Leonid Toptunov and the Chernobyl Button Press

At 1:23:40 in the morning on Saturday, April 26th, 1986, a 25-year-old senior reactor control engineer named Leonid Toptunov pressed the AZ-5 emergency shutdown button on the control panel of Reactor 4 at the Chernobyl Nuclear Power Plant in northern Ukrainian SSR. The button was meant to drop all 211 control rods into the core, halting the chain reaction. Instead, the reactor power output, which had been hovering near 200 megawatts, climbed to roughly 30,000 megawatts — about ten times its design rating — in approximately four seconds. The first explosion, a steam blast, lifted the 1,000-tonne upper biological shield off the top of the reactor and rotated it ninety degrees. Two to three seconds later, a second explosion — best estimates around 225 tons of TNT equivalent, possibly a small nuclear excursion — blew the roof off the reactor building and sprayed graphite, fuel, and burning radioactive debris across the surrounding kilometre. Toptunov was alive when the explosions hit; he would die of acute radiation syndrome on May 14th, 1986, eighteen days later, age 25. Two plant workers died on the night itself: Valery Khodemchuk, a circulation pump operator, was crushed in the rubble and never recovered, and Vladimir Shashenok, an automatic-systems adjuster, was pulled out of the debris alive but with a broken spine and burns and died at the on-site medical unit before sunrise.

The cause was a safety test gone wrong. The RBMK-1000 reactor at Chernobyl, designed in the early 1970s, had a known safety concern: in the event of an external power failure, the cooling pumps would stop, and there would be a gap of about 70 seconds before the diesel backup generators could spin up to full output. Soviet engineers had calculated that the residual rotational energy in the spinning steam turbine, after steam was cut off, might be sufficient to power the coolant pumps through that 70-second gap. They wanted to verify it. Reactor 4 had been scheduled to run the turbine-coastdown test during a maintenance shutdown originally planned for April 25th, 1986. The test had been postponed once before in 1982 and once again in 1985 without success. The shift on April 25th began the test correctly. Then a regional power dispatcher delayed the shutdown by ten hours. The test was therefore handed over from the trained day shift to the less-prepared night shift, with the deputy chief engineer Anatoly Dyatlov in nominal command and the unit shift supervisor Aleksandr Akimov, age 33, executing the test.

The reactor refused to cooperate. While the night shift was lowering reactor power for the test, around 12:30 a.m. on April 26th, the control system overshot. Power dropped to approximately 30 megawatts — far below the test's required 700 megawatts and into a regime where xenon-135, a fission byproduct, was poisoning the reactor. To recover power, Akimov and Toptunov withdrew almost all of the manual control rods from the core, far below the operational safety minimum. By 1:23 a.m. they had stabilised at around 200 megawatts and decided to proceed with the test. They closed the steam isolation valves to begin turbine coast-down. Coolant flow dropped, steam voids in the core grew, the positive void coefficient inherent to the RBMK design caused power to begin climbing, and Toptunov pressed AZ-5. The graphite tips at the bottom of the control rods, a design flaw the Soviet nuclear establishment had known about since 1983 and had not corrected, displaced cooling water at the bottom of the core as the rods inserted, briefly increasing reactivity rather than decreasing it. Power surged. The fuel channels deformed and jammed the rods partway in. The remaining centre of the core ran away.

The first responders were the plant's own fire brigade, led by 23-year-old Lieutenant Volodymyr Pravyk. They arrived at the reactor 4 building within two minutes of the second explosion, climbing onto the roof of the adjacent reactor 3 building to fight fires that threatened to spread laterally. The roof was strewn with chunks of graphite from the destroyed core, dose rates approaching tens of roentgens per second. None of the men were equipped with radiation dosimeters that could read above 4 roentgens per hour. Most thought they were fighting a conventional industrial fire. Pravyk led his team across the burning roof for approximately three hours. He was hospitalised that morning at Pripyat Medical Unit Number 126, transferred to Hospital Number 6 in Moscow, and died of acute radiation syndrome on May 11th, 1986. Of the fire crews that responded that night, 28 men died of ARS within three months, including most of Pravyk's first crew.

The town of Pripyat, three kilometres from the plant, had a population of 49,360, mostly families of Chernobyl plant workers and their children. The Soviet authorities did not begin evacuation until 36 hours after the explosion. On the afternoon of Sunday, April 27th, 1986, 1,200 buses arrived from Kiev and elsewhere, and Pripyat residents were told to bring documents and necessities for a three-day absence. They never returned. Some children playing outside on the morning of April 26th had received radiation doses high enough that their parents would later seek lifelong medical follow-up; thyroid cancer rates among children of the affected region rose roughly tenfold over the following decade due to iodine-131 in milk and air. The 30-kilometre Exclusion Zone, formalised over the following weeks, eventually displaced over 350,000 people from northern Ukraine and southern Belarus.

The cleanup was conducted by people who, in many cases, did not survive it. Valery Legasov, the deputy director of the Kurchatov Institute and the senior scientist on the government commission, identified within hours that the burning graphite core was injecting cesium-137, iodine-131, and strontium-90 directly into the atmosphere; the Forsmark nuclear plant in Sweden detected radiation on workers' shoes on April 28th, which is how the Soviet government's denial collapsed and the world learned that Chernobyl had happened. The fire in the open core had to be smothered before it could be sealed. From April 27th to May 10th, approximately 600 Soviet military helicopter pilots flew sorties over the open reactor crater, dropping 5,000 tonnes of sand, lead pellets, clay, and boron carbide directly into it from altitudes around 200 metres. The dose rate over the crater was high enough that pilots flew with lead plates strapped under their seats and rotated through ten or fifteen sorties before being permanently grounded. Several developed leukemia in the years after.

