In my view, Chernobyl didn’t explode because of one mistake but because of a chain of failures. After working my way through a lot of documentaries, I’ve put together a simple explanation, from how a nuclear power plant works to the explosion itself, minute by minute. If you’re ever planning a trip to Chernobyl, read it before you go: the zone will make far more sense.
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Why did Chernobyl explode? It’s such a complex question that there’s no one-sentence answer. As with any disaster, the explosion at the Chernobyl nuclear power plant wasn’t caused by a single failure but by several in a row. Let’s go through it minute by minute in this post.
The Chernobyl disaster explained: why it exploded and how it happened
I’ve seen plenty of documentaries and decided to spell it all out in the simplest possible terms, so that anyone can understand why Chernobyl exploded. To understand it myself first, though, I had to watch several Chernobyl documentaries dubbed into Czech and Slovak.
My English is very good, but when it comes to terms like graphite rods, I’m lost. So, why did Chernobyl explode? By the way, I’ve written several posts about Chernobyl on this site. You’ll find them all in the Chernobyl blog section.
I’d also recommend my article What to see in Chernobyl. On a full-day trip I visited a range of places there, including a true phenomenon: the antenna, also known as the Duga radar, the largest radar in the world. I’ve also written a detailed guide on how to plan a trip to Chernobyl, so give it a read. In it I mention the tour operator I went with, because Chernobyl without a tour operator is practically impossible: you need permits to enter the military zone. Since February 2022, however, after Russia’s invasion of Ukraine, tourist trips to the zone have been suspended (2026).
The Chernobyl explosion had several causes
If I had to sum it up in one sentence, it was a failed experiment to find out how long the plant could keep cooling the reactor without electricity until the backup power kicked in. Chernobyl killed 31 people directly and thousands indirectly. It all happened on 26 April 1986, when Chernobyl exploded. Indirectly, though, the disaster began a day earlier, and the regime of the time played its part too: a system where things got done just so they could be ticked off.
For example, the reactor roof was supposed to be made of non-flammable material, which wasn’t available at the time. But the roof had to be built anyway, so they used flammable materials. Accidents were common, and they were covered up.
How a nuclear power plant works
A nuclear power plant works on the principle of heating water, which turns into steam. The steam then drives turbines connected to an electricity generator. Nuclear power plants are (supposedly) green (until they explode), because they use incomparably less material and produce only about 3.5 tonnes of waste a year. What they release into the air is steam. Coal-fired power stations, by comparison, produce up to 35 million tonnes of waste a year and release smoke into the air. On the other hand, nuclear power plants produce spent radioactive waste, which I definitely don’t consider green. But that’s a long discussion to have with Greenpeace.
The Chernobyl explosion minute by minute
25 April 1986, the day before the disaster
- 1am: Staff at reactor 4 start reducing the power to prepare for the reactor test. The test is meant to show whether, in a power cut, the Chernobyl nuclear power plant can keep the turbines pumping enough cold water to cool the reactor and prevent an explosion. The critical gap is the 40 seconds before the backup power supply kicks in.
- 2pm: The cooling system of reactor 4 is switched off so that it can’t spoil the test results. But the test is pushed back, so that the surrounding towns don’t lose power during the day.
- 11.10pm: The staff get permission to continue the reactor 4 test, but in the meantime there has been a shift change. The workers who started the test go home and a new shift takes over. They have less experience, don’t know all the details and were never given proper instructions on how to carry out the reactor test. The deputy chief engineer, Anatoly Stepanovich Dyatlov, ran the test. His arrogance and ‘I’m the best here, you’ll do what I tell you’ attitude contributed heavily to the explosion. What’s more, he had already caused a small nuclear explosion once before and received a high dose of radiation.
