Chernobyl - A Timeline of The Worst Nuclear Accident in History

Over 30 years ago, a series of missteps caused the worst nuclear accident in history, and its effects are still being felt to this day.
Marcia Wendorf

Located 65 miles north of Kiev, Ukraine, the V.I. Lenin Nuclear Power Station at Chernobyl was a model of Soviet engineering. Its four RBMK nuclear reactors produced enough electricity for 30 million homes and businesses.

The RBMK reactor is a class of graphite-moderated nuclear power reactors that were designed and built by the Soviet Union. Certain aspects of the design contributed to the Chernobyl disaster, and there were calls for the reactors to be decommissioned. However, the reactors were redesigned, and as of 2019, ten are still in operation.

1,600 Radioactive U-235 Fuel Rods

In 1986, Chernobyl had four working reactors, with two new ones under construction. The newest of the four, Reactor No. 4, contained 1,600 radioactive uranium-235 fuel rods. Because U-235 is unstable, its atoms spontaneously release neutrons, which hit other U-235 nuclei, causing them to release neutrons. This is what is called a chain reaction.

The byproduct of a chain reaction is the release of enormous quantities of heat and energy, and this heat is what's used to turn water into steam, which drives a turbine, that generates electricity.

Chernobyl - A Timeline of The Worst Nuclear Accident in History

To keep a chain reaction from running away with itself and becoming a nuclear bomb, control rods containing a neutron-absorbing substance are inserted between the fuel rods. Reactor No. 4 had 211 control rods made of the element boron. If you raise the control rods, the chain reaction accelerates, if you lower the control rods, the chain reaction slows.

Friday, April 26, 1986, 11:45 p.m.

176 workers were coming in for their shifts from the neighboring town of Pripyat. Built in 1970 as a company town for the power station, Pripyat had a population of 50,000 and enjoyed many of the luxuries denied to other Soviet citizens, such as well-stocked supermarkets, good schools, and plentiful sports facilities.

This night involved the continuation of a test that was begun twelve hours earlier. It was a test of the plant's ability to keep Reactor No. 4 cool in the event of a power failure. Whether the plant’s still-spinning turbines could produce enough electricity to keep the coolant pumps running during the brief gap before the emergency generators would kick in. To perform the test, they would have to shut the reactor down, and by the time the night shift arrived, the reactor was operating at 50% power.

"The odds of a meltdown are one in 10,000 years." -- Vitali Sklyarov, Minister of Power and Electrification of Ukraine

Saturday, April 26, 1986, 00:28 a.m.

The night shift foreman, Alexander Akimov, began arguing with Chernobyl’s Deputy Chief Engineer, Anatoly Dyatlov, over how low the amount of electrical power the reactor was generating should be taken. Akimov cited a manual that stated that it should not be less than 700 megawatts, while Dyatlov insisted that 200 megawatts were safe. Since Dyatlov outranked him, Akimov had to agree.

1:19 a.m.

The Reactor Control Engineer, Leonid Toptunov, blocked the automatic shutdown of the reactor due to a low water level and raised the reactor's power up to 7 percent by removing all but six of the control rods. The reactor was now growing unstable.

Chernobyl - A Timeline of The Worst Nuclear Accident in History
Reactor No. 4 schematic Source:Alex.Bikfalvi/Wikimedia Commons

1:23:40 a.m.

Readings showed the reactor's temperature had climbed to 4,650 C, almost as hot as the surface of the sun. An engineer who had been on a catwalk above the reactor ran into the control room, shouting that the fuel rod caps were jumping in and out of their sockets. These caps weigh 350kg (772 lb) each.

Alarmed, Akimov pressed a button to reinsert the control rods, but instead of dropping their full seven meters, they stopped at between two and 2.5 meters.

1:23:45 a.m.

The reactor reached 120 times its full power, and its radioactive fuel disintegrated. There was a long, low, almost-human sounding moan, then an explosion lifted the 1,000-ton concrete shield that was above the reactor and wedged it at an angle.

This allowed air to reach the reactor, and the oxygen in the air started a fire in the reactor's graphite. The air also caused the metal in the fuel tubes to react with the water in the reactor to produce hydrogen gas. Hydrogen gas is highly flammable, and it exploded, blasting debris into the air and onto the roof of the neighboring Reactor No. 3.

