Reactor 4 was running a safety test. The test was supposed to demonstrate that, in the event of a power loss, the residual spin of the steam turbine could keep cooling water flowing for the few seconds before backup diesel generators came online. The test had been postponed earlier in the day; the night shift, less experienced and less briefed, was running it. The reactor had been throttled down to dangerously low power levels. The shift supervisor was Aleksandr Akimov; the senior reactor operator was Leonid Toptunov. The deputy chief engineer in charge, Anatoly Dyatlov, ordered the test to proceed despite the unsafe conditions. At 1:23:40 AM local time, an emergency shutdown was initiated. The RBMK-1000 reactor design had a flaw that almost no one outside a small Soviet circle understood: at low power, inserting the control rods initially increased reactivity rather than decreasing it. Within seconds, the reactor power surged to many times its rated maximum. The fuel rods shattered. A steam explosion blew the 1,000-ton concrete lid off the reactor. A second explosion seconds later, possibly hydrogen, threw burning graphite and reactor fuel into the night sky.

Two plant workers died in the immediate explosion. One, Valery Khodemchuk, was killed instantly in the reactor hall and his body was never recovered; he is entombed in the reactor ruins. The other, Vladimir Shashenok, was pulled out alive but died of injuries and burns hours later. The firefighters arrived within minutes. They were professional but conventional firefighters, not nuclear specialists. The fires on the roof, which they fought through the night, were burning graphite from the reactor core. They had no protective equipment beyond standard fire turnout gear. Many of them, like Vasily Ignatenko, picked up fragments of graphite with their hands. Twenty-eight of them, mostly the firefighters of Pripyat and Chernobyl plus several plant operators, died over the following months from acute radiation syndrome. Many were treated in Moscow's Hospital No. 6 by Soviet doctors and the visiting American specialist Robert Peter Gale, who supervised bone marrow transplants. Most of the transplants failed. The 28 deaths from acute radiation are documented and uncontested. Two more deaths in the first hours bring the immediate toll to 31.

Pripyat was a closed city of about 49,000 people, built three kilometers from the plant to house its workers and their families. Its average resident age was 26. Its children went to school under murals of atomic energy as the future. For a day and a half after the explosion, residents were not told what had happened. Children played outside. A wedding was held that Saturday afternoon. Soviet authorities radioed instructions for evacuation only on the afternoon of April 27. Buses, eventually 1,200 of them, arrived in the early afternoon and the entire city was evacuated in roughly three hours. Residents were told to bring documents and a small bag, that they would return in three days. Most never returned. Lyudmila Ignatenko, separated from her husband by the radiation barriers but determined to be with him, talked her way past Soviet doctors and stayed at his bedside through the days when his skin sloughed off in sheets. The eventual evacuation zone was the 30-kilometer Chernobyl Exclusion Zone. About 135,000 people were ultimately moved out as additional contaminated areas were identified. A new city, Slavutych, was built across the Dnieper marshes to house plant workers.

The cleanup was done by people the Soviets called liquidators. Estimates of their total number range from about 600,000 to 800,000, drawn from the military, miners, factory workers, scientists, and conscripts from across the Soviet Union. Coal miners from the Donbas tunneled under the reactor in dangerous heat to install a heat exchanger. Helicopter pilots dropped sand, lead, and boron onto the burning core; many flew dozens of missions through the radioactive plume. Soldiers known as bio-robots, after Soviet-built robots failed in the high radiation, shoveled the most contaminated debris off the roof of Reactor 3 in 40 to 90-second shifts. They averaged 250 millisieverts per worker, far above any peacetime safety limit. Three plant workers, often called the suicide squad, dove into a flooded basement chamber to manually open valves and drain water that, had it contacted the molten reactor core, could have produced a steam explosion many times worse than the original. Their names were Alexei Ananenko, Valeri Bezpalov, and Boris Baranov. Contrary to early Western reports, all three survived the dive itself; Baranov died of unrelated causes years later, the other two were still alive in the late 2010s. Their work likely prevented a catastrophe of an order of magnitude greater. The original concrete sarcophagus enclosing Reactor 4 was erected in November 1986, designed for a thirty-year life and built in radiation conditions where no design lasts forever.

How many people the Chernobyl disaster killed in total is one of the most contested numbers in modern public health. The 31 immediate deaths are firm. Beyond that, every estimate carries assumptions that experts disagree about. The Chernobyl Forum, organized by the IAEA and WHO, has projected up to 4,000 eventual deaths from radiation-induced cancers among the most exposed populations. The TORCH report of 2006 estimated 30,000 to 60,000 worldwide. A Greenpeace study reached as high as 200,000 across Belarus, Russia, and Ukraine. The publication Chernobyl: Consequences of the Catastrophe for People and the Environment claimed 985,000, a figure widely criticized for methodology. What is well documented: more than 6,000 cases of childhood thyroid cancer linked to iodine-131 ingestion in contaminated dairy, with very high survival rates but a small number of deaths; a 350,000-person evacuated population whose mental health and life outcomes were measurably damaged by displacement and uncertainty; an unknown number of induced abortions across Europe driven by radiophobia rather than actual risk. Belarus received about 60 percent of the contamination that landed on the former Soviet Union. The Republican Scientific and Practical Center for Radiation Medicine in Gomel, Belarus, has been studying long-term health effects in the affected populations for nearly four decades.

Valery Legasov was the first deputy director of the Kurchatov Institute and the senior Soviet scientist sent to Chernobyl in the days after the explosion. He is the figure who, more than any other Soviet official, told the international community what had happened. At the IAEA conference in Vienna in August 1986, he gave a remarkably frank presentation about the accident, the design flaws of the RBMK reactor, and the human errors that had triggered it. The Soviet establishment did not forgive him for that frankness. He was passed over for honors, blocked from advancement, and increasingly marginalized within the Soviet scientific community. On April 27, 1988, the second anniversary of the day Pripyat had been evacuated, Legasov hanged himself in his Moscow apartment. He left tape-recorded accounts that became part of the historical record. Anatoly Dyatlov, the deputy chief engineer who had pushed through the test, was tried in 1987, sentenced to ten years in prison, and released early; he died in 1995, partially paralyzed by radiation exposure, having spent his final years insisting that the reactor design itself was at fault. Both men were partly right.

The original sarcophagus was always temporary. By the early 2000s it was visibly deteriorating. The international Chernobyl Shelter Fund was established in 1997 to design and build a permanent successor. Donor governments, most notably the European Bank for Reconstruction and Development and many European states, ultimately contributed 864 million euros. Construction of the New Safe Confinement began in 2010. It is an enormous steel arch, 105 meters tall and 257 meters across, the largest moveable land-based structure ever built. Construction took place adjacent to the reactor site to keep workers out of the highest radiation. In November 2016 the completed arch was slid on rails over the existing sarcophagus, sealing the destroyed reactor under a new outer shell designed to last 100 years and to permit eventual remote dismantling of what remains inside. It was an extraordinary engineering achievement, undertaken by an international coalition for the benefit of every country downwind of the site. In February 2025 a Russian drone strike during the ongoing war damaged the New Safe Confinement, and by December 2025 the IAEA reported the structure had lost its primary safety function. The work of confining what happened on April 26, 1986 is, forty years on, still not done.