Underneath the reactor, a different problem emerged. The molten core — uranium fuel mixed with the graphite moderator, melted concrete, sand, and steel — flowed downward through the destroyed reactor structure into the basement spaces. By early December 1986, plant workers exploring the lower levels with remote cameras and shielded rovers found, in steam distribution corridor 217/2, a black, glass-like mass shaped like a slumped foot at the base of a wall. They named it the Elephant's Foot. The corium pillar weighed several tonnes and emitted about 8,000 roentgens per hour, lethal in two minutes. Photographers who took the now-iconic image had to use a mirror around a corner because direct exposure was unsurvivable. The Elephant's Foot is still in the basement; it has cooled, and the dose rate has dropped by orders of magnitude, but its existence is one of the reasons the original sarcophagus, which was hastily constructed of steel and concrete poured by remote-controlled equipment and frequently by hand, was always considered a temporary structure.

That sarcophagus — formally Object Shelter — was completed by November 30th, 1986, just over seven months after the accident. Approximately 600,000 cleanup workers, called liquidators, rotated through the site over the following years. Soviet records eventually attributed dozens to hundreds of immediate deaths to acute radiation syndrome and direct accident response; longer-term mortality estimates remain contested. The 2005 Chernobyl Forum, a joint study by the IAEA, WHO, and several UN agencies, projected approximately 4,000 eventual deaths attributable to radiation among the most exposed populations. The Union of Concerned Scientists later estimated up to 27,000 worldwide. By contrast, the Soviet government's initial position in 1986 was 31 dead.

The replacement structure, the New Safe Confinement, was a 36,000-tonne steel arch 108 metres tall and 162 metres long, assembled in halves on a clear field beside the reactor and slid into place over the original sarcophagus on November 29th, 2016. It was designed to last 100 years, contain the reactor while it was dismantled internally, and prevent any further atmospheric release. On February 14th, 2025, during the war in Ukraine, a Russian Shahed-type drone struck the upper section of the New Safe Confinement, puncturing the outer cladding and starting a small contained fire that emergency crews extinguished without measurable atmospheric release. As of the most recent reporting, the structure remains operational and the work of dismantling reactor 4 from inside it continues, almost forty years after Leonid Toptunov pressed AZ-5.

The other three reactors at the Chernobyl plant continued generating electricity for the Soviet and then Ukrainian power grid for years after the disaster. Reactor 2 was shut down after a turbine hall fire in October 1991. Reactor 1 was decommissioned in November 1996 as a condition of international financing for the New Safe Confinement. Reactor 3, the last operating unit, generated commercial power until December 15th, 2000, when Ukrainian President Leonid Kuchma personally pressed the shutdown button on a televised ceremony, fourteen years and seven months after the explosion in the unit next door. Roughly 3,000 workers continued daily operations on the site through that period, riding buses each morning from the purpose-built town of Slavutych, which had been constructed in 1986–1988 as a replacement for evacuated Pripyat.

Of the senior personnel from the night of April 25th–26th, the trials began in 1987. Six men were prosecuted in a closed Soviet court at the Palace of Culture in Chernobyl town. Plant director Viktor Bryukhanov, chief engineer Nikolai Fomin, and deputy chief engineer Anatoly Dyatlov each received ten-year sentences for "gross violation of safety regulations creating conditions that led to an explosion." Three other operators received shorter terms. Aleksandr Akimov, the unit shift supervisor who had run the test from the control panel, and Leonid Toptunov, the engineer who pressed AZ-5, had both already died of acute radiation syndrome in May 1986 and were not prosecuted. Dyatlov served three years of his ten-year sentence before being released on health grounds with terminal radiation-induced damage; he died in December 1995. Bryukhanov was released in 1991 after Soviet collapse and lived until 2021. Valery Legasov, the senior scientist whose blunt August 1986 report to the IAEA in Vienna disclosed that the RBMK had a positive void coefficient and a graphite-tipped control rod design — facts the Soviet nuclear ministry had classified as state secrets — was professionally ostracised in Moscow, denied a promised promotion, and on April 27th, 1988, the second anniversary of the accident, hanged himself in his apartment. He left behind audio cassettes detailing the design flaws, which were transcribed and partially published.

The exclusion zone has, in the absence of human residents, become one of Europe's largest involuntary wildlife reserves. Studies by ecologists from the Polesie State Radioecological Reserve on the Belarusian side and from international teams have documented thriving populations of European bison reintroduced in 1996, Przewalski's horses introduced in 1998 (about 30 originally, an estimated 150 by 2020), grey wolves at densities estimated seven times those of comparable unprotected reserves elsewhere in Europe, and substantial populations of brown bear, lynx, beaver, and wild boar. Long-term genetic studies of voles and birds from the most contaminated forests have produced disputed findings on heritable mutation rates. The trees of the so-called Red Forest, the patch of pine immediately west of the reactor that absorbed the heaviest fallout in the first hours and turned literally red within days, were bulldozed and buried in 1986; the soil there remains the most radioactive in the zone and is closed to all entry without specific dosimetric authorisation.