In a study commissioned by the U.S. government on the Chernobyl disaster, Richard Wilson of Harvard University described this second explosion as a small nuclear explosion.

01:26:03 a.m.

"Call everybody, everybody" -- Chernobyl Dispatch

The first fire alarm came into Paramilitary Fire Station Number Two, based on the grounds of the power plant. The firemen scrambled to Reactor No. 4 and climbed up onto its ruined roof. Afraid to use water because of the exposed electrical cables, the firemen threw sand and used their canvas hoses to beat out the flames.

Chernobyl - A Timeline of The Worst Nuclear Accident in History
Reactor No. 4 burning core Source: Igor Kostin/Wikimedia Commons 

In Reactor No. 4's control room, two trainees, Aleksandr Kudryavtsev and Viktor Proskuryakov, were sent to assess the damage. They made it to the reactor hall where they observed the 1,000-ton Upper Biological Shield jammed at an angle in the reactor shaft, and blue and red flames raging in the reactor itself. Both Kudryavtsev's and Proskuryakov's bodies immediately darkened with what is known as "nuclear tan" as they received a fatal dose of radiation.

2:50 a.m.

Workers inside the plant had accounted for all the staff with the exception of Valery Khodemchuk. At the time of the accident, Khodemchuk had been in the main circulating pump room, which was close to the explosion. Unbeknownst to his coworkers, he had been vaporized by the explosion.

Chernobyl's doctor, Valentyn Belokon, who had been treating injured workers realized that they were suffering from radiation poisoning. He called the hospital in Pripyat and requested potassium iodide tablets. Potassium iodide blocks radioactive iodine from being absorbed by the thyroid gland.

7:30 a.m.

Akimov and Toptunov entered Reactor No. 4 in an attempt to bring water into the ruined reactor. This would cost them their lives. Akimov died on May 11th, having said that his conscience hurt more than his injuries, and  Toptunov died three days later.

8:00 p.m.

Inhabitants of Pripyat gathered on a railway bridge that had a view of the nuclear power plant to view the beautiful flames of all colors caused by the burning graphite. A breeze from the power plant swept over them, carrying a radiation dose of 500 Roentgens. No one standing on the bridge survived, and it is called the "bridge of death".

Sunday, April 27, 1986, 10:00 a.m.

The first of what will become almost 1,800 helicopter flights above the reactor began. The heliocopters dropped sand, lead, clay, and neutron-absorbing boron onto the burning reactor, but practically none of the neutron-absorbing materials reached the core.

Chernobyl - A Timeline of The Worst Nuclear Accident in History
Reactor No. 4 roof collapsed Source: Wikimedia Commons

Due to the radiation, the helicopter crews were aware that theirs was most likely a suicide mission, but they went anyway.

2:00 p.m.

The authorities began an evacuation of the city of Pripyat. Via loudspeaker, they told people to take enough food and clothing for three days and to leave their pets behind. In total, almost 350,000 people were evacuated, and they would never return. The pets left behind were shot.

Chernobyl - A Timeline of The Worst Nuclear Accident in History
Pripyat Source: Jason Minshull/Wikimedia Commons

Monday, April 28, 1986

On his way out of the plant after a shift, a worker at the Forsmark Nuclear Power Plant in Sweden went through a radiation detector, and the detector went off. It was quickly determined that a cloud of radioactive gas had drifted across all of Scandinavia, Germany, and Czechoslovakia. Pharmacies in Denmark quickly sold out of potassium iodide tablets.

Tuesday, April 29, 1986

A U.S. reconnaissance satellite showed the roof of Reactor No. 4 blown off, and the glowing mass inside still smoking.

In an attempt to protect them from thyroid cancer, Polish authorities began distributing potassium iodine tablets to children living in the northeastern part of that country.

Friday, May 2, 1986

There were two floors of pools containing water directly beneath Reactor No. 4, plus the basement was flooded with water from ruptured pipes that were used by the firefighters. The smoldering graphite, nuclear fuel, and other materials had formed a mass called corium, which is a radioactive version of lava.

Chernobyl - A Timeline of The Worst Nuclear Accident in History
Source: Wikipedia

The mass was burning at a temperature of more than 1,200 degrees C., and if it melted through the reactor hall floor and into the pools of water, a steam explosion would hurl the mass into the air and would eject even more radioactive material.

