Okay, just finished HBO’s Chernobyl and have fallen down the deep deep rabbit hole of trying to understand how nuclear reactors work, why water is needed, what a meltdown is and what happened at Chernobyl.
I think I have a good handle on it, but one thing I’m still not sure on: what initially “turns on” a nuclear reactor?
I get that the uranium fuel is encased in the zirconium rods, and that when it’s up and running the splitting uranium releases neutrons that split even more uranium and so on.
And I get that uranium is unstable on its own and eventually decays.
What I don’t understand is what begins the initial fission process that produces the heat that steams the water that turns the turbine.
I mean, the whole point of submerging the rods in water (or encasing them in graphite) is to ensure that the reactor doesn’t overheat, but what starts the reaction that such submerging or encasement is necessary? When a the rods are being assembled, they aren’t already producing that kind of heat, are they? If they are, then how is construction of a reactor even possible?
Again, how does a nuclear reactor “turn on”, and, by extension, how is it “turned off”?
In: Chemistry
Some background: A sustained nuclear fission reaction occurres when a radioactive element decays, emitting neutrons, and these neutrons are captured by another atom of the same element, which causes it to become unstable, fall apart, and release more neutrons which repeat the process.
What starts the reaction?
The radioactive fuel source emits neutrons naturally and when sufficient quantity of the fuel is arranged in a certain way under certain conditions, criticality occurres, resulting in a sustained chain reaction.
Nuclear reactors use control rods to turn the reactor on and off and control the rate of reaction, and thus the power output.
These rods are usually made of a material that can absorb neutrons, such as boron.
When the rods are pulled all the way out (or up), nothing is there to absorb the neutrons emitted by the fuel and prevent them from causing other atoms to fall apart, and the reactor is at full power.
When the control rods are all the way in, they absorb enough neutrons to stop the chain reaction, and the reactor is off.
The Chernobyl reactor was not designed to safely operate at full power but that is the state it ended up in.
There is more to the story. I will share if you are interested.
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