Chernobyl: Toptunov’s Penciled-In Nightmare
This investigation details the final hour before the Chernobyl disaster, focusing on the 10-hour delay that placed the reactor in the hands of an inexperienced night shift. Listeners learn how xenon poisoning stalled the reactor's power, why the AZ-5 button caused a positive scram, and the reality behind the 3.6 roentgen reading that misled Anatoly Dyatlov. It documents the first fatality, Valery Khodemchuk, and the crew's futile attempts to pump water into a core that had already been destroyed.
On April 26, 1986, a 10-hour delay forced 25-year-old Leonid Toptunov to execute a reactor safety test he had only seen in penciled-in notes. Senior engineers Anatoly Dyatlov and Alexander Akimov oversaw a catastrophic power stall caused by xenon poisoning, leading to the ill-fated press of the AZ-5 emergency button. This sequence of human error and design flaws examined the fatal graphite tips that turned a shutdown command into a massive explosion. The 2,000-ton biological shield nicknamed Elena was tossed into the air, while pump operator Valery Khodemchuk became the disaster's first unrecovered victim beneath the radioactive lava.
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The walls of Reactor Four are bending like rubber. Alexander Yuvchenko looks up and sees the stars through the shattered ceiling. It is April twenty-sixth, nineteen eighty-six.
Welcome to Pod This and The Discovery Hour. Daniel is a nuclear historian, and he is here to help us examine the disaster at Chernobyl.
The Poisoned Core
Leonid Toptunov adjusts his glasses. He is staring at the scribbled handover notes left by a day shift that should have finished the test ten hours ago. Toptunov is only twenty-five, and he has been in this post for just three months. The Kiev grid controller’s voice crackles through the intercom, sounding like a death knell for his preparation. He looks at the circular power meter. The needle is hovering at a level he was never trained to handle. He notices the test procedure on his desk has been crossed out and rewritten in pencil.
Those penciled-in changes on Toptunov's desk feel like a nightmare. It is unsettling to think that a twenty-five-year-old with only three months of experience was left holding the keys to a reactor. The rules were being rewritten on the fly.
The timing was a catastrophe because the test was never supposed to happen on his watch. The day shift was the one trained for this specific procedure. However, the Kiev grid controller stepped in and demanded a ten-hour delay. They needed to keep the lights on and the ovens running for the evening rush in the city. The reactor stayed at half-power all day, and that fundamentally changed the chemistry inside the core.
The pressure to meet the city's power grid demands pushed the most dangerous part of the job onto the least experienced crew.
That delay created a phenomenon called xenon poisoning. As the reactor sat at half-power for those ten hours, a byproduct called Xenon-one-thirty-five built up. It began absorbing neutrons like a sponge. When Toptunov finally tried to drop the power to the test level of seven hundred megawatts, the xenon caused the thermal power to crash completely. It bottomed out at a stalling thirty megawatts.
At thirty megawatts, the reactor is basically a car that is about to stall on the highway. Why didn't they just turn it off and walk away?
The pressure to complete the test was immense. Instead of shutting down, they tried to force the engine back to life. To fight the xenon and get the power back up, the operators did something unthinkable. They pulled two hundred and four of the reactor's two hundred and eleven control rods almost entirely out of the core.
If those rods are the brakes of a nuclear reactor, they were essentially flying down a mountain with only seven tiny pins left to stop them.
By removing nearly all the brakes, they created a hair-trigger environment. The reactor was deceptively quiet but inherently unstable. They managed to get the power back to two hundred megawatts. This was still far below the safety limit, but they decided to proceed with the test anyway.
The power was artificially propped up by removing the brakes. The test finally runs. At one-twenty-three and forty seconds in the morning, Shift Supervisor Alexander Akimov pushes the emergency stop button. He is expecting a routine shutdown.
The Ghost Reactor
Akimov presses the A-Z-five button. He expects the control rods to slide in and kill the reaction. But what happens instead?
The rods had a design flaw. They were tipped with four and a half meters of graphite. As they entered the core, that graphite pushed out the water that was supposed to slow things down. It acted like a match instead of an extinguisher. Power surged to thirty-three thousand megawatts in seconds. That is ten times the normal operating capacity.
The safety system itself became the detonator.
The steam pressure grew so immense it tossed the two-thousand-ton upper biological shield right into the air. They called that shield Elena. It flipped like a coin and crashed back down. That left the core wide open to the sky.
In that moment of total rupture, they lose their first man.
Valery Khodemchuk. He was in the pump room directly below the explosion. He was entombed instantly under radioactive debris. He is still there today.
While he is buried, the men in the control room are looking at dials. They think they are still in control.
Anatoly Dyatlov looked at the dosimeter and saw it reading three point six roentgens per hour. He assumed the reactor was intact because that was the maximum the meter could show. It was pegged at its limit. The actual level was fifteen thousand roentgens.
They were walking into a fire they could not even see.
Akimov and his crew spent that first hour standing in twenty centimeters of radioactive water. They were desperately turning valves to pump thousands of gallons of water into the core. But the core was gone.
They were trying to save a ghost.
The tragic irony is that their training told them the machine was fail-safe. They died because they believed the emergency button was a shield. In reality, it was the trigger. It forced them to spend their final hours fighting for a reactor that had already been destroyed.
Alexander Akimov presses the A-Z-five emergency button. It should have been a calm conclusion to a long night of testing. On the console, the power needles are supposed to fall. But as the graphite-tipped rods displace the water, the dials sweep past the red lines to thirty-three thousand megawatts. The reactor doesn't shut down. Instead, there is a low, earth-shaking thud that rattles the pens on the desk.
It is hard to imagine these men staring at their control panels and watching the needles flatline, while the roof above them had already vanished into the night sky.
That is the tragedy of April twenty-sixth. The night shift was fighting their own belief that the system was infallible. When Akimov pressed the emergency button, he thought he was choosing safety. He had no way of knowing that the graphite tips would act like a match to a powder keg. By the time they were hauling hoses to pump water into the core, there was no core left to save. They were sacrificing their lives to hydrate a ghost.
They chose to trust the dials over the evidence of their own eyes. Daniel, thank you for walking us through those final, surreal minutes of the disaster. If this story changed how you think about human error, please share this episode with a friend. Until next time, keep questioning, and keep discovering.
Sources & further reading
Midnight in Chernobyl
This definitive account provides a meticulously researched minute-by-minute breakdown of the events leading up to the disaster.
Chernobyl: The History of a Nuclear Catastrophe
Plokhy examines the political and technical failures of the Soviet system that contributed to the reactor's instability.
How this episode was made
Created by Martin Andersen with PodThis. The research and script are AI-generated with Google Gemini from the creator's topic, and the voices with Gemini or Cartesia. PodThis editors have not reviewed this episode.
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