The Chernobyl Disaster
The Reactor Test, the Evacuation, and the Exclusion Zone
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The explosions destroyed the reactor and released radioactive material into the atmosphere. Firefighters and engineers worked for days to contain the fire and build a concrete sarcophagus around the damaged reactor. Pripyat was evacuated within hours, though many residents didn't know why. The Soviet government delayed announcing the disaster for nearly 36 hours.
The accident created an exclusion zone around the plant that remains largely uninhabited today. Recovery efforts included building new containment structures and managing radioactive waste. This detailed account covers each step from reactor cooling to long-term effects on health and environment. Anyone interested in nuclear safety or Cold War history will find this essential reading.
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When a nuclear reactor shuts down, the fission process stops, but heat keeps coming from radioactive decay, which continues even after the reaction has ended. For reactors like those at Chernobyl, coolant must keep flowing to prevent the core from overheating or melting down. The RBMK reactors used water as coolant, pumped by electric motors. Reactor number four had 1,661 fuel channels, and needed more than 45 million liters of coolant every hour to stay safe.
When a reactor shuts down, it continues to produce heat that must be removed or the core could melt. Each of Chernobyl’s reactors had three backup diesel generators to provide power in case of a total loss, but those took sixty to seventy-five seconds to reach full load and generate the five point five megawatts needed to run one main pump. In the time it took for the generators to start up, special counterweights on each pump provided coolant via inertia to keep things going. However, if a station blackout occurred at the same time a coolant pipe ruptured, the emergency core cooling system would be required to pump additional water into the core.
The plan was based on the idea that the turbine’s rotational energy could power the emergency cooling system long enough to keep the reactor safe. The steam turbine would slow down as it gave up its energy, but engineers calculated there might be enough stored momentum to run the coolant pumps for about forty-five seconds. That wouldn’t fully cover the time between losing external power and the emergency generators coming online, but it could help ease the situation in those critical moments.
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A safety test was needed to confirm the turbine’s energy capability, but earlier attempts had failed. In 1982, a test showed the excitation voltage was too low. The system was adjusted and tested again in 1984, but still didn’t work. A third try in 1985 also failed, this time due to recording equipment issues. The test was scheduled for 1986, planned during a controlled power-down of reactor number four, ahead of a maintenance outage.
The test that was planned had a procedure written for it, but those who wrote it didn’t understand how the RBMK-1000 reactor would behave under the conditions being tested. It was treated only as an electrical test of the generator, even though it involved key parts of the reactor system. The rules at the time didn’t require approval from the chief design authority for the reactor, known as NIKIET, or from the nuclear safety regulator. The plan included turning off the emergency core cooling system, which is meant to pump water into the core during a loss-of-coolant accident. Still, the site’s chief engineer had given permission, following the regulations in place.
The reactor's thermal power was lowered to 700-1,000 megawatts to ensure proper cooling while the turbine spun at operating speed but remained disconnected from the power grid. This test simulated a power outage to check how the reactor responded under these conditions, verifying that the system could handle a loss of power without overheating. Engineers wanted to confirm the reactor could safely shut down and cool itself when the turbine was running but not connected to the grid. The test was part of a larger plan to evaluate the reactor's performance during emergency conditions, designed to mimic a real-world scenario where power might be lost suddenly. The turbine would spin at full speed without generating electricity, allowing engineers to study system behavior during a simulated emergency. This experiment validated safety protocols and was not intended to be routine operation but rather a controlled test under specific conditions. The outcome would determine whether the reactor could handle similar situations safely, with results crucial for understanding risks. The test was scheduled for a specific time with strict procedures followed by operators, carefully monitored from start to finish. It represented a critical moment in the reactor's operational history that would shape how the facility responded to emergencies in the future.
Four of eight main circulating pumps were scheduled to be powered by off-site electricity while the remaining four drew power from the turbine itself. This arrangement was part of a safety test shutting down main power supply and relying on backup systems to keep the reactor cooled, simulating a blackout scenario. The test checked how well backup systems performed under stress, evaluating whether the plant could function safely without external power. These pumps played a key role in circulating coolant through the reactor core. The turbine had to be ready to take over when needed. This was not just a routine check but a critical evaluation of safety protocols determining whether the reactor could handle complete power loss. It was a high-stakes experiment in nuclear safety requiring careful coordination between turbine and off-site grid.
The team was running a safety test, trying to understand how the reactor would respond under specific settings. They needed to know if the system could shut down properly when the steam was cut off. In ordinary circumstances, closing the steam supply to the turbine generator would trigger an automatic reactor shutdown. The engineers were watching closely as the process unfolded, waiting for the results that would determine whether the reactor could handle such a scenario safely. This was not just a routine check—it was part of a larger experiment meant to assess the reactor’s behavior during a controlled situation.
The test involved measuring the voltage from the turbine that was still spinning, as well as checking the voltage and revolutions per minute of the four main circulating pumps that were being driven by that same turbine. These pumps were crucial to the reactor’s operation, and their performance during the test would help determine whether the system could safely handle a loss of power. The readings were carefully monitored to assess how the reactor responded under these specific conditions.
