How a Nuclear Reactor works

Transcript: But you don't need to be a nuclear scientist to understand what happened at Chernobyl. You only need to know is this:
- Uranium fuel. As Uranium atoms split apart and collide, reactivity goes up. But Boron control rods, they reduce reactivity. But there's a third factor to consider: Water.
- Cool water takes heat out of the system. The higher the reactivity, positive void coefficient, which means more heat which means. But if you don't balance the reactivity it never stops rising. Nuclear fuel, as it does it turns to steam or what we call a void. It gets hotter, it gets less reactive.
- In an RBMK reactor of the type used at Chernobyl, fuel increases reactivity. The rise in temperature reduces it which means uranium fuel is now unchecked by the negative temperature coefficient.
- Xenon reduces activity. The reactor isn't hot enough to produce when the core is running at full power, sufficient steam burns the Xenon away. Xenon is still being created. Steam increases it. It built up, cause a problem. It'd appear we have a vicious cycle on our hands, like brakes on a car. What does that mean?
- At 30 Megawatts, the reactor is drowning in poison. But the more steam present within the system, to make matters worse, unchecked by control rods, our reactors do not have containment buildings, poisoning the core. None of it is burning away. The reactor is now primed to slow down.
- As for the fuel, its gone cold. They begin pulling control rods out. It means we don't use properly enriched fuel in our cores. The balance immediately swings in the opposite direction like brakes on a car. Every last molecule of liquid water instantly converts to steam were it not for this. We're starting to lose balance.
- The control rods are out, the pumps are shut down, which expands and ruptures a series of fuel rod channels. Reactivity increases. But even still, the Xenon poisoning is so strong which means more steam. The best they can do is raise the power.
Taken from HBO's Chernobyl (2019).