Chernobyl: What Happened?
, Why Reactor 4 Exploded—and What the Disaster Changed
In the early hours of 26 April 1986, engineers at the Chernobyl Nuclear Power Plant were conducting a test on Reactor No. 4.
Within seconds, the test became a catastrophe.
A sudden and enormous increase in reactor power was followed by explosions that destroyed the reactor core and building.
A fire burned in the exposed graphite moderator.
Radioactive material was released into the atmosphere and carried across large areas of Ukraine, Belarus, Russia and eventually much of Europe. (World Nuclear Association)
The accident became the world’s most notorious nuclear disaster.
But Chernobyl was not caused by one reckless operator pressing the wrong button.
It was the product of a dangerous reactor design, inadequate safety information, procedural violations, institutional secrecy and a safety culture that failed at almost every level. (IAEA Publications)
Understanding Chernobyl therefore requires looking beyond the explosion itself.
The more important question is:
How did an operating nuclear reactor reach a condition in which a safety shutdown could help trigger its destruction?
What was the Chernobyl Nuclear Power Plant?
The Chernobyl Nuclear Power Plant stood near the city of Pripyat in what was then the Ukrainian Soviet Socialist Republic, part of the Soviet Union.
The plant contained several RBMK-1000 reactors.
RBMK reactors were large graphite-moderated, water-cooled reactors developed by the Soviet Union.
They possessed certain advantages.
They could produce substantial electrical power.
They could be refuelled while operating.
And their design suited Soviet industrial and nuclear requirements.
But they also possessed serious weaknesses.
Two became particularly important at Chernobyl:
the positive void coefficient
and
the design of the control rods.
Those characteristics helped turn a badly managed reactor test into a catastrophic accident. (IAEA Publications)
What test was being carried out?
Reactor No. 4 was due to shut down for maintenance.
Before doing so, operators planned to conduct an electrical test.
The purpose was to determine whether the momentum of the turbine generator, after steam supply had been cut off, could temporarily provide enough electrical power to operate essential equipment until emergency diesel generators reached full output.
The idea itself was not inherently unreasonable.
Previous attempts had not provided satisfactory results.
So another test was planned during the April 1986 shutdown.
The problem was not merely the existence of the test.
It was the condition into which the reactor was allowed to move while the test was prepared.
The delayed shutdown
The reactor was initially being reduced in power when electricity demand elsewhere caused the grid controller to request that Chernobyl continue generating.
The test was therefore delayed.
Reactor No. 4 continued operating at reduced power for several additional hours.
Eventually, permission came to continue the shutdown.
By then, the reactor was in a different condition from the one originally anticipated when the test had been planned.
That mattered because nuclear reactors do not simply behave according to the position of a power lever.
Their behaviour depends on the condition of the fuel, coolant, neutron absorbers and numerous other factors inside the core.
The unexpected power drop
As power was reduced, Reactor No. 4 suffered a much greater fall in output than intended.
One factor was the accumulation of xenon-135, a powerful neutron absorber produced inside an operating reactor.
This phenomenon is commonly called xenon poisoning.
When reactor power falls, xenon can suppress the nuclear chain reaction and make it difficult to restore power.
Operators attempted to raise the reactor’s output.
To do so, they withdrew a large number of control rods.
The reactor eventually stabilised at a power level far below that originally intended for the test.
But it was now operating in an unstable and dangerous configuration.
Too few control rods
Control rods absorb neutrons and therefore help regulate the nuclear chain reaction.
There were operating limits governing how many should remain inserted into the RBMK core.
During the preparations for the test, too many rods were withdrawn.
That reduced the reactor’s safety margin.
Operators violated procedures.
But later investigations also found that the personnel were not adequately informed about some of the most dangerous characteristics of the RBMK reactor under precisely these operating conditions. (IAEA Publications)
This distinction matters.
Chernobyl cannot honestly be explained either as:
“the operators caused it”
or
“the reactor caused it.”
Both human actions and serious engineering weaknesses interacted.
What was the positive void coefficient?
This is one of the most important technical features of the accident.
In many reactor designs, the formation of steam bubbles in the coolant tends to reduce the nuclear reaction.
That provides a natural stabilising effect.
The RBMK could behave differently.
Under certain operating conditions, when water inside the reactor turned into steam, neutron absorption by the water decreased while the graphite continued moderating neutrons.
The nuclear reaction could therefore increase.
More heat produced more steam.
More steam could produce more reactivity.
More reactivity produced still more power.
This is known as a positive void coefficient.
At low power and with many control rods withdrawn, that characteristic could become extremely dangerous. (IAEA Publications)
The test begins
Despite the unstable reactor condition, the test proceeded.