When full power returned from the backup generators, the plan was for the turbine to keep spinning on its own without any resistance. This process, known as free-wheeling, would allow the system to slow down gradually. The operators expected this controlled descent to happen smoothly, as part of a larger safety test being conducted at the reactor. They had calculated that the energy stored in the spinning turbine could be safely dissipated this way, without causing any immediate danger. This was not an emergency situation, but rather a routine procedure meant to verify the system’s response under specific conditions. The team trusted the design and the sequence of events they had laid out.
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The test was scheduled for the day-shift of April 25, 1986, as part of a planned reactor shutdown. The day shift had been briefed ahead of time on how to handle the reactor during the test, and a group of electrical engineers was on hand to carry out the electrical portion once conditions were right. At 1:06 AM on April 25, the power output began to drop gradually, and by the start of the day shift, the reactor had reached 1600 MW out of its normal 3,200 MW thermal level.
The day shift at Chernobyl was set to run a reactor test at 14:15, with preparations already underway, including shutting down the emergency core cooling system. But at around 14:00, the Kiev electrical grid controller called in and asked that the planned reduction in power output from the plant be delayed. The reason was urgent—power was needed to meet peak evening demand. That request came as another regional power station unexpectedly went offline, adding pressure to the situation.
Soon, the day shift gave way to the evening shift. Even with the delay, the emergency core cooling system stayed turned off. It had to be shut down manually, using a large valve wheel about the size of a sailboat's helm, which meant that two or three people had to turn it by hand throughout their entire shift. The system didn’t play a role in the disaster itself, but letting the reactor run for eleven hours without that protection during the test showed how deeply unsafe practices had become. It was a sign of a broader failure in safety culture.
At 23:04, the Kiev grid controller gave the go-ahead for the reactor to shut down again. By then, the day shift had already left, and the evening shift was getting ready to head out too. The night shift wouldn’t start until midnight, well into their assigned time. The test was supposed to wrap up during the day shift, so the night crew would only have needed to keep the decay heat cooling systems running in a plant that was otherwise shut down.
The night shift at Chernobyl had very little time to prepare for the experiment they were about to conduct. Anatoly Dyatlov, who served as deputy chief-engineer, was present to supervise the test. He had been one of the main architects of the procedure and was the most senior individual on site. Aleksandr Akimov led the Unit 4 night shift, while Leonid Toptunov, the senior reactor control engineer, was responsible for overseeing the reactor’s operations, including the movement of control rods. Toptunov was only twenty-five years old and had been working independently as a senior engineer for roughly three months.
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At 00:05 on 26 April, the reactor reached 720 MW as planned, but then things took an unexpected turn. The process of fission produced xenon-135, a substance that absorbs neutrons and slows the reaction. Normally, this wouldn’t be a problem, because in steady operation, xenon-135 gets “burned off” just as fast as it forms, turning into the stable isotope xenon-136. But with power reduced, iodine-135—already present—was decaying into xenon-135 faster than the lower neutron levels could handle. This caused a drop in power that wasn’t anticipated, a condition known as reactor poisoning. Though difficult to control, it was a predictable outcome during this kind of power reduction.
When the reactor power had dropped to about 500 megawatts, control switched from a local automatic regulator to the automatic regulators, in an attempt to manually keep the power level stable. AR-1 then activated, pulling out all four of its control rods automatically, but AR-2 failed to activate because of a problem with its ionization chambers. In response, Toptunov reduced power to try and stabilize the sensors. That action led to a sudden and unexpected drop in power, down to just 30 megawatts thermal or less. The exact cause remains unclear. Most reports blame Toptunov’s error, though Dyatlov said it was due to a fault in the AR-2 system.
The reactor had dropped to just 5% of the minimum power needed for the test, a level so low that it stopped the natural burn-off of xenon-135 in the core and kept power from rising. Workers in the control room responded by pulling out many of the control rods. It took several minutes before the reactor reached 160 megawatts at 00:39. At that point, most of the rods were pushed up as far as they could go, though the setup still fell within normal limits—equivalent to having more than 15 rods fully inserted. In the following twenty minutes, power was raised again, up to 200 megawatts.
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At 01:23:40, as the experiment was coming to an end, an operator pressed the emergency AZ-5 button to start a scram, or emergency shutdown, of the reactor. Some believe this was done perhaps in preparation for scheduled maintenance. The reason for the timing remains unclear, since both Akimov and Toptunov would die shortly after the alarm sounded. Eyewitnesses said the control room felt calm at that moment, and there were no active emergency signals. Later, members of the reactor’s design team argued that the button must have been pressed only after the reactor had begun to self-destruct.
The AZ-5 button triggered a mechanism to fully insert all control rods, including manual ones that had been pulled out earlier. In the RBMK design, each control rod had a graphite moderator attached to its end. When the rod was fully withdrawn, this graphite extension would sit in the core's water, increasing reactor power by displacing neutron-absorbing water.
