Chernobyl disaster

Background Reactor cooling after shutdown shown as % of thermal power from time of sustained fission shutdown using two different correlations. Due to decay heat, solid fuel power reactors need high flows of coolant after a fission shutdown for a considerable time to prevent fuel cladding damage, or in the worst case, a full core meltdown. In nuclear-reactor operation, most heat is generated by nuclear fission, but over 6% comes from radioactive decay, which continues after the reactor shuts down. Continued coolant circulation is essential to prevent core overheating or a core meltdown. RBMK reactors, like those at Chernobyl, use water as a coolant, circulated by electrically driven pumps. Reactor no.4 had 1,661 individual fuel channels, requiring over 45 million litres of coolant per hour for the entire reactor. In case of a total power loss, each of Chernobyl's reactors had three backup diesel generators, but they took 60–75 seconds to reach full load and generate the 5.5 MW needed to run one main pump. Special counterweights on each pump provided coolant via inertia to bridge the gap to generator startup. However, a potential safety risk existed in the event that a station blackout occurred simultaneously with the rupture of a coolant pipe. In this scenario the emergency core cooling system (ECCS) is needed to pump additional water into the core. Safety test The turbine run-down energy capability still needed to be confirmed experimentally, and previous tests had ended unsuccessfully. An initial test carried out in 1982 indicated that the excitation voltage of the turbine-generator was insufficient. The electrical system was modified and the test was repeated in 1984, but again proved unsuccessful. In 1985, the test was conducted a third time, but also yielded no results due to a problem with the recording equipment. The test procedure was to be run again in 1986 and was scheduled to take place during a controlled power-down of reactor no.4, which was preparatory to a planned maintenance outage. As planned, a gradual reduction in the output of the power unit began at 01:06 on 25 April, and the power level had reached 50% of its nominal 3,200 MW thermal level by the beginning of the day shift. Preparations for the test were carried out, including the disabling of the emergency core cooling system. and an output of 720 MW was reached at 00:05 on 26 April. Xenon poisoning in this context made reactor control more difficult, but was a predictable phenomenon during such a power reduction. When the reactor power had decreased to approximately 500 MW, the reactor power control was switched from local automatic regulator to the automatic regulators, to manually maintain the required power level. Several minutes elapsed before the reactor was restored to 160 MW at 00:39, at which point most control rods were at their upper limits, but the rod configuration was still within its normal limit, calculated as equivalent to having more than 15 rods inserted. Over the next twenty minutes, reactor power would be increased further to 200 MW. resulting in the inserted control rod equivalent value falling below its required level of 15. This was not apparent to the operators, because there were no instruments capable of calculating this value in real time. These actions left the reactor in an extremely unstable state. Nearly all of the 211 control rods had been withdrawn, and excessive coolant flow prevented the water from adequately cooling between cycles, causing it to re-enter the core at close to boiling point. Unlike other light-water reactor designs, the RBMK design had a positive void coefficient, meaning that voids formed during boiling had the effect of increasing rather than decreasing core reactivity. More boiling and hence more voids tended to intensify the nuclear chain reaction, which caused yet more boiling. Unknown to the operators, this positive feedback loop had little to restrain it, and the reactor became highly sensitive to void formation, Four of the eight main circulating pumps (MCP) were to be powered by voltage from the coasting turbine, while the remaining four pumps received electrical power from the grid as usual. The steam to the turbines was shut off, beginning a run-down of the turbine generator. The diesel generators started and sequentially picked up loads; the generators were to have completely picked up the MCPs' power needs by 01:23:43. As the momentum of the turbine generator decreased, so did the power it produced for the pumps. The water flow rate decreased, leading to increased formation of steam voids in the coolant flowing through the fuel pressure tubes. an operator pressed the emergency AZ-5 button to initiate a scram (emergency shutdown) of the reactor, perhaps in preparation for scheduled maintenance. The reason for the timing is uncertain, as both Akimov and Toptunov would die shortly thereafter. The atmosphere in the control room at that point was calm, according to eyewitnesses, and there were no active emergency signals at that time. Representatives of the reactor's design team would later argue that the button must have been pressed only after the reactor had begun to self-destruct. The AZ-5 button engaged a drive mechanism to fully insert all control rods, including the manual control rods that had been withdrawn earlier. In the RBMK design, each control rod had attached to its end a graphite neutron moderator extension, the purpose of which was to boost reactor output by displacing neutron-absorbent water when the main part of the control rod was fully withdrawn. When the control rod was at maximum extraction, the graphite extension sat within the