Chernobyl: Why Did the Nuclear Reactor Explode and Could It Happen Again?

What is a positive void coefficient and how did it contribute to the reactor explosion on April 26, 1986?

Jackson Ryan Former Science Editor
Jackson Ryan was CNET's science editor, and a multiple award-winning one at that. Earlier, he'd been a scientist, but he realized he wasn't very happy sitting at a lab bench all day. Science writing, he realized, was the best job in the world -- it let him tell stories about space, the planet, climate change and the people working at the frontiers of human knowledge. He also owns a lot of ugly Christmas sweaters.
Jackson Ryan
10 min read

Chernobyl, a bleak and brutal miniseries co-produced by HBO and Sky UK, is likely to go down as one of the best TV shows this year and maybe even of all time. It tells the true story of the world's worst nuclear disaster, which occurred in a Soviet nuclear power plant in April 1986.

Written by Craig Mazin and directed by Johan Renck, Chernobyl stoically adheres to the era and crisis it portrays like radiation clinging to discarded fireman uniforms. It may have taken some artistic liberties for the sake of story, but refused to sweep the truth of the catastrophe under the rug. It rendered historical truths, and the countless lies, in a harrowing light.

At every step, Chernobyl touched on the ineptitude of Soviet governance, the uncompromising courage of the liquidators tasked with cleaning up the site, the weight that hung over the shoulders of every scientist investigating the disaster and the stark reality of atomic power.

But the crowning achievement of the Chernobyl series is how it inspired an immense scientific curiosity in its viewers through the horror. We know the Chernobyl disaster really happened -- and the hard-nosed, honest approach to the meltdown only served to heighten that curiosity. Google Trends shows a huge spike in searches for terms related to the science of the show: "RBMK reactor," "nuclear reactor" and "radiation sickness" have all seen huge leaps since Chernobyl's TV debut.

Over its five episodes, Chernobyl constantly moved toward answering one question -- "How?" -- and we've wanted to skip ahead and find the answers out for ourselves. The final episode, which aired on June 3, finally revealed the truth of that April morning in 1986.

Scene from Chernobyl showing a burning building

Moments after the reactor explosion, Chernobyl burns.


Valery Legasov, the chief of the commission tasked with investigating the disaster, takes part in the trial of three power plant officials responsible for the explosion and its immediate aftermath. Along with politician Boris Shcherbina and physicist Ulana Khomyuk, the trio detail the key reasons behind the disaster and squarely point to the failings of those officials, including chief engineer Anatoly Dyatlov, as the cause for the plant's explosion.

But we're talking about nuclear physics here. Things are messy and confusing. The term "positive void coefficient" gets thrown around, and that's not a term you hear every day. Even Chernobyl's engineers couldn't fully grasp the consequences of their actions. So we've dug through the radioactive quagmire to bring you the science behind Chernobyl's RBMK reactor explosion -- and the reasons we're not likely to see it happen again.

What is an RBMK reactor?

The Soviet Union's nuclear program developed the technology for RBMK reactors throughout the '50s, before the first RBMK-1000 reactor began construction at Chernobyl in 1970. RBMK is an acronym for Reaktor Bolshoy Moshchnosti Kanalniy, which translates to "high-power channel-type reactor."

In the simplest terms, the reactor is a giant tank full of atoms, the building block that makes up everything we see. They are themselves composed of three particles: protons, neutrons and electrons. In a reactor, the neutrons collide with atoms another, splitting them apart and generating heat in a process known as nuclear fission. That heat helps generate steam, and the steam is used to spin a turbine which, in turn, drives a generator to create electricity in much the same way burning coal might.

The RBMK reactor that exploded at Chernobyl, No. 4, was a huge 23 feet (7 meters) tall and almost 40 feet (12 meters) wide. The most important segment of the reactor is the core, a huge chunk of graphite, sandwiched between two "biological shields" like the meat in a burger. You can see this design below.

Schematic showing the graphite core and the biological shields

A schematic of the plant used in HBO's Chernobyl showing the graphite core and the biological shields.