When the control rods slid into the reactor at 0.4 meters per second, it took about twenty seconds to reach the bottom of the core. As they moved down, their graphite extensions first pushed out water that absorbed neutrons and replaced it with graphite that slowed neutrons, which actually caused the reaction rate to rise at first. This unexpected behavior had been seen before—in 1983, a similar situation occurred at the Ignalina Nuclear Power Plant. That incident showed the same effect, yet no safety changes were made. Later, the IAEA report INSAG-7 noted that people believed the conditions for this power spike would never happen, but they did occur during the Chernobyl disaster, matching almost every detail of what had been observed earlier.
Just seconds after the scram began, a sudden power spike triggered overheating in the core, leading to fractures in some fuel rods. Experts have theorized that this event may have jammed the control rods, leaving them only one-third inserted. By the time three seconds had passed, the reactor’s output had surged past 530 MW.
The instruments didn’t capture what happened next; scientists had to rebuild the sequence using math and simulations. A sudden spike in power led to a rise in fuel temperature and steam buildup, which quickly increased steam pressure. That pressure caused the fuel cladding to break down, sending fuel elements into the coolant and rupturing the channels where they were stored.
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As the scram continued, the reactor output surged to around 30,000 MW thermal, ten times its normal operational output, the final reading on the control panel. Some estimates suggest the spike may have been even higher. It was not possible to fully reconstruct the exact sequence that led to the reactor’s destruction and the power unit building's collapse, but a steam explosion appears to have followed. Explosive steam pressure from damaged fuel channels escaping into the reactor’s exterior cooling structure caused the blast that destroyed the reactor casing. The upper plate, known as the upper biological shield—which held the entire reactor assembly—was torn away and launched through the roof of the reactor building. This is believed to be the first explosion many heard.
The explosion damaged more fuel channels and cut most of the coolant lines that supplied the reactor chamber. With those lines severed, the remaining coolant turned to steam and leaked out of the core. The reactor lost all its water, and because of a high positive void coefficient, the heat increased even more. This caused the thermal power to rise rapidly, worsening the situation inside the reactor.
A second, more powerful explosion followed just seconds after the first, packing the force of about 225 tons of TNT. It ended the nuclear chain reaction and sent damaged pieces of the reactor core flying through the air. People outside Unit 4 saw burning chunks and sparks rising from the reactor, some landing on the machine hall roof and starting fires. Around a quarter of the graphite blocks and overheated fuel material were thrown out. The blast damaged the building enough to let air flow through the core, which helped spread fires among the graphite and greatly increased the release of radioactive material into the environment.
After the larger explosion, some employees went outside to get a better look. One survivor, Aleksandr Yuvchenko, described what he saw when he looked up toward the reactor hall. He said he saw a "very beautiful" beam of blue light, like a laser, caused by ionized air. The glow seemed to be "flooding up into infinity."
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There were several ideas about what caused the second, bigger explosion. One idea suggested that hydrogen gas, created either by the reaction of overheated steam with zirconium or by hot graphite reacting with steam, ignited and caused the blast. Another theory came from Konstantin Checherov, who published his thoughts in 1998. He believed the second explosion was a thermal explosion of the reactor, triggered by the uncontrolled release of fast neutrons after all the water disappeared from the core.
In 2009, Sergei A. Pakhomov and Yuri V. Dubasov examined the explosion's power and the release of xenon radioisotopes following the accident. They proposed that the second blast might have resulted from a rapid nuclear event caused by melted core material lacking coolant and moderator. According to their findings, there was no gradual rise in power, but rather an immediate and uncontrollable prompt criticality—akin to what happens when a nuclear weapon fails to fully detonate.
Physicists at the V.G. Khlopin Radium Institute in Cherepovets, a city 1000 km northeast of Chernobyl, detected unusual levels of xenon-135 four days after the disaster. This short-lived isotope suggested that nuclear activity inside the reactor may have sent xenon high into the atmosphere, far more than what the subsequent fire could have done. That theory offered an alternative to the widely accepted idea that a steam explosion caused a positive-feedback power surge, leading to the reactor’s self-destruction. The second explosion, responsible for most of the damage, released around 40 billion joules of energy—about 10 tons of TNT—according to estimates by Pakhomov and Dubasov.
In 2017, Lars-Erik De Geer, Christer Persson, and Henning Rodhe looked into the nuclear fizzle hypothesis as a possible cause of the first explosion. They concluded that such an event was more likely than others to have triggered the initial blast. Both analyses suggest that if a fizzle occurred—whether causing the first or second explosion—it would have involved a rapid, limited chain reaction within only a small part of the reactor core. In fizzle events, self-disassembly happens quickly, which supports this theory.
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The reactor building and turbine hall had been constructed with bitumen, a flammable material, against safety rules. When debris from the explosion set fire to the roof of reactor no. 3, which was still operating, workers needed to act quickly to extinguish the flames and protect its cooling systems. Yuri Bagdasarov, the chief of the night shift, pushed for an immediate shutdown, but chief engineer Nikolai Fomin opposed it. Operators were handed respirators and potassium iodide tablets before being instructed to continue their work. At 5:00, Bagdasarov made the decision to shut down the reactor on his own, a move later validated in writing by Dyatlov and Station Shift Supervisor Rogozhkin.
