Chernobyl: The 25-Year-Old at the Poisoned Core

You are going to explain why the emergency shutdown button was actually the thing that triggered the explosion. This is Pod-This and Learn With Me. I am Marcus, and I am joined by Sofia.

It wasn't supposed to be them. The day shift had the plan. But the Kiev grid controller called because they needed the power for the evening peak. They delayed the test by ten hours.

They inherited a situation that was already falling apart. Alexander Akimov and his operator, Leonid Toptunov, took over the shift. Toptunov was only twenty-five. They were running a reactor that had been at half-power for far too long.

That's a common misunderstanding. They weren't looking for a thrill. They were trying to recover from a stall. Because of that ten-hour delay, the reactor's chemistry shifted. The power plummeted to thirty megawatts, which is dangerously low.

They were trying to restart a flooded engine while their boss, Anatoly Dyatlov, pressured them to ignore the warning lights. The flooding was a buildup of Xenon one-thirty-five. It's what they call a neutron poison.

It acts like a sponge that soaks up the neutrons needed for the nuclear chain reaction. A real sponge eventually gets saturated and stops. Xenon is different. It eventually burns away if the power stays high enough. If this Xenon sponge is soaking up all your neutrons and the power is dropping, how would you get the reaction moving again?

They pulled out almost all of them. The safety rules, the Operational Reactivity Margin, required a minimum of fifteen rods to stay in the core at all times. By the time they were done, they only had about six to eight left.

That is the situation they were in. The reactor was like a car with no brakes. It was idling in a way it was never meant to. Now we have to look at how they actually started the test in that state, and what happened when they finally tried to stop.

We are at the moment of the test now, at one twenty-three in the morning. When things started to look unstable, Akimov pressed the A-Z five button. That was the emergency shutdown. What is your intuition about what an emergency button like that actually does?

That is a common guess, but it is wrong. In this specific design, pressing that button actually triggered the explosion. Remember how the core was poisoned and unstable from the previous shift? When those control rods started moving down, they had graphite tips. For a few seconds, those tips displaced the water that was supposed to be cooling the core. Instead of slowing down, the reaction accelerated.

That is the terrifying reality of the R-B-M-K design. Think of it like slamming on the brakes in a car. Except, for the first two inches of travel, the pedal actually floors the accelerator. The analogy stops working because a car engine does not usually vaporize when you speed up. In four seconds, the power jumped to thirty-three thousand megawatts. That is ten times the normal operating limit. What do you think happens to a machine when it hits ten times its capacity in seconds?

It exploded. But the weirdest part is what happened next. The operators looked at their meters, and the radiation readings said three point six Roentgens per hour. That is high, but not immediately lethal. Do you see a problem with... well, with trusting that number right after a massive explosion?

It was closer to fifteen thousand Roentgens. The meters they were using were low-range models. They literally could not count higher than three point six. They just maxed out. Even when workers saw glowing chunks of graphite on the ground—which only exists inside the core—the supervisor, Dyatlov, insisted it was just insulation. He refused to believe the reactor was gone.

It was a mix of cognitive bias and equipment failure. They stayed in the ruins for hours. They were trying to pump water into a core that did not exist anymore.

You have captured the situation perfectly.

That really gets to the heart of the tragedy behind that design flaw. This week, try to explain to a friend how a safety mechanism could actually function as a detonator. Keep learning and keep growing. Thanks for listening.