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
Chernobyl is a great show, but you **really need to watch the last episode**. It has some wonderful “Why did we build such a dangerous thing? Because we are Soviets/Russians and it was cheaper.” What you see, which is great, is **NOT** how nuclear reactors work in civilized countries, frankly they don’t even work that way is Russia anymore.
When Uranium decays, it emits a fast neutron. These are good for heat, bad for living things, but not good for chain reactions. The Graphite in the reactor slows down the neutrons, making them much more reactive and likely to trigger another atom of Uranium to split. These slow neutrons lead to chain reactions. Chain reactions decay Uranium much, much faster than it naturally decays – making much, much more heat.
The half-life of U235 “on the shelf” is 700M years (you should not have U235 on a shelf anywhere near you). That means half of your lump will decay in 700M years. In a reactor, you can cause half the U235 to decay in 2-3 years, thanks to chain reactions. Or, if you lose control of it in 2-5 seconds blowing the whole darn thing up.
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