Shortly after the accident, firefighters arrived at the scene. The first to respond was a brigade from the Chernobyl Power Station, led by Lieutenant Volodymyr Pravyk, who would later die from radiation sickness two weeks after the disaster. These firefighters were not told how dangerous the radioactive smoke and debris were, and may not have understood that what they were dealing with was more than just an ordinary fire. As Grigorii Khmel, the driver of one of the fire engines, recalled: "We didn't know it was the reactor. No one had told us."
We arrived at around two in the morning, just ten or fifteen minutes before. The scene was littered with graphite. Misha asked, “Is that graphite?” I kicked it aside, but one of the firefighters on the other truck picked some up. “It’s hot,” he said. The pieces varied in size—some large, others small enough to handle. No one knew much about radiation back then. Not even the people working there had a clear understanding. Misha filled a cistern and aimed the water at the top. Then Vashchik, Kolya, Volodya Pravik, and others climbed up the ladder. I never saw them again.
Anatoli Zakharov, a fireman who was stationed in Chernobyl, gave a description in 2008 of what happened during the disaster. He remembered joking with his fellow firefighters, saying, "There must be an incredible amount of radiation here. We'll be lucky if we're all still alive in the morning." He also said that everyone knew the risks, and that if they had followed proper regulations, they never would have gone near the reactor. Instead, he explained, there was a sense of moral duty — a feeling that they were acting like kamikaze pilots, knowing the danger but moving forward anyway.
The main goal was to put out the fires on the roof and around the building housing reactor four, to keep the flames from spreading to reactor three. By five o’clock, the fires were under control, though many firefighters were exposed to dangerous levels of radiation. The fire inside reactor four kept burning until May tenth, and it’s believed that more than half of the graphite core may have burned away.
In an effort to control the blaze and prevent radiation from spreading, crews dropped over 5,000 tonnes of materials—including sand, lead, clay, and boron—from helicopters onto the reactor. Nearly none of it reached the core. Around 600 Soviet pilots took part in the dangerous missions, flying thousands of flights while exposed to high levels of radiation.
One firefighter later said the radiation tasted like metal and made his face tingle as if pins and needles were running through it. This description matches what another man experienced—Louis Slotin, a physicist who worked on the Manhattan Project. He died a few days after a fatal radiation exposure during a criticality accident.
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The authorities in Moscow ran the plant, which meant that the government of Ukraine did not get timely updates about what had occurred. For hours, people in Pripyat remained unaware of the danger. Within a few hours, dozens began to fall ill. They suffered from severe headaches, a metallic taste in their mouths, and were overcome by fits of coughing and vomiting. The town's residents were not told anything during the night.
Valentyna Shevchenko, at the time Chairwoman of the Presidium of Verkhovna Rada of the Ukrainian SSR, recalled that Vasyl Durdynets, the acting Minister of Internal Affairs of Ukraine, contacted her during the workday at 09:00. He began the call by discussing routine matters, and only toward its end did he bring up the incident at the Chernobyl nuclear power plant. He said the fire had been put out and everything was under control. When Shevchenko inquired about the people, Durdynets assured her there was no need for worry, saying, "Some are celebrating a wedding, others are gardening, and others are fishing in the Pripyat River."
Afterward, someone made a phone call to Volodymyr Shcherbytsky, who held the position of General Secretary of the Communist Party of Ukraine and effectively served as the region's leader. Shcherbytsky indicated that he expected a group from the state commission to arrive soon. This delegation was to be led by Boris Shcherbina, who was the deputy chairman of the Council of Ministers of the USSR.
A commission was formed later that day to look into what happened, led by Valery Legasov, who was the First Deputy Director of the Kurchatov Institute of Atomic Energy. His team included nuclear expert Evgeny Velikhov, hydro-meteorologist Yuri Izrael, radiologist Leonid Ilyin, and others. They traveled to Boryspil International Airport and reached the power plant on the evening of April 26. By then, two people had already died and 52 were in the hospital. The group found clear proof that the reactor was completely destroyed and that dangerous radiation levels had sickened several people. Early the next morning, on April 27, they ordered the evacuation of Pripyat.
The City Council addressed Pripyat residents on 27 April 1986, informing them that radioactive conditions were worsening due to the Chernobyl Power Station accident. The Communist Party, its officials, and armed forces were taking action to deal with the situation. To ensure people's safety and health—especially children—the city needed to evacuate residents to nearby towns in the Kiev region. Starting at 14:00, buses would be available for each apartment block, supervised by police and city officials. Citizens were advised to bring documents, essential belongings, and some food. Senior leaders of public and industrial facilities had chosen which employees would remain in Pripyat to keep operations running. During the evacuation, all houses would be guarded by police. Those leaving were told to turn off lights, electrical equipment, water, and close windows. The council urged everyone to stay calm and orderly during this short-term relocation.
By 3 p.m., 53,000 people had been evacuated to the Kiev region, told only to bring what was necessary and that they would remain evacuated for about three days. Most personal belongings were left behind, and residents could only recover certain items months later. The next day, discussions began about evacuating people from the 10 kilometer zone. Ten days after the accident, the evacuation area was expanded to 30 kilometers. The Chernobyl exclusion zone has remained ever since, although its shape has changed and its size has expanded.