Steam supply to the turbine was reduced.
As the turbine slowed, the pumps driven by the electrical system began to behave differently.
Coolant flow changed.
More steam voids developed within parts of the reactor.
Because of the RBMK’s positive void coefficient, those changes could increase reactivity.
The reactor began moving towards a condition in which small changes could produce very large effects.
Then came the moment that has become central to the Chernobyl story.
AZ-5 — the emergency shutdown
Operators pressed the AZ-5 emergency shutdown button.
In principle, this should have shut down the reactor by inserting all control rods into the core.
But the RBMK control rods had a dangerous design characteristic.
Their lower sections incorporated graphite displacers.
When fully withdrawn rods first began moving into the core, the graphite tips could initially increase reactivity in parts of the reactor rather than immediately reducing it.
Under normal conditions the effect might have been manageable.
In the unstable condition of Reactor No. 4 that night, it was disastrous.
The insertion of the rods contributed to an extremely rapid surge in power.
Fuel channels ruptured.
Steam pressure rose violently.
Within seconds, the reactor was being destroyed. (IAEA Publications)
The explosions
Two major explosions followed.
The precise physical sequence has been analysed extensively, but the result is beyond dispute.
Reactor No. 4 was torn apart.
The massive upper biological shield was displaced.
The core was exposed.
Graphite began burning.
Unlike many Western nuclear power stations, the RBMK reactor did not sit within a full-pressure containment structure capable of containing such a catastrophic release.
Radioactive material was therefore able to escape directly into the atmosphere.
The disaster had entered an entirely new phase.
The firefighters
Firefighters arrived quickly.
Many did not initially understand that they were entering an environment contaminated by intense radiation.
Their immediate task was familiar:
put out fires and prevent them spreading to neighbouring reactor buildings.
But this was not an ordinary industrial fire.
Pieces of highly radioactive material from the reactor core had been thrown around the site.
Firefighters and plant personnel received enormous radiation doses.
Their actions nevertheless helped prevent fires from spreading further through the facility.
Two workers died as an immediate result of the accident, and 28 more people died within weeks from acute radiation syndrome, according to widely accepted assessments. (World Nuclear Association)
Pripyat
The nearby city of Pripyat had been built largely to house workers from the nuclear plant and their families.
After the explosion, residents initially continued with ordinary life.
Children went outside.
People watched activity around the plant.
Few understood the radiation levels surrounding them.
The city was not evacuated immediately.
Approximately thirty-six hours after the accident, evacuation finally began.
Residents were told to take essential belongings and expect to return within a short period.
Most never lived there again.
Pripyat became one of the most haunting symbols of the disaster:
a modern city suddenly abandoned.
Soviet secrecy
For the Soviet authorities, Chernobyl created both a nuclear emergency and a political crisis.
The Soviet system placed great importance on secrecy and the appearance of technological competence.
Initially, information about the accident was severely restricted.
But radioactive contamination could not be contained by censorship.
On 28 April, elevated radiation levels were detected at the Forsmark nuclear power plant in Sweden.
The contamination was traced not to Sweden itself but towards the Soviet Union.
International pressure mounted.
The Soviet authorities could no longer conceal that a major nuclear accident had occurred.
Chernobyl demonstrated a fundamental truth about large technological disasters:
physical evidence does not respect political borders.
The evacuation zone
Authorities eventually created an exclusion zone around the damaged plant.
Settlements were evacuated.
Land was abandoned.
Agricultural production ceased across heavily contaminated areas.
The evacuation affected tens of thousands of people immediately and many more through later relocations.
The most heavily contaminated regions extended across Ukraine, Belarus and Russia.
For the displaced population, Chernobyl was not simply an accident involving a reactor.
It meant the loss of homes, communities, livelihoods and places to which families had belonged for generations.
The liquidators
Hundreds of thousands of people participated in the emergency response and cleanup over subsequent years.
They became known collectively as the liquidators.
They included firefighters, soldiers, engineers, miners, plant workers, helicopter crews, construction workers, medical staff and many others.
Their tasks varied enormously.
Some removed highly radioactive debris.
Some built containment structures.
Some decontaminated roads and buildings.
Others monitored radiation or worked on the damaged reactor site.
The conditions could be extremely hazardous.
The liquidators became central to preventing an already enormous disaster from becoming worse.
The first sarcophagus
The destroyed Reactor No. 4 needed to be enclosed rapidly.
Workers constructed a huge concrete and steel structure around the remains.
Often called the sarcophagus, it was officially known as the Shelter Object.
It was built under extraordinary conditions and completed within months.