HBO/Annotated by CNET

The core is where the fission reaction takes place. It has thousands of channels which contain "fuel rods," composed of uranium which has atoms "easy" to split. The core also has channels for control rods, composed of boron and tipped with graphite, designed to neutralize the reaction. Water flows through the fuel rod channels and the entire structure is encased in steel and sand.

The water is critical to understanding what happened at Chernobyl. In an RBMK reactor, water has two jobs: keep things cool and slow the reaction down. This design is not implemented in the same way in any other nuclear reactors in the world.

The fuel rods are the powerhouse of the core and are composed of uranium atoms. The uranium atoms cast a net in the core, and as rogue neutrons ping around inside they pass through the solid graphite that surrounds them. The graphite "slows" these neutrons down, much like the water does, which makes them more likely to be captured by the uranium atoms net. Colliding with this net can knock more neutrons loose. If the process occurs over and over in a chain reaction, it creates a lot of heat. Thus, the water in the channel boils, turns to steam and is used to create power.

Unchecked, this reaction would run away and cause a meltdown but the control rods are used to balance the reaction. Simplistically, if the reactor is generating too much power, the control rods are placed into the core, preventing the neutrons from colliding as regularly and slowing the reaction.

In a perfect world the systems, and people controlling the systems, ensure that the scales never tip too far one way or the other. Control rods move in and out of the reactor, water is constantly pumped through to keep the whole thing cool and the power plant produces energy.

But if the plant itself loses power, then what happens? That's one of the RBMK reactor's shortcomings. No power means water is no longer being pumped to cool down the reactor -- and that can quickly lead to disaster. In the early hours of April 26, 1986, the reactor was undergoing a safety test which aimed to fix this issue.

The safety test

Scene showing Valery Legasov testifying

Valery Legasov testifies before the commission, in front of the three power plant officials responsible for the disaster. 


The safety test is the starting point for a chain of errors which ultimately resulted in reactor 4's explosion.

The facts are so:

  • In the event of a blackout or loss of power to the plant, the RBMK reactor will stop pumping water through the core.
  • A backup set of diesel-fueled generators kick in after 60 seconds in such an instance -- but this timeframe risks putting the reactor in danger.
  • Thus, the test was hoping to show how an RBMK reactor could bridge the 60 seconds and keep pumping cool water into the system by using spare power generated as the plant's turbines slowed down.
  • The test was originally scheduled for April 25 but was delayed for 10 hours by power grid officials in Kiev.
  • The delay meant a team of nightshift staff would have to run the test -- something they had not been trained to do.
  • To perform the test, the reactor had to be put into a dangerous low-power state.

The low-power state in the RBMK reactor is not like putting your computer in sleep mode. It cannot be returned to its usual power state quickly. However, the team in the control room at Chernobyl attempted to do just that and disregarded the safety protocols in place.

To attempt to get the power back up to an acceptable level, the workers removed the control rods in the core, hoping to kickstart the reaction again and move the power back up. But they couldn't do it. During the 10-hour delay, the core's low-power state caused a buildup of xenon, another type of atom that in essence blocks the nuclear fission process. The core temperature also dropped so much it stopped boiling water away and producing steam. 

The usual course of action with such low power would be to bring the core's power level back up over 24 hours. The power plant chief, Dyatlov, did not want to wait and so forged ahead with the safety test.

"Any commissioning test involving changes to protection systems has to be very carefully planned and controlled," explains Tony Irwin, who advised Soviet Union scientists and engineers on safe operating practices of RBMK reactors in the wake of Chernobyl.

"In this accident they were operating outside their rules and defeating protection which was designed to keep the reactor safe."

A disregard for the rules -- and the science -- exposed them to the RBMK's great danger: the positive void coefficient.

The positive void coefficient

We hear the term "positive void coefficient" bellowed by Jared Harris' Legasov in Chernobyl's final episode, and it is key to the explosion -- but it's not exactly explained.

Recall how the water both cools the core and "slows"the reaction down. However, when water turns to steam it lacks the ability to effectively do both of those things, because it boils away and becomes bubbles or "voids." The ratio of water to steam is known as the "void coefficient." In other nuclear reactors, the void coefficient is negative -- more steam, less reactivity.

In the RBMK reactor, it's the opposite: More steam results in higher reactivity. This positive void coefficient is unique to RBMK reactors.

