Until mid-April 1986, Chernobyl was a small city in the erstwhile Soviet Union that lay some 93 km north of the Ukrainian capital Kyiv and about 350 km from the Belarusian capital Minsk. But on the intervening night of April 25-26, 1986, the name entered global infamy with what would be remembered as the worst disaster in the history of nuclear power generation.So, what happened in Chernobyl on April 26, 1986, and why does it count as the costliest man-made disaster of all time? We explain.Situated in present-day Ukraine, the so-called Chernobyl nuclear power station was located 16 km northwest at Pripyat, an industrial town that housed close to 50,000 residents — including most of the workers at the power station (Chernobyl itself had 12,500 residents). Operational since 1977-83, the station had four reactors, each of which could produce 1,000 megawatts of electricity.Although the accident happened on April 26, the chain of events leading up to it began the day before, when plant technicians tried to execute an experiment with the Unit 4 RBMK reactor. An RBMK reactor is a Soviet-designed, graphite-moderated nuclear power reactor that uses enriched uranium fuel, with water flowing through individual pressure tubes, boiling directly to drive turbines.As part of the experiment, workers shut down Unit 4’s power-regulating systems and its safety features and withdrew most of the control rods from the core, even as the reactor continued running at 7% power. Mistakes by other operators as well as a flawed reactor design and the absence of a work safety culture (as it exists today) contributed to what happened next.Also in Explained | Looking back at the Chernobyl disaster: Who was responsible?Around 1:23 am on April 26, the chain reaction in Unit 4’s core went out of control, leading to a series of explosions. This partially damaged the core and triggered a large fireball, which blew off the reactor’s heavy steel and concrete lid.Since RBMK reactors lacked a pressure-retaining containment structure to limit releases during severe core damage, this absence of a robust final physical barrier allowed large amounts of radioactive material to be released directly into the atmosphere. The situation was further exacerbated when the resulting fire in the graphite reactor drove prolonged radioactive emissions over several days, where it was carried great distances by air currents.Story continues below this adIn all, about 3.5% of Unit 4’s nuclear fuel was dispersed outside during the accident, contaminating large territories of Ukraine, Belarus, and Russia. Some 150,000 square km in these countries were contaminated, stretching northward of the accident site as far as 500 kilometres.The aftermath, and the cleanupWithin 36 hours of the accident, the entire town of Pripyat was completely evacuated. A cover-up was reportedly attempted, but monitoring stations in Sweden recorded abnormally high levels of wind-transported radioactivity on April 28. When pressed for an explanation, the Soviet government admitted to the Chernobyl accident, provoking international outcry over the dangers posed by radioactive emissions.Close to 67,000 people were evacuated and relocated from their homes in contaminated areas during the subsequent months — a factsheet by the International Atomic Research Agency (IAEA) puts the total number of people relocated due to the accident at around 200,000.Following the accident, “liquidators” — also called recovery operation workers — were drafted from all over the Soviet Union to help clean up the plant premises and the surrounding area. These were mostly plant employees, Ukrainian firefighters, and soldiers and miners from Russia, Belarus, and Ukraine. According to IAEA, although the exact number of liquidators is unknown, Russian registries list approximately 400,000 liquidators (as of 1991) and about 600,000 people were granted the status of “liquidator”.Story continues below this adMore in Explained | Why did Russian troops seize control of Chernobyl nuclear disaster site?These liquidators worked on decontamination and major construction projects, which included building settlements and towns for plant workers and those evacuated. They also constructed waste repositories, dams, water filtration systems, and the “sarcophagus”, which entombs the Unit 4 reactor to contain the remaining radioactive material.An area spanning 30 kilometres around the plant site was marked as the “exclusion zone”. As part of efforts to decontaminate this exclusion zone to reduce radioactive exposure to people, heavily contaminated topsoil was removed and buried in shallow trenches in 1987. To prevent contaminants from reaching the Pripyat-Dnieper river system, hydraulic barriers and wells were also built in 1986.A report in the New Scientist magazine dated August 17, 1991, had put the cost of clean-up in the five years since the disaster at a total of 533 million roubles (about £533 million), citing figures released by the Soviet government’s “state commission for extraordinarysituations”. This included outlays on new settlements for evacuees, entertainment of foreign scientists, the purchase of condemned meat, and to “cover the costs” incurred by the nuclear energy ministry. The last expenditure appeared to include additional safety measures and training at other RBMK nuclear reactors.Story continues below this adIn 2016, a report by Jonathan Samet, then director of the University of South California’s Institute for Global Health, put the cost of the Chernobyl accident as exceeding $700 billion over three decades. Despite the unavailability of more recent estimates, this figure makes the 1986 incident the costliest disaster in history.IAEA noted that between 1991 and 2005, in Belarus, Ukraine, and in the most affected regions of Russia, there were at least 5,000 documented cases of thyroid cancer in children who were between 0 and 14 years of age when the accident occurred, which is far higher than normal. However, health studies of the “liquidators” had failed to show any direct correlation between their radiation exposure and an increase in other forms of cancer or disease.“Health represents the largest proportion of the indirect costs. These costs greatly exceed those directly related to the plant because this price tag spans a lifetime and possibly even reaches to the next generation. Neuropsychological effects, such as depression, are among the most widespread and expensive of the long-term consequences,” Samet said in an accompanying press statement.In 2019, BBC reported that the Kyiv-based National Research Centre for Radiation Medicine estimated that around 5 million citizens of the former Soviet Union, including 3 million in Ukraine, have suffered as a result of the “largest anthropogenic disaster in the history of humankind”, while some 800,000 people in Belarus were registered as being affected by radiation following the disaster.