Its purpose was to reduce further releases of radioactive material and isolate the damaged reactor from the environment.
But it was never intended to be a permanent solution.
The structure aged.
Water entered.
Sections became unstable.
Eventually, a much larger engineering solution would be required.
The New Safe Confinement
Decades later, an enormous arch-shaped structure known as the New Safe Confinement was built beside Reactor No. 4 and then moved into position over the old shelter.
It was designed to isolate the destroyed reactor for many decades and provide space in which the unstable original shelter and reactor remains could eventually be dismantled safely.
Chernobyl therefore remains an active engineering and decommissioning project.
The accident happened in 1986.
Managing its physical consequences continues generations later.
Who was blamed?
The initial Soviet explanation placed substantial responsibility upon the reactor operators.
Several were prosecuted.
There had unquestionably been violations of operating procedures.
But as more information became available, that explanation became increasingly inadequate.
The International Atomic Energy Agency’s later INSAG-7 reassessment substantially revised the understanding of the accident.
It emphasised serious RBMK design weaknesses, inadequate safety information, poor regulation, deficient operating procedures and a deeply flawed safety culture alongside operator actions. (IAEA Publications)
That later reassessment is vital.
The men in the control room made mistakes.
But they were operating a reactor whose most dangerous characteristics had not been communicated to them adequately.
The deeper failure — safety culture
Perhaps the most important lesson from Chernobyl is not about graphite or control rods.
It is about safety culture.
A strong safety culture encourages people to question assumptions.
It rewards reporting problems.
It ensures that known design weaknesses are communicated clearly.
It recognises that operating procedures must reflect actual engineering risk.
And it allows inconvenient information to travel upwards through an organisation rather than being suppressed.
Chernobyl possessed almost the opposite environment.
Design weaknesses existed.
Information about them was fragmented or withheld.
The regulatory system lacked sufficient independence.
Operators worked inside an organisational structure that encouraged production and compliance with authority.
The accident was therefore not simply a failure of machinery.
It was a failure of the system surrounding the machinery. (World Nuclear Association)
How many people died because of Chernobyl?
This is one of the most difficult and frequently misunderstood questions.
The immediate deaths can be counted relatively clearly.
Two plant workers died as a direct result of the explosion, and 28 emergency workers subsequently died from acute radiation syndrome within the following weeks. (World Nuclear Association)
Long-term health effects are far more difficult to quantify.
Radiation exposure can increase the probability of cancers that also occur naturally within populations.
That means scientists must estimate excess cases statistically rather than identify every individual cancer as being caused by Chernobyl.
Different organisations and studies have therefore produced different projections depending on populations, dose assumptions and methodology.
One effect, however, is particularly well established:
a substantial increase in thyroid cancer among people exposed as children or adolescents, largely associated with radioactive iodine.
The human consequences should therefore be discussed carefully rather than reduced either to exaggerated claims of hundreds of thousands of proven radiation deaths or to assertions that the accident caused virtually no long-term harm.
Why were children particularly vulnerable?
Radioactive iodine released from the reactor contaminated pasture and food supplies, particularly milk.
The thyroid gland naturally absorbs iodine.
Children’s thyroid glands are smaller and biologically active, making them particularly vulnerable to radioactive iodine exposure.
Consumption of contaminated milk therefore became an important pathway of radiation exposure.
Had authorities communicated the danger more rapidly and introduced protective measures sooner, some exposure could have been reduced.
Again, secrecy and delay became part of the accident’s consequences.
What happened to the other Chernobyl reactors?
The destruction of Reactor No. 4 did not immediately close the entire power station.
Other Chernobyl reactors eventually returned to or continued operation because the Soviet and later Ukrainian electricity systems still required their output.
Reactor No. 2 closed after a fire in 1991.
Reactor No. 1 ceased operating in 1996.
Reactor No. 3, the final operating unit, shut permanently in December 2000.
The site then entered the long process of decommissioning and radioactive-waste management.
That work continues today. (Chornobyl NPP)
Were RBMK reactors changed after Chernobyl?
Yes.
The accident forced substantial modifications to remaining RBMK reactors.
Changes were made to reduce the dangerous positive void behaviour, modify the control rods, increase the number of rods required in the core and improve operating procedures and safety systems.
The broader Soviet and international nuclear industries also strengthened safety cooperation and scrutiny. (World Nuclear Association)
In other words, engineers did not conclude that Chernobyl was simply the result of irresponsible operators who could be replaced.
They changed the reactors.
That fact alone demonstrates how important the design deficiencies were.
Chernobyl today
Chernobyl is no longer an operating nuclear power station.
