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
Latest Answers