26 April 1986
- 12.38am: First, the reactor’s power had to be lowered again so that the tests could begin. At low power, the water level in the reactor was always hard to control. Then, completely unexpectedly, the reactor shut down and stalled. Dyatlov gave the order to raise the power and pull out all the control rods that kept the reactor in check. A nuclear power plant works like this: the uranium fuel rods release enormous heat, which turns the water entering the reactor from below into steam. The steam rises and drives a huge turbine that generates electricity. The control rods regulate the whole process. Pull some of them out and the power, the heat and the amount of steam rise immediately. Pull them all out and the reactor becomes hard to control. Dyatlov ordered them all out so that the shut-down reactor would start up again quickly. The workers didn’t argue because they were afraid of losing their jobs, even though it went against every regulation.
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12.52am: An alarm went off because there was too little water in the reactor. The shortage set off more and more alarms, but nobody believed that Chernobyl could explode. Meanwhile, pressure was quietly building in the reactor, because the control rods were out and the rising heat was turning the remaining water into steam. With all the alarms going off, nobody noticed.
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1.12am: His staff warned him, but Dyatlov was determined to go ahead with the reactor test anyway. During the test, the drive of the huge turbine that forces water into the reactor is disconnected, and the turbine keeps turning on its own momentum as it runs down. Within 40 seconds, the backup power sources should start, get the turbine going again and keep the cooling running. The aim of the test was to find out whether the reactor could stay cool enough for those 40 seconds without any power supply. If not, it would overheat in an instant. Dyatlov, though, considered this risk purely theoretical.
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1.23am: The reactor test officially begins. As soon as the pumps slowed down, there wasn’t enough water in the reactor, and what was left turned into steam and built up enormous pressure. The pressure was so high that it lifted the 350-kilogram caps on the fuel rods. The whole reactor began to shake. On command, the control rods went back in, but the pressure in the reactor was so great that they broke. That is why they pressed the emergency shutdown button, AZ-5. But that is exactly what set off a disaster there was no coming back from. The emergency button inserts the safety rods into the reactor (they work like control rods, but they are more powerful and can stop the reactor immediately). These rods, however, had graphite tips. As soon as they entered the reactor, the graphite caused a sharp surge in power. The power doubled every second.
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1.23am and 38 seconds: The steam pressure in the reactor was so enormous that Chernobyl exploded. The explosion tore off the reactor’s 1,000-tonne armoured lid, and the metallic taste of death spread through the air.
What Chernobyl looks like today
Today, more than 40 years after the Chernobyl explosion, life in the area is slowly returning to normal. But it won’t be truly normal for the next 1,000 years, which is how long the radioactive mass inside the plant will stay active.
After the explosion, the original reactor 4 was covered with a concrete sarcophagus to contain the radiation, which still leaks out today. That sarcophagus was very makeshift, though, and built in a hurry. Later, another sarcophagus went up over the original reactor and the old concrete sarcophagus. They slid the new shelter (New Safe Confinement) over the reactor in November 2016. In February 2025 a drone strike damaged it. They patched the hole left by the drone with a large panel, but further repairs are still pending (2026). Radiation in the area has dropped, but it’s still there. About 2,250 employees still work at the plant (2026), making sure the area around it is cleaned up. They constantly measure contamination and work to make Chernobyl a safer place.
Why did Chernobyl explode?
There are still highly radioactive places in the area. One of the most radioactive is the basement of the hospital in Pripyat, the town right next to the plant. This is where they brought the firefighters. They had spent the whole night fighting the fire at reactor number four, and nobody had told them what danger they were in. Many of them later died in a hospital in Moscow, but their firefighting suits were so radioactive that nobody knew what to do with them. So they were thrown into the hospital basement, where they still lie today. It’s one of the most radioactive accessible places in the world.
Here are all my Instagram stories from my day in Chernobyl:
Tip from Milan On Omio you can compare flights, trains and buses for your next trip.
You may also want to read my article People, don’t turn a Chernobyl tour into Disneyland just because it’s cool.
A Slovak travel blogger who has been getting Slovaks out of their office chairs since 2013. Author of the book Influencer and founder of the Bez Mapy blog. "Bez Mapy" is Slovak for "without a map".