Its work now centres on decommissioning, spent fuel, radioactive-waste management and maintaining the structures surrounding Reactor No. 4. (Chornobyl NPP)
The site has also been affected by Russia’s war against Ukraine.
Russian forces occupied the Chernobyl site and parts of the Exclusion Zone in 2022.
In February 2025, Ukraine reported that a drone strike damaged the New Safe Confinement over Reactor No. 4.
Restoration and assessment work continues, and international support remains involved in protecting the facility. The Chornobyl plant reported in 2026 that work was continuing to address damage to the New Safe Confinement. (Chornobyl NPP)
So even forty years after the accident, Chernobyl remains a place requiring active international attention.
Why Chernobyl still matters
Chernobyl changed the nuclear industry.
It demonstrated the consequences of combining dangerous engineering characteristics with weak regulation and poor safety culture.
It exposed the limits of secrecy during technological disasters.
It showed that emergency planning cannot stop at national borders.
And it demonstrated how difficult it is to recover public trust once authorities appear to have hidden the truth.
But perhaps the most important lesson is broader than nuclear energy.
Complex systems rarely fail because of one mistake.
Disasters develop when:
engineering weaknesses exist,
warnings are misunderstood or suppressed,
procedures do not match reality,
organisations discourage challenge,
and
individual decisions interact with weaknesses already built into the system.
Chernobyl was an extraordinary nuclear accident.
The organisational lessons behind it are painfully ordinary.
Frequently Asked Questions
What happened at Chernobyl?
During a reactor test in the early hours of 26 April 1986, Reactor No. 4 at the Chernobyl Nuclear Power Plant experienced an uncontrollable power surge. Explosions destroyed the reactor and exposed its radioactive core, resulting in a major release of radioactive material. (World Nuclear Association)
What caused the Chernobyl disaster?
The disaster resulted from a combination of major RBMK reactor design deficiencies, dangerous operating conditions, procedural violations, inadequate safety information and a poor safety culture. Later international assessments placed substantially more emphasis on reactor-design and institutional failures than the earliest Soviet explanation had done. (IAEA Publications)
Did the operators cause Chernobyl?
Operator actions contributed to the accident, including operating the reactor in an unsafe configuration. However, later investigations established that major design deficiencies and inadequate information supplied to operators were also fundamental causes. (IAEA Publications)
Why did pressing the emergency shutdown button make things worse?
Because of the RBMK control-rod design, inserting fully withdrawn rods could initially increase reactivity in parts of the core. In the extreme conditions present immediately before the accident, this contributed to the enormous power surge. (IAEA Publications)
What was wrong with the RBMK reactor?
Among its important safety weaknesses were a large positive void coefficient under certain operating conditions and a problematic control-rod design. Those characteristics made the reactor especially unstable in the condition reached during the test. (World Nuclear Association)
How many people died immediately after Chernobyl?
Two workers died as a direct result of the accident, and 28 people died within several weeks from acute radiation syndrome. The long-term health impact is more complicated and is assessed statistically rather than through a single universally agreed death toll. (World Nuclear Association)
Is Chernobyl still radioactive?
Yes. Radioactive material remains within the destroyed reactor, waste facilities and areas of the Exclusion Zone. The site continues to require long-term decommissioning, monitoring and radioactive-waste management. (Chornobyl NPP)
Is the Chernobyl power station still operating?
No. The final operating reactor at Chernobyl was shut down permanently in December 2000. The site is now undergoing decommissioning and radioactive-waste management.
Can Chernobyl simply be cleaned up and abandoned?
No. Managing the destroyed reactor, spent fuel, contaminated materials and radioactive waste is a long-term engineering undertaking likely to continue for many decades.
Read More
CHERNOBYL
In his investigative nonfiction work, Bill Stewart examines the Chernobyl disaster not simply as a reactor explosion, but as a chain of technical, organisational and human failures.
The investigation explores the RBMK reactor, the failed safety test, the decisions made inside the control room, the design weaknesses that operators did not fully understand, the Soviet response, the liquidators and the lessons subsequently identified by international investigators.
At its centre is a question that applies far beyond nuclear engineering:
How does a complex organisation reach the point where known weaknesses, flawed assumptions and human decisions combine to create catastrophe?
Read the book →
About the Author
Bill Stewart is a Scottish author and former Marine Auditor and Lead Accident Investigator.
Drawing on more than four decades at sea and professional experience involving accident investigation, safety management and complex operational systems, his investigative nonfiction examines major disasters, engineering failures, aviation and maritime history, intelligence operations and significant historical events.
His approach is straightforward:
Tell the story honestly. Respect the facts. Never lose sight of the people.