Scene showing a scientist in a lab

Emily Watson is riveting as a nuclear physicist who represents all of the real life scientists that worked to unravel how Chernobyl exploded.


Once the plant workers shut down the reactor at 1:23:04 a.m., water is no longer pumped into the core. The catastrophic cascade at Chernobyl is set in motion.

The safety test shuts down the reactor and the remaining water boils away. Thus, more steam.

The steam makes the nuclear fission more efficient, speeding it up. Thus, more heat.

More heat boils the water away faster. More steam.

More steam… you get the point.

If we freeze-frame right here, the scenario is grim. The core is quickly generating steam and heat in a runaway reaction. All but six of the 211-plus control rods have been removed from the core and the water is no longer providing any cooling effects. The core is now a giant kid's ball pit in an earthquake, with neutrons bouncing around the chamber and constantly colliding with one another.

The only thing the plant workers could do was hit the emergency stop button.

The Chernobyl explosion

At 1:23:40 a.m., the emergency stop button was pressed by chief of the night shift, Alexander Akimov. This forces all of the control rods back into the core.

The control rods should decrease the reaction but because they are tipped with graphite, they actually cause the power to spike even more. Over the next five seconds, the power increases dramatically to levels the reactor cannot withstand. The caps on the top of the reactor core, weighing more than 750 pounds, begin to literally bounce in the reactor hall.

A grid of steel blocks

The 700-plus pound steel blocks resting on top of the reactor core started rumbling around and being lifted into the air in the moments before the explosion.


Then, at 1:23:45 a.m., the explosion occurs. It's not a nuclear explosion, but a steam explosion, caused by the huge buildup of pressure within the core. That blows the biological shield off the top of the core, ruptures the fuel channels and causes graphite to be blown into the air. As a result, another chemical reaction takes place: air slips into the reactor hall and ignites causing a second explosion that terminates the nuclear reactions in the core and leaves a mighty hole in the Chernobyl reactor building.

Could it happen again?

It's kind of insane to think that humans can control the power of the atom. The Fukushima disaster that affected a Japanese nuclear plant in 2011 demonstrates that catastrophes still lurk within reactors around the world and we are not always prepared for them.

After Chernobyl, a number of changes were implemented in the RBMK reactors across the former Soviet Union. Today, eight such reactors still exist in operation in Russia: three in Kursk, three in Smolensk and two in Leningrad -- the only place where they are currently operating. They are expected to be shut down by 2034. 

Those sites were retrofitted with safety features which aim to prevent a second Chernobyl. The control rods were made more plentiful and can be inserted into the core faster. The fuel rods feature slightly more enriched uranium which helps control the nuclear reactions a little better. And the positive void coefficient, though it still exists in the design, has been dramatically reduced to prevent the possibility of a repeat low-power meltdown.

Of course, the one thing that hasn't changed is us. Chernobyl was a failure on the human scale, long before it was a failure on the atomic one. There will always be risks in trying to control nuclear fission reactions, and those risks can only be mitigated -- not reduced to zero. Chernobyl and other nuclear reactors aren't nuclear bombs waiting to detonate. The HBO series teaches us that they can become dangerous if we fail to understand the potential of atomic science.

So can this kind of nuclear catastrophe happen again? Yes. As long as we try to harness the power of the atom, the odds will fall in favor of disaster. But should we stop trying to do so? No. Harnessing the power of the atom and mitigating the risks of nuclear energy as best we can is one of the ways to a cleaner energy future.

According to the World Nuclear Association, nuclear energy accounts for approximately 11% of all energy generated on the Earth. Across the planet, 450 reactors are currently in operation -- only eight of them are RBMK reactors with enhanced safety features -- and as we look at ways to reduce our reliance on harmful fossil fuels, nuclear energy must be considered as a viable alternative. We can't continue to burn coal like we do and expect the climate crisis to disappear.

So we will continue to harness the power of the atom and we will get better. We have to.

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Originally published June 4, 2019.

Correction, April 25, 2023: The Chernobyl nuclear plant was commissioned by the Soviet Union. Also, since this article was published, two RBMK reactors have been decommissioned, meaning there are now eight RBMK reactors still in operation.