water of the core, its neutron-moderating effect operating to increase reactor power. Explosions As the scram continued, the reactor output jumped to around 30,000 MW thermal, 10 times its normal operational output, the last indicated reading on the control panel. According to some estimates, the power spike may have been 10 times higher than that. It was not possible to reconstruct the precise sequence that led to the destruction of the reactor and the power unit building, but a steam explosion appears to have been the next event. Explosive steam pressure from the damaged fuel channels escaping into the reactor's exterior cooling structure that caused the explosion that destroyed the reactor casing, tearing off and blasting the upper plate called the upper biological shield (to which the entire reactor assembly was fastened) through the roof of the reactor building. This is believed to be the first explosion that many heard. This explosion ruptured further fuel channels, as well as severing most of the coolant lines feeding the reactor chamber. As a result, the remaining coolant flashed to steam and escaped the reactor core. The total water loss combined with a high positive void coefficient further increased the reactor's thermal power. occurred two or three seconds after the first, dispersing the damaged reactor core and effectively terminating the nuclear chain reaction. The blast further compromised the reactor containment vessel, ejecting red-hot fragments of graphite moderator and damaged fuel channel material. Observers outside Unit 4 reported burning lumps of material and sparks shooting into the air above the reactor, some of which fell onto the roof of the machine hall and started fires. Approximately 25% of the graphite blocks and overheated material from the fuel channels were expelled. Possible causes of the second explosion There are several hypotheses about the nature of the second, larger explosion. One view was that the second explosion was caused by the combustion of hydrogen, which had been produced either by the overheated steam-zirconium reaction or by the reaction of red-hot graphite with steam that produced hydrogen and carbon monoxide. Another hypothesis, by Konstantin Checherov, published in 1998, was that the second explosion was a thermal explosion of the reactor due to the uncontrollable escape of fast neutrons caused by the complete water loss in the reactor core. The force of the second explosion and the ratio of xenon radioisotopes released after the accident led Sergei A. Pakhomov and Yuri V. Dubasov to theorize in 2009 that the second explosion could have been an extremely fast nuclear power transient resulting from core material melting in the absence of its water coolant and moderator. Pakhomov and Dubasov argued that there was no delayed supercritical increase in power but a runaway prompt criticality, similar to the explosion of a fizzled nuclear weapon. This was an alternative to the more accepted explanation of a positive-feedback power excursion where the reactor disassembled itself by a steam explosion. The energy released by the second explosion, which produced the majority of the damage, was estimated by Pakhomov and Dubasov to be at 40 billion joules, the equivalent of about 10 tons of TNT. Both analyses argue that the nuclear fizzle event, whether producing the second or first explosion, consisted of a prompt chain reaction that was limited to a small portion of the reactor core, since self-disassembly occurs rapidly in fizzle events. Grigorii Khmel, the driver of one of the fire engines, described what happened: block ejected from the core Anatoli Zakharov, a fireman stationed in Chernobyl, offered a different description in 2008: "I remember joking to the others, 'There must be an incredible amount of radiation here. We'll be lucky if we're all still alive in the morning. The immediate priority was to extinguish fires on the roof of the station and the area around the building containing reactor no. 4 to protect no. 3. The fires were extinguished by 5:00, but many firefighters received high doses of radiation. The fire inside reactor no. 4 continued to burn until 10 May 1986; it is possible that well over half of the graphite ultimately burned out. Around 600 Soviet pilots risked dangerous levels of radiation to fly the thousands of flights needed. One firefighter described his experience of the radiation as "tasting like metal", and feeling a sensation similar to pins and needles all over his face. This is consistent with the description given by Louis Slotin, a Manhattan Project physicist who died days after a fatal radiation overdose from a criticality accident. Radiation levels The explosion and fire threw hot particles of the nuclear fuel and more dangerous fission products into the air. The ionizing radiation levels in the worst-hit areas of the reactor building have been estimated to be 5.6 roentgens per second (R/s), equivalent to more than 20,000 roentgens per hour. A lethal dose is around 500 roentgens (~4.4 Gray (Gy) in modern radiation units) over five hours. In some areas, unprotected workers received fatal doses in less than a minute. A dosimeter capable of measuring up to 1,000 R/s was buried in the rubble of a collapsed part of the building, and another one failed when turned on. Most remaining dosimeters had limits of 0.001 R/s and therefore read "off scale". The reactor crew could ascertain only that the radiation levels were somewhere above 0.001 R/s (3.6 R/h), while the true levels were vastly higher in some areas. Accident investigation The IAEA had created the International Nuclear Safety Advisory Group (INSAG) in 1985. INSAG produced two significant reports on Chernobyl: INSAG-1 in 1986, and a revised report, INSAG-7, in 1992. According to INSAG-1, the main cause of the accident was the operators' actions, but according to INSAG-7, the main cause was the reactor's design. Both reports identified an inadequate "safety culture" at all managerial and operational levels as a major underlying factor. == Crisis management ==