In the years following 1986, as officials mapped out areas contaminated by radiation beyond the initial exclusion zone, they eventually identified enough hotspots to warrant the evacuation of 135,000 people in total. From 1986 through 2000, nearly three times as many residents were permanently relocated from the most severely affected regions—bringing the overall number of resettled individuals to around 350,000. To replace the city of Pripyat and house workers for the Chernobyl Nuclear Power Plant, a new settlement called Slavutych was constructed on the Dnieper marshes, complete with a direct rail link to the power plant.
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Evacuation started more than a day before the Soviet Union admitted there had been an accident. On 28 April, alarms went off at the Forsmark Nuclear Power Plant in Sweden, over a thousand kilometers from Chernobyl. Workers there told the Swedish Radiation Safety Authority, which traced the radiation back to somewhere else. That day, the Swedish government asked the Soviet Union if there had been a nuclear accident. The Soviets initially said no. It was only after Sweden threatened to inform the International Atomic Energy Agency that the Soviet government admitted an accident had happened at Chernobyl.
At first, officials said only that a small accident had happened, but as they began moving over 100,000 people from the area, it became clear how serious things were. At 21:02 on April 28, a short message was read during the TV news program Vremya. The announcement said, "There has been an accident at the Chernobyl Nuclear Power Plant. One of the nuclear reactors was damaged. The effects of the accident are being remedied. Assistance has been provided for any affected people. An investigative commission has been set up."
The Soviet Union had never before officially acknowledged a nuclear disaster, making this moment significant in itself. The official news agency, TASS, went on to reference the Three Mile Island incident and other U.S. nuclear failures. *The New York Times*' Serge Schmemann described this approach as typical of Soviet whataboutism. At the same time, the decision to form a commission showed how seriously the situation was being taken. Later, radio broadcasts switched to classical music — a familiar signal in the USSR that a major tragedy was about to be announced.
As the world took in the gravity of what had happened, ABC News shared the details of the catastrophe. Not long after, Shevchenko arrived at the scene—she was among the first top Ukrainian officials to reach the site on the morning of April 28. Later that day, near midnight, she made her way back home, stopping briefly at a radiological checkpoint in Vilcha, one of the first such stations established right after the accident occurred.
There was a message from Moscow saying there was no need to delay the May Day festivities in Kiev. On April 30, members of the Communist Party's Political Bureau met to talk about how to celebrate. Scientists told them the radiation levels in the city were normal. Because of that, they decided to cut the usual three and a half to four hours of celebration down to less than two hours.
While most of Pripyat was evacuated, a few key buildings remained open for use by workers continuing their involvement with the plant. Among them were the Jupiter factory and the Azure Swimming Pool. The latter served as a vital spot for recreation, offering relief to Chernobyl liquidators during the demanding clean-up operations.
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Beneath the reactor were two floors of bubbler pools, filled with water and designed to support emergency cooling pumps. These pools also acted as a pressure suppression system, ready to condense steam if a small pipe broke. Above them, on the third floor, was a steam tunnel. If steam escaped from a broken pipe, it was meant to flow into this tunnel and then bubble through the water in the pools below.
The burning materials inside the reactor—steel, fuel, serpentinite and more—reached temperatures over 1,200 degrees Celsius, melting into a mixture like corium, a radioactive semi-liquid similar to lava. This molten mass pooled on the floor of the sub-reactor area. There was concern that if it flowed into the water-filled bubbler pools, the reaction would create dangerous steam and possibly trigger another explosion, so engineers considered draining the water. Those fears didn’t come true, though, because the corium instead dripped harmlessly into the flooded pools. When the molten fuel hit the water, it cooled into a light-brown ceramic pumice that floated due to its low density.
The government commission, unaware of the danger, ordered that the bubbler pools be drained by opening their sluice gates. The valves were in a flooded corridor deep underground, in a subterranean annex next to the reactor building. Engineer Oleksiy Ananenko was chosen for the task because he had worked in those corridors before. He reported to his superior, Boris Baranov, who decided to go with him. They also selected Valeri Bespalov to join them, carrying a dosimeter. Dressed in waterproof gear and armed only with a flashlight and dosimeters, they made their way through the dark, highly radioactive tunnels. There, they opened all the valves for the sluice gates. Many reports later claimed all three men died within days. In fact, they survived and were each awarded the Order for Courage in May 2018.
Once the gates of the bubbler pool were opened, the fire brigade began using pumps to drain the basement. The effort continued until 8 May, when the operation finally wrapped up. Over the course of the work, around 20,000 tonnes of water were removed from the area.
At Shcherbina's direction, helicopter crews from the Kiev Military District's Air Forces were sent to cover the damaged reactor number four with sand. General-leutenant N.P. Kryukov and General-mayor Nikolay Antoshkin arrived at the plant early on April 27. Kryukov was the Commander of the Air Forces, while Antoshkin served as Chief of Staff. They were part of the military response to contain the disaster.
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The government commission worried that the molten core might burn down into the ground and taint the groundwater. So they decided to freeze the soil right beneath the reactor to stop that from happening and to help hold up the foundations. They started pumping liquid nitrogen into the earth using oil-well drilling tools on May 4. They figured they'd need about twenty-five tonnes of liquid nitrogen every day to keep the ground frozen at minus one hundred degrees Celsius. But the plan was dropped quickly.
