radioactive material gets hit by the radiation of a different atom, it might trigger the release early. Think of it like a pebble rolling down a mountain, hitting other pebbles, eventually causing an avalanche. There are a bunch of equations that predict exactly how much radioactive stuff you need to start that avalanche.
Nuclear fuel is made up of radioactive elements that are ready to go. They get hit by the radiation of other atoms nearby, and they give off their own increased radiation, and it starts a chain reaction. This is done under controlled circumstances — the scientists know exactly how much material to add to create energy, but not create a big kaboom.
Control rods are made of stuff that absorbs radiation. You insert the control rods and they start absorbing all the radiation flowing around inside the reactor. This means less radiation hitting the radioactive material, which slows down the reaction and stops the avalanche. There are also some equations behind this, so the scientists know how much of the control rods to put inside the reactor. Put them in all the way and you’ll basically kill the reaction completely. Put them in a little bit and you just reduce it to a lower level.
Nuclear reactors are basically big sticks of bags of bouncy balls. If you hit a bag of bouncy balls with another ball, the bag splits open… And balls bounce everywhere.
Balls flying everywhere makes things hot.
Control rods are sticky, and bouncy balls stick to them.
What you want is enough sticks to stick balls to that all the bags of balls don’t split really really fast.
If you want to turn the reactor off, you put in more control rods. They catch all the balls, and stop more bags of balls from splitting.
So look at a pool table.
You rack up all the balls on one side, and the cue ball on the other right?
If you shoot the cue ball into the other balls they scatter and bounce around.
This is basically what a nuclear reaction does. A neutron (cue ball) shoots into a Uranium atom (racked balls) and the energy splits it apart to make more reactions.
Now you put a big piece of foam rubber or something between the cue ball and the racked balls.
You shoot the cue ball, the foam rubber absorbs the kinetic energy, the racked balls don’t burst apart.
The control rods are basically made from a special element that absorbs the random neutrons that zing around and prevent them from starting chain reactions.
Imagine uranium like a bunch of mousetraps with ping pong balls on them. The ping pong ball is like a neutron. If you fire a ping pong ball at one of the mouse traps, it’ll cause a chain reaction launching the other ping pong balls.
Imagine the mouse traps reload automatically, so without intervention the reaction will continue until something intervenes.
The carbon rods would be like lowering dividers between the mousetraps so the balls in motion will get deflected and stop triggering the other mousetraps.
Real ELI5 answer, since most of the answer here are a bit lofty:
Reactors have a very angry metal inside of them. Think of the angry metal as clusters of billiards balls. Think of how the game billiards/pool works: you hit a ball into the rack and the balls explode in all directions. This isn’t a perfect analogy, but it gets us there.
Now think of a reactor as full of thin rods of this angry metal. Think in 2D, so that we’re on a pool table and we’re looking at “slices” of these thin rods. You have a very big billiards table full of multiple racks, all waiting to be hit by balls.
Now imagine that this is billiards table is also frictionless, and the racks somehow ADD energy to the system when they get split (unlike in real pool where the momentum is distributed over the balls so that over all they all sorta average out). The balls will go for a long ways and will hit other racks and split them and release more balls, causing a chain reaction. This can quickly get out of hand if none of the balls are removed from the table.
Besides balls just flying off of the table (leakage), or hitting other racks and not splitting them (not every absorption of a neutron in fuel causes a fission), there needs to be a way to “control” the amount of balls on the table. Now interspersed among all of these rods of billiards balls ready to be split, you have little pockets, and they can absorb a nearly infinite amount of billiards balls.
This is basically how a reactor works. Control rods keep the amount of neutrons in check, so that the reactor doesn’t go supercritical. Well, if the reactor is gaining power it is *technically* supercritical, so there’s another concept involved called “prompt criticality” that I can also ELI5 if anyone actually reads this or is interested, but there’s a difference between a nuclear reaction and a *nuclear reaction*, and that’s what prompt criticality is.
Watch Chernobyl on Max…last couple of episodes the scientist really had to dumb it down for the party 🙂
All atoms contain two main parts, the nucleus made up of protons and neutrons and the electrons that orbit around it.
The particles within the nucleus want to repel each other but are held together with a strong force. That means there’s a lot of energy contained within the nucleus. Imagine you had a load of springs in a box. As soon as you open it all of the springs will bounce out. The nucleus is the same, if you can get rid of the strong force the nucleus will split apart and release energy.
The isotopes used for nuclear reactions are very unstable. Imagine a glass that’s completely full of water, it only takes a tiny bit more water for the glass to overflow. It’s similar with things like Uranium. If you introduce another neutron to the nucleus it “overflows” and splits apart releasing energy.
The most important part is that when the nucleus splits it releases more neutrons. These then impact nearby nuclei and cause them to split as well leading to a chain reaction. In the case of Uranium-235 each split releases three neutrons. That means that every time a nucleus splits it’ll then cause three more to split. If you let this happen unrestricted then the number of splits (and therefore energy output) will increase exponentially to a massive amount, that’s basically a nuclear bomb.
What control rods do is safely absorb neutrons before they can cause another split. If you put a control rod into the reactor it’ll absorb a certain percentage of the free neutrons. As an example let’s say one control rod absorbs 1/3 of the neutrons. If you add one control rod then each split will only lead to two further splits, rather than three. This means that the energy output will increase more slowly. If you add a second rod each split will only lead to one more split, that means that the energy output will remain constant. If you have three rods all of the neutrons will be absorbed and the reaction will stop.
I’ll explain it in monkey terms
You know how a bunch of monkeys love to play a game where they throw bananas to each other? Now, imagine if every time a monkey catches a banana, it splits into two bananas, and then those bananas are thrown to other monkeys who haven’t caught one yet. This makes the game faster and faster because more bananas are being thrown around.
But what if the game gets too wild and we need to calm it down? That’s when we introduce banana catchers (these are our control rods). These are special monkeys whose job is to catch the bananas but they don’t throw them back.
So, the more banana catchers we have, the slower the game becomes, because fewer bananas are being thrown around. If the game is going too slow, we can ask some banana catchers to leave. If it’s going too fast, we can bring in more banana catchers.
In a nuclear reactor, the splitting of atoms (uranium or plutonium) is like the bananas splitting, and the control rods are like the banana catchers. Their job is to slow down the reaction and keep everything under control.
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