35 Years On, What Caused The Chernobyl Nuclear Disaster?


Tom Hale

Senior Journalist

clockApr 26 2021, 15:55 UTC

The fateful Reactor Control Room in Chernobyl Exclusion Zone. Image credit: CE85S/

On April 26, 1986, the worst nuclear disaster in history rapidly unfolded and continues to gently rumble today. Known simply as Chernobyl, the nuclear disaster became a watershed moment in the Cold War, the dying days of the USSR, and humanity’s use of nuclear power. It’s also become an infamous reminder of the perils of toying with nuclear power, as well as the unfortunate consequences that human error and bureaucratic incompetence can have on events.


Where Is Chernobyl?

The disaster took place at the Chernobyl Nuclear Power Plant near the city of Pripyat in what was then Soviet Ukraine. Today, the site is found in northern Ukraine near the Ukraine–Belarus border.

What Happened At CHERNOBYL?

In the early hours of Saturday, April 26, 1986, engineers at the plant were (somewhat ironically) carrying out a safety test designed to see how to cool the core of reactor 4 in the event of a power outage.

In its simplest sense, a nuclear reactor is an atomic-powered steam engine: heat caused by fission (the splitting of atoms) is used to heat water to create steam, which spins a turbine to generate electricity. To control the rate of fission, boron carbide control rods can be dunked into the reactor to absorb neutrons released during fission and slow down the reaction. Pumped cooling water is also used to manage the levels of heat and steam produced. It gets a lot more complicated than this (atomic physics is not known for its simplicity), but this is the bare bones of what you need to know. 

If a blackout occurs, then the cooling pumps are also without power and the system could potentially overheat. To address this problem, the reactor had several diesel pumps that can cool the core by pumping water around the core, but these engines took around 1 minute to get going. The purpose of this test was to see how to maintain sufficient cooling while waiting for the backup diesel pumps to kick in. 

Why Did Chernobyl Explode?

The experiment, however, was riddled with mismanagement, run by inadequately trained personnel, and skimped on the vital safety precautions. During the test, the operators turned off automated control systems, going against the safety regulations. It's also noted that the test was initially planned for the previous afternoon but after some delays, it fell into the hands of the less experienced night-time crew. 

Late on April 25, just before midnight, the grid controller agreed that the reactor could reduce its power. Although power was low, xenon was still being created and started to build up. Xenon is a byproduct of the fission reactions carried out in the reactor and is particularly troublesome as it absorbs neutrons, further reducing reactivity in the reactor as it builds up. 

Just past midnight on April 26, operators noted the reactor was running at low power. To regain the power and stability needed for the upcoming test, they should have slowly raised power over the course of hours or days, but they acted rashly. The build-up of xenon meant the only way to increase power was by removing the control rods. Under orders of Anatoly Dyatlov, the deputy chief engineer of the plant, the operators removed almost all but eight out of the 200 control rods from the reactor — far too many than permitted by the safety regulations — to generate some much-needed power.


“It was like airplane pilots experimenting with the engines in flight,” Valery Legasov, a Soviet chemist, said in 1987.

The abandoned town of Pripyat. Image credit: Oriole Gin/

Under these shaky conditions, the safety test began at around 1:23 am and the water coolant stopped pumping around the reactor, as you would be expected to see during a blackout.

Heat and steam quickly built. Less than a minute into the test, an operator pressed the emergency shutdown button in response to the mounting steam, causing the control rods to enter the reactor. This aimed to halt and cool the reactor, but it inadvertently sparked a surge of power. A significant design flaw of the control rods, namely their "graphite tips," initially increased the rate of fission after they were inserted and resulted in a sharp increase in heat.


Like water bubbling in a boiled kettle, intense pressure built up and the reactor blew up with tremendous force. Oxygen from the air rushed into the reactor and met with the piping hot contents of the reactor, causing a combustion reaction and another explosion. Fires lit up much of the nuclear power plant. Many of these fires were put out over the course of the next day, but the blaze in the reactor core continued to burn for days. One notorious relic of the fire is the so-called "Elephant's foot," a cooled lump of radioactive corium that lies in the basement of Chernobyl Nuclear Power Plant to this day. 

How many people died in Chernobyl?

Worsening the disaster, the notoriously problematic RBMK reactors do not have what is known as a containment structure designed to keep radiation inside the plant in the event of such an accident. As a result, radioactive debris was scattered out of the reactor over a wide area, forcing thousands of people to evacuate. The immediate death toll was 54, mostly from acute radiation sickness, with thousands more succumbing to radiation-related sickness like cancer. The World Health Organisation estimates that the Chernobyl nuclear incident contributed to the death of up to 9,000 people, but some estimates have gone as high as 60,000 deaths. 

Due to the secretive nature of the USSR and the paranoia of the Cold War, many of the details behind the disaster were hidden from the wider world. However, after the collapse of the Soviet Union, more information regarding the catastrophe came to light. Even today, 35 years on, scientists are still learning more about the disaster. Last week, two studies were released detailing the long-term consequences of exposure to ionizing radiation, revealing that children of those who were affected by the catastrophe display no additional DNA damage, with levels of genetic mutations that match those of the general population.


The Ukrainian government is now petitioning the disaster site be listed as a UNESCO World Heritage Site, to commemorate the event and the mishandling, the history and people's rights.

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