To prevent the reactor from overheating, workers dug a tunnel beneath it to house a cooling system. This system used coiled pipes filled with water, topped by a thin layer of graphite that could handle high temperatures. The graphite was meant to stop the concrete above from melting. It was then sealed between two concrete layers, each meter thick, to keep everything stable. This layered design, a graphite-concrete sandwich, was similar to what later became part of many nuclear reactor safety systems.
As air temperatures fell and reports indicated the fuel melt had ended, plans for the graphite cooling plate and earlier nitrogen-injection ideas were dropped. Later analysis showed the fuel had moved down three floors, with several cubic meters settling at ground level. The underground channel built for precaution, which included active cooling, was no longer seen as needed. To reinforce the structure beneath the reactor, the area was filled with concrete instead.
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With the fire put out from the open-air reactor, the next task was to stop contamination from spreading through wind or birds. There was also the risk that rainwater could wash radioactive material into the underground water supply, letting it spread beyond the site. The falling rain could speed up rusting of the steel inside the damaged reactor structure. Another serious issue was the high levels of gamma radiation being released, which posed a real danger to the workers near reactor no. 3.
The plan was to build a massive steel and concrete structure to cover the damaged reactor, and it came to be called the Sarcophagus. Work began on May 20, 1986, just 24 days after the accident, and lasted until late November. The team had to move fast, all while dealing with intense radiation levels.
The workers building the sarcophagus had to be protected from radiation, so crane operators worked from lead-lined cabins. They erected walls around the reactor, cleared and concrete-surfaced the ground to reduce radiation and let big machines move in, and built a thick shielding wall to protect those working on reactor no. 3. A high-rise buttress was added to support parts of the old structure, and they constructed a full roof over the site. They also installed a ventilation-extraction system to catch airborne contamination inside the shelter.
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After the disaster at Chernobyl, a group of scientists and engineers came together under the name Field Integrated Scientific and Technical Brigade No. 9, or PKNTB-9, to assess the damage. This team was made up of workers from NIKIET and the V. G. Khlopin Radium Institute. They were led by G. S. Sinitsyna and S. S. Kovalenko. Their task was to carry out dosimetric surveys across several key areas of the plant, including Unit 3, Unit 4, the Vent Block, and the Deaerator block. The team also worked to evaluate the extent of the damage and locate any remaining nuclear fuel.
They didn’t get very far at all. The team could only measure radiation levels down to the floor on the +12.5 level. Beyond that, the radiation was too intense to go further. It was later discovered that the extreme levels came from corium on the +9.0 level, part of a horizontal flow. Their work was also slowed by an electrical fire on May 23.
Following models created by Oleksandr Borovoi and Valery Legasov, they figured that 90% of the nuclear fuel stayed inside the building. To find exactly where, members of the Kurchatov Institute went to Chernobyl on June 6. They were led by Konstantin Checherov, who began trying to enter the high-radiation zones. As earlier estimated, they couldn’t reach those lower levels from +12.5 because of the intense radiation coming from staircase 257. Then, on June 10, Checherov measured a level of 11,400 roentgens per hour inside an open hatch in the Southern Main Circulation Pump Hall. That radiation was later determined to come from corium.
Around June 15, Mikhail Kostyakov and Vladimir Kabanov tried to enter the lower levels of the reactor from the south. They started in corridor 017/2 on the +0.0 level and headed up a staircase toward corridor 217/2 on the +6.0 level. But during the climb, their dosimeter broke, so they turned back. They never realized they had just come across the Elephant’s Foot.
After the construction of the Sarcophagus began, all exploration work had to pause because concrete was being poured into the buildings. The project moved forward steadily, with the structure finally completed in November. Only after that milestone could the investigations resume. The timing was critical, as the Sarcophagus needed to be in place before any further access to the reactor area was possible. Once the concrete had set and the enclosure was secure, the teams returned to assess the site. That pause in activity was necessary to ensure safety and structural integrity. The work resumed once the Sarcophagus was finished, marking a turning point in the response efforts. The November completion allowed for renewed access to the reactor area.
In the aftermath, Konstantin Checherov joined with Oleksandr Borovoy and others to form the "Complex Expedition." Their task was to examine the damaged building and locate the reactor fuel. They needed to determine if the fuel could go critical again. A key worry for the team was the molten fuel's composition. If it contained enough fissile material and a moderator, it could theoretically cause further damage or even another explosion.
The work resumed in late November 1986 when the Complex Expedition started assessing the damage at the site. At the same time, teams from the Khoplin Radium Institute began searching for fuel rods, collecting any they could find and transporting them back to their laboratory for further analysis.
In December 1986, Vasya Koryagin stumbled upon something unexpected during his work at the reactor site—a massive, irregular object that had once been molten lead and now sat in the core. People gave it the name “The Elephant’s Foot” because of its wrinkled look. Later analysis revealed it was made from melted sand, concrete, and a large quantity of nuclear material that had leaked from the reactor. The floor beneath the reactor was still scorching hot, cracked open by what had once been lava, and filled with strange crystal formations called chernobylite. Experts determined there was no longer any danger of another explosion.
