What “turns on” a nuclear reactor?

Question

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

Answer 1

So I hope this ELI5 enough but a startup neutron source is used to reliably initiate the separation of neutrons within the fuel, there’s wiki if you want to read more:

[https://en.wikipedia.org/wiki/Startup_neutron_source](https://en.wikipedia.org/wiki/Startup_neutron_source)

Also “cooling water” is a relative term. The cooling water in some designs are recirculated through one or more heat exchanges to heat up another separate source of water to create steam and spin a turbine in to generate electricity. Essentially two separate closed systems or water loops. The cooling water that surrounds the fuel and control rods in the reactor however is only “cool” relative to the equipment submerged within it. the fuel rods are not “hot” when they are made, although they maybe radioactive depending on the fuel type. the reaction can be controlled using control rods with elements that will absorb the neutrons bouncing around inside the reactor that if left to their own devices would continue to knock neutrons off the fuel until it was depleted or the containment was lost (a melt down like Chernobyl)