Evacuation with the Chernobyl Nuclear Power Plant in the distance The nearby city of Pripyat was not immediately evacuated, and the townspeople were not alerted during the night to what had just happened. Within a few hours, dozens of people fell ill. Later, they reported severe headaches and metallic tastes in their mouths, along with uncontrollable fits of coughing and vomiting. As the plant was run by authorities in Moscow, the government of Ukraine did not receive prompt information on the accident. Valentyna Shevchenko, then Chairwoman of the Presidium of Verkhovna Rada of the Ukrainian SSR, said that Ukraine's acting Minister of Internal Affairs Vasyl Durdynets phoned her at work at 09:00 to report current affairs; only at the end of the conversation did he mention that there had been a fire at the Chernobyl nuclear power plant, adding that it was extinguished and everything was fine. When Shevchenko asked "How are the people?", he replied that there was nothing to be concerned about: "Some are celebrating a wedding, others are gardening, and others are fishing in the Pripyat River". The Chernobyl exclusion zone has remained ever since, although its shape has changed and its size has expanded. The surveying and detection of isolated fallout hotspots outside this zone over the following year eventually resulted in 135,000 long-term evacuees in total. A new city of Slavutych has been built across the Dnieper marshes to house Chernobyl Nuclear Power Plant employees instead of Pripyat, with a direct rail connection to the Chernobyl NPP. Official announcement on 1 May 1986 Evacuation began one and a half days before the accident was publicly acknowledged by the Soviet Union. On the morning of 28 April, radiation levels set off alarms at the Forsmark Nuclear Power Plant in Sweden, At first, the authorities conceded only that a minor accident had occurred, but once they began evacuating more than 100,000 people, the scale of the situation became widely known. At 21:02 the evening of 28 April, a 20-second announcement was read in the TV news programme Vremya: "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." This was the first time the Soviet Union had officially announced a nuclear accident. The Telegraph Agency of the Soviet Union (TASS) then discussed the Three Mile Island accident and other American nuclear accidents, which Serge Schmemann of The New York Times wrote was an example of the common Soviet tactic of whataboutism. The mention of a commission also indicated to observers the seriousness of the incident, and subsequent state radio broadcasts were replaced with classical music, a common method of preparing the public for an announcement of a tragedy in the USSR. Shevchenko was the first of the Ukrainian state top officials to arrive at the disaster site early on 28 April. She returned home near midnight, stopping at a radiological checkpoint in Vilcha, one of the first that were set up soon after the accident. started to pool on the floor of the sub-reactor space, creating corium, a radioactive semi-liquid material comparable to lava. It was feared that if this mixture travelled through the pipes into the pool of water, the resulting steam production would further contaminate the area or even cause another explosion, and that it would therefore be necessary to drain the pool. These fears ultimately proved unfounded, since corium began dripping harmlessly into the flooded bubbler pools before the water could be removed. The molten fuel hit the water and cooled into a light-brown ceramic pumice, whose low density allowed it to float. Once the bubbler pool gates were opened, fire brigade pumps were then used to drain the basement. The operation was not completed until 8 May, after 20,000 tonnes of water were pumped out. At Shcherbina's instructions, helicopter units of the Air Forces of the Kiev Military District were directed to seal the crippled reactor unit no. 4 with sand. Generál-leytenánt ("two stars") N.P. Kryukov and General-mayor ("one star") Nikolay Antoshkin, respectively the Commander and Chief of Staff of the Air Forces of the Kiev Military District, arrived at the plant in the early morning of 27 April. Foundation protection measures The government commission was concerned that the molten core would burn into the earth and contaminate groundwater. To reduce the likelihood of this, it was decided to freeze the earth beneath the reactor, which would also stabilize the foundations. Using oil-well drilling equipment, injection of liquid nitrogen began on 4 May. It was estimated that 25 tonnes of liquid nitrogen per day would be required to keep the soil frozen at -100 C. As an alternative, subway builders and coal miners were deployed to excavate a tunnel below the reactor that would make room for a cooling system. The final makeshift design for the cooling system incorporated a coiled formation of pipes cooled with water, with a thin top cover of thermally-conductive graphite. The graphite layer would prevent the concrete above from melting, and was to be encapsulated between two concrete layers, each 1 m thick, for stabilisation. This graphite-concrete "sandwich" would be similar in concept to later core catchers now part of many nuclear reactor designs. Following a drop in air temperatures, and