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The concrete sarcophagus built after the accident had a planned lifespan of just thirty years. On February 12, 2013, a section of the turbine-building’s roof measuring six hundred square metres collapsed near the structure. At first, it was thought the collapse was caused by snow weight, but the amount wasn’t unusual. A Ukrainian fact-finding panel later concluded that poor repair work and structural aging were to blame. The sarcophagus itself was already showing signs of serious deterioration, with experts warning it might soon collapse entirely.
In 1997, the international Chernobyl Shelter Fund was formed to build a more stable covering for the dangerous sarcophagus around Reactor No. 4. That year, it received €864 million from donors across the world. The project was overseen by the European Bank for Reconstruction and Development. Construction of the new structure began in 2010. It was called the New Safe Confinement, a metal arch standing 105 meters high and stretching 257 meters wide. Built on rails beside the reactor building, it was designed to slide into place over the old sarcophagus. The shelter was finished in 2016 and placed over the reactor on November 29 of that year.
In February 2025, a Russian drone hit the shelter over Reactor No. 4, starting a fire that damaged the outer and inner protective layers of the NSC and the insulating materials inside. By December 2025, the IAEA said this strike meant the structure could no longer do its main job of keeping radiation contained. IAEA Director General Rafael Grossi said an inspection in late November confirmed the protective structure had lost its primary safety functions, including its confinement ability. Still, the mission found no permanent damage to the load-bearing parts or monitoring systems of the building.
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The used fuel from reactors one through three was initially kept in the cooling ponds at those sites, as well as in a second interim storage pond called ISF-1. That facility now holds most of the spent fuel from units one to three, which allowed the reactors to be decommissioned under less strict conditions. About fifty of the fuel assemblies from units one and two were damaged and needed special care during handling. Moving the fuel to ISF-1 happened in three steps: first, fuel from unit three was transferred; then all the undamaged fuel from units one and two; and finally, the damaged fuel from those same units. The entire process was completed by June 2016.
A need arose for a larger, long-term solution for radioactive waste at the site, leading to the design of a new facility called ISF-2. It was meant to store used fuel assemblies from reactors one through three, along with other operational wastes and materials from decommissioning those units. In 1999, a contract was signed with Areva NP, formerly known as Framatome, to build it. By 2003, when much of the structure was already under construction, technical problems in the original design became clear. Areva withdrew from the project in 2007, and Holtec International was then brought in to create a new plan. That updated design was approved in 2010, work began in 2011, and the facility was finished in August 2017.
ISF-2 stands as the world’s largest nuclear fuel storage facility, designed to hold over 21,000 fuel assemblies for at least a century. The site features a processing plant capable of cutting down the fuel assemblies and placing them into canisters. These canisters are then filled with inert gas and sealed shut. Afterward, they’re moved to dry storage vaults, where the containers will be safely enclosed for up to 100 years. The facility is built to process 2,500 fuel assemblies each year.
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In the basement of the reactor building, three types of lava can be found: black, brown, and a porous ceramic form. These materials are silicate glasses containing inclusions of other substances. The porous lava is brown lava that fell into water and cooled quickly. It's unknown how long the ceramic form will slow the release of radioactivity. Between 1997 and 2002, several studies suggested that the self-irradiation of the lava would turn all 1,200 tonnes into a submicrometre, mobile powder within weeks.
According to a published paper, the degradation of the lava inside the reactor is likely slow and gradual. That same paper says the amount of uranium leaking from the wrecked reactor is only ten kilograms per year, which shows the lava is holding up against its environment. It also notes that when the shelter is improved, the rate of leaching will go down. As of 2021, some fuel had already broken down significantly. The 'elephant’s foot', which at first was so solid it took an armor-piercing AK-47 round to chip off a piece, had softened into something like sand.
Before the New Safe Confinement building was finished, rainwater falling on the remaining fuel acted as a neutron moderator, which caused more fission and raised the risk of a critical reaction. To stop this, scientists used a solution of gadolinium nitrate to absorb neutrons and slow the process down. Even after the building was completed, fission may have continued to rise. From 2017 through late 2020, neutron density doubled in the sub-reactor area, then leveled off early in 2021. This unexpected increase happened as water levels dropped, which is the opposite of what was expected and different from other fuel areas. Such changes raised fears that a self-sustaining reaction might occur, spreading more radioactive material inside the confinement. In early 2021, a ChNPP press release said the rise in neutron levels had stopped.
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In 1991, the United Nations created the Chernobyl Trust Fund to assist people impacted by the nuclear accident. The fund is overseen by the United Nations Office for the Coordination of Humanitarian Affairs. That office is responsible for planning strategy, raising resources, and promoting the needs of those affected. Beginning in 2002, the United Nations Development Programme took over management of the fund’s activities. This change marked a shift from providing emergency help to supporting long-term recovery and development for the victims.
In 1997, at the G8 summit held in Denver, the Chernobyl Shelter Fund was created to support the Shelter Implementation Plan, or SIP. The plan aimed to make the site environmentally safe by stabilizing the damaged sarcophagus and building a new structure called the New Safe Confinement. When the SIP was first outlined, it was estimated to cost $768 million. By 2006, that figure had risen to $1.2 billion.
