How do control rods in nuclear reactors work?
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They get in between the Uranium fuel rods and absorb neutrons. Since neutrons are what powers a nuclear reaction, fewer neutrons means a weaker reaction.
Basically, the fuel in a nuclear reactor is sustaining its fission reaction because the uranium fuel shoots out neutrons as it “burns”. These neutrons hit other uranium atoms causes them to split, and so on. The control rods are made from a material that tends to absorb neutrons, so inserting them amongst the uranium kills the fuel’s ability to sustain their reactions.
They work like the brakes on a car. Without them, the accelerator would be smashed to the floor and the reaction would get out of control. They insert/remove control rods to increase or decrease the reaction. The more that are inserted, the more the brakes are pressed, and the slower the reaction gets, causing less power output.
Uranium 235 needs a neutron added to it with a lot of energy to cause fission. When it splits, it emits 2-3 neutrons with enough energy to cause more fission. If left uncontrolled, the reactor will get got enough that a nuclear meltdown will occur, and liquid uranium will burn its way out of the reactor.
Control rods are made of materials that are really good at absorbing neutrons. That absorption means those neutrons can’t go on to cause more fission, thus slowing down the reaction.
Subcritical – lots of control rods, the nuclear reaction is slowing down. If left untouched, it will eventually stop
Critical – a balanced amount of control rods, the nuclear reaction is maintaining its rate. If left alone, it will continue reacting at the same rate
Supercritical – few control rods, the nuclear reaction is speeding up. If left alone, it will eventually go into meltdown. Supercritical is a necessary step to getting the reactor up an running
Here’s how a nuclear reaction works, in simple terms.
An atom is made up of smaller particles. Some of these particles are stuck together in what’s called the nucleus. It’s possible to hit them with other particles, causing them to break apart and fly everywhere. Then, as they fly around, they might hit other nuclei, causing them to break apart and go on to hit more and more nuclei. (“Nuclei” is the plural form of “nucleus”.)
It’s like bowling pins, if you push the ones in the front, as they fall they knock the others down too. Or like dominoes, as the first one falls, it hits the domino after it, which causes more and more to fall.
Here’s where control rods come in. They absorb the particles, so then they can’t hit more nuclei and break them apart. It’s like if you put something between the dominoes, it will stop the ones after it from being knocked over.
If you have enough control rods, you can stop the reaction altogether, or if you have only a few, it will slow things down.
As for why we want nuclei to break apart in the first place, it’s because when they break apart, they produce lots of energy.
You have a box full of bouncy balls, by shaking the box the balls jump around like crazy in there, you wanna limit how much they move around so you implant some sticks in the walls of the box.
Nuclear fission works by a neutron striking a nucleus of uranium-235.
When a neutron with the right amount of energy (not too little, not too much) hits it, it causes the nucleus to disintegrate, producing some smaller nuclei, lots of energy and typically three neutrons.
Each of these neutrons could go off and cause another U-235 nucleus to disintegrate (so 3 becomes 9, becomes 27, becomes 81, 243, …) this is a run-away chain reaction and causes the reactor to overheat and melt down, which is typically not ideal.
To prevent this, control rods (usually made of boron) are inserted and they absorb some of the neutrons, trying to make it so that, on average, each U-235 disintegrated produces one viable neutron to disintegrate another U-235 atom, keeping the reaction – and heat produced – in check.
Looking at the other responses, may I say I’m pleased to see some of my colleagues here from the Kurchatov Institute and Minenergo…..*but I digress*
Nuclear reactor makes heat by smashing little particles called neutrons into the fuel over and over. Control rods are made of a material that absorbs a lot of neutrons, so less of them hit the fuel. Insert control rods, fewer neutrons hit fuel, less heat produced. Insert the control rods far enough and not enough neutrons hit the fuel to sustain the reactor, and it essentially “turns off.”
Best I could do for a 5 year old.
The HBO miniseries Chernobyl has a scene that explains to a layman how a nuclear reactor works better than anyone in here could. Google “chernobyl testimony red blue cards” and watch the 3:33 scene
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