What “turns on” a nuclear reactor?


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


This is a good question.

The fuel in the reactor has enough reactivity/energy to run for up to 2 years. The only thing holding it back are the control rods which inhibit the reaction. The control rods absorb the neutrons in the core so they can’t split atoms in any meaningful amount.

To start up the reactor, we slowly remove control rods and monitor the core. As you remove control rods, the neutrons are able to interact more and start splitting atoms, and the neutron counters in the core go up. But the core isn’t online yet. After a short time the core steadies out and neutron counts stop rising.

Eventually you finally pull enough rods that the core is slightly supercritical. Neutron counts continue to rise exponentially even with no operator action. Power keeps exponentially increasing until there is enough fission to heat the fuel up (called the Point of Adding Heat). The fuel heat up slows the reaction down and brings the core to an exactly critical (steady state) condition. From here we can continue to withdraw rods to raise power.

To shut it down, we insert the control rods. We can either scram, which is an immediate insertion of all rods, or slowly insert them. These rods absorb neutrons in the core and within a couple seconds will have the reactor shut down after a scram signal. For a soft shutdown, we simple insert rods reverse sequence until the core is shut down due to lack of neutrons.

It’s all done by controlling the core neutron count (which impacts the fission rate)

When you first assemble the reactor, you have neutron absorbers in the core (either control rods, or liquid boron in the water) which prevents the core from starting up.

No single fuel rod can start up on its own, it’s just the design of the fuel. You need a group of them in precise configurations with a moderator around them and no neutron absorbers before the fuel can start up.

I hope this helps

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:


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)

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.

To start a fission reaction you need to get enough radioactive material, close enough together, so that it reaches critical mass. The radioactive materials shoot particles at each other which causes the other one to shoot particles back.

It could be as simple as having 2 buckets of radioactive material and moving the 2 buckets next to each other.

Nothing. Or well, anything.

Uranium, on it own, occassionally emits a neutron. Normally, thd neutron flies off, and nothing much happens.

However, when you put the right amount of it, with the right KIND, in the right geometry together, a neutron is likely to hit another Uranium atom, which will split, releasing 2 (or more) neutrons. Those neutrons, because they are ALSO in the right (or wrong, depending on your perspective) shape/size/amount, will ALSO likely hit other atoms, releasing 4.. etc.

Just getting the uranium in the right orientation is enough to start it.

Alternately: you can have something JUST BARELY in the right orientation, and have it filled with things that harmlessly absorb neutrons (control rods). By moving those neutron absorbs in and out of the blib, you can control how much of that runaway process happens.

But really, once the pile is in the right orientation, the process will start on its own

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.