Three engineers volunteered to drain the water from the pools - Alexei Ananenko, Valeri Bezpalov, and, Boris Baranov. Their mission was a success, but all three died from radiation sickness.

The China Syndrome

The molten mass still posed a threat if it melted down to groundwater below the reactor building. This is the so-called "China Syndrome". At first, workers tried freezing the ground beneath the reactor by injecting it with liquid nitrogen. Then, they filled the reactor room with concrete.

In the "China Syndrome", the core components of a nuclear reactor meltdown, burning through the containment vessel and the building housing the reactor, then they burn through the Earth's crust and body until reaching the opposite side of the planet, which in the U.S. is colloquially referred to as China.

In reality, a core couldn't penetrate the several-kilometer thickness of the Earth's crust, and it certainly couldn't travel back upwards against the pull of gravity. Also, China is not the antipode of any landmass in North America.

May 6, 1986

Authorities closed the schools in the cities of Gomel and Kiev, and they began relocating children. Kiev radio warned people not to eat leafy vegetables, and to stay indoors as much as possible.

December 14, 1986

Work started on a concrete "sarcophagus" that would completely encase Reactor No. 4, and protect the environment from radiation for what was hoped to be at least 30 years. 300,000 tons of concrete and 6,000 tons of metal were used to build the sarcophagus.

September 17, 2007

After realizing that the sarcophagus might not be sufficient, work began on a new confinement structure designed by a French consortium called Novark. That structure was comprised of a 150 by a 257-meter arch that would be slid into place. Construction costs were estimated at €432 million euros, with a project time of five years.

Chernobyl - A Timeline of The Worst Nuclear Accident in History
Containment structure Source:Tim Porter/Wikimedia Commons

The Aftermath

In the immediate aftermath of Chernobyl, a total of 31 firemen and plant workers died. Some of their bodies were so radioactive, they had to be buried in lead coffins. A report by the World Health Organization estimated that 600,000 people within the Soviet Union were exposed to high levels of radiation, and of those, 4,000 would die. Those who lived near the Chernobyl site have reported increased instances of thyroid cancer, and they have an increased risk of developing leukemia.

Chernobyl - A Timeline of The Worst Nuclear Accident in History
Exclusion zone Source:CIA Factbook/Wikimedia Commons

Anatoly Dyatlov and the director of the Chernobyl plant, Viktor Bryukhanov, were sentenced to ten years each in prison for their roles in the disaster.

The "Liquidators"

Scores of people stepped up to contain the disaster, and they came to be called the "Liquidators". They include: * Yuri Korneev, Boris Stolyarchuk, and Alexander Yuvchenko who are the last surviving members of the Reactor No. 4 night shift * The firefighters who immediately responded to the reactor accident * The Civil Defense Troops of the Soviet Armed Forces who removed contaminated materials and deactivated the reactor itself * Internal Troops and the police who provided security, access control, and population evacuation * Military and civil medical and sanitation personnel * The Soviet Air Force and civil aviation units who performed critical helicopter-assisted operations on the reactor building, air transportation, and aerial radiation monitoring * A team of coal miners who built a protective foundation to prevent radioactivity from entering the aquifer below the reactor * Construction professionals * Media and performing artists who risked their lives to document the disaster and to provide on-site entertainment for the liquidators * The photographers Igor Kostin and Volodymyr Shevchenko took photos in the immediate aftermath, including photos of Liquidators conducting highly-hazardous manual tasks.

Chernobyl - A Timeline of The Worst Nuclear Accident in History
Liquidator medals Source: Wikimedia Commons

700 Million Years

The Chernobyl accident is one of only two nuclear energy accidents that is classified as a "Level 7 Event," the highest classification. The other is the 2011's Fukushima disaster in Japan. At the lowest level of Reactor 4 lies the famous "elephant's foot", a several-meter-wide mass of corium that is still giving off lethal amounts of radiation. The half-life of radioactive elements is defined as the amount of time it takes for the radioactivity to fall to half its original value. The half-life of U-235 is 700 million years.

Chernobyl - A Timeline of The Worst Nuclear Accident in History
The elephant's foot Source: Unexplained Mysteries/Youtube


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