In 2003, the United Nations Development Programme began the Chernobyl Recovery and Development Programme, or CRDP, to help Ukraine deal with the lasting effects of the disaster. The program started in February 2002, based on a report about the human impact of the accident. It focused on four Ukrainian regions most affected: Kyivska, Zhytomyrska, Chernihivska, and Rivnenska. The goal was to support the government in handling the long-term social, economic, and environmental damage caused by Chernobyl.
Since 1990, over eighteen thousand children from Ukraine who were impacted by the disaster have received medical care in Tarará, a resort town located in Cuba.
Following the disaster, a program was established to study health impacts caused by radiation exposure, particularly from iodine-131. It was funded with twenty million US dollars, mainly provided by Japan. The financial support was distributed among Ukraine, Belarus, and Russia to investigate how the accident affected people’s well-being in those countries.
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The explosion sent radioactive materials flying across the landscape, carried by wind and spreading contamination far beyond Ukraine. The area affected included Belarus, Russia, and much of Europe. Chernobyl released an estimated 400 times more radiation than the bombs dropped on Hiroshima and Nagasaki. Roughly 100,000 square kilometres of land became contaminated, mostly in Belarus, Ukraine, and Russia. A forest directly downwind from the plant, later called the Red Forest, was killed instantly by the radiation.
The only deaths definitively linked to the disaster occurred among plant workers and emergency responders: two engineers killed instantly by the explosions, and twenty-eight others who died from acute radiation syndrome within three months. Effects on the broader population are still disputed. The most commonly cited projection comes from the Chernobyl Forum's 2005 report, which estimates as many as four thousand eventual deaths among around six hundred thousand people who received the highest doses. That same report also documented about six thousand cases of thyroid cancer, roughly fifteen of which were confirmed fatal by 2011, mostly in individuals who were children or adolescents at the time. Other analyses, using different approaches and covering larger groups, have produced estimates ranging from a few thousand to tens of thousands of future cancer-related deaths. A small number of studies have even suggested figures reaching into the hundreds of thousands, though those claims have been widely contested due to methodological concerns.
The accident's true cost is put at between US$235 billion and US$700 billion, making it one of history’s most expensive disasters. That massive financial toll added to the economic pressures already weakening the Soviet Union, helping push it toward collapse. Around the world, nuclear energy policies shifted dramatically in response. New reactor projects slowed, international safety agreements were signed like the Convention on Early Notification of a Nuclear Accident, and anti-nuclear activism grew stronger across many European nations.
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The lack of trust in the Soviet government, which had tried to hide the truth, led to widespread discussion in the First World during the early hours of the disaster. Reporters questioned experts and shared their doubts with the public, who in turn began to lose faith in those they once relied on. This growing skepticism shaped how people understood what had happened.
The accident brought existing worries about nuclear reactors to a head across the world. Even though most attention focused on the unique design of the Chernobyl reactor, hundreds of other reactor projects—including ones still under construction at Chernobyl, reactors numbers 5 and 6—were eventually called off. Rising costs from new safety requirements, along with growing public anxiety and legal challenges, led to a sharp decline in new reactor builds after 1986.
The accident brought widespread concern about how little regard the Soviet nuclear industry had for safety, which slowed the growth of that sector and pushed the government to open up more about its operations. The cover-up of the Chernobyl disaster helped spark glasnost, a policy that "paved the way for reforms leading to the Soviet collapse." It turned out that issues with the reactor’s design and construction had been known since at least 1973. The KGB was aware of these problems and passed them along to the Central Committee, but no action was taken, and the information was kept secret.
In the wake of the Chernobyl disaster, Italy held a nuclear power referendum in 1987, which led to the country beginning to phase out its nuclear plants in 1988. That decision was later reversed in 2008, but a new referendum in 2011 brought back the public's opposition to nuclear energy, effectively nullifying the government’s earlier move.
In Germany, the Chernobyl accident brought about the creation of a federal environment ministry, and the German environmental minister was given authority over reactor safety, a responsibility the minister still holds today. The disaster also strengthened the anti-nuclear movement in Germany, which led to the decision made by the Schröder government between 1998 and 2005 to end the use of nuclear power. That policy was temporarily reversed, but it ended again after the Fukushima nuclear disaster.
In the wake of the Chernobyl disaster, the International Atomic Energy Agency called a conference in 1986 to address nuclear safety. That effort led to the creation of a treaty known as the Convention on Early Notification of a Nuclear Accident. The agreement requires member countries to inform others about any nuclear or radiation accidents that might impact neighboring states. It also established the Convention on Assistance in the Case of a Nuclear Accident or Radiological Emergency, which outlines how countries should help each other during such events.
Chernobyl has been studied in research looking into why such disasters happen, with sleep deprivation and mismanagement among the factors examined.
The State Archives of Ukraine contain about a thousand documents that were once classified, covering the building of the power station, the accident, and what came after, with records going as far as the early 2000s. In 2017, UNESCO recognized this collection by adding it to its Memory of the World International Register, acknowledging its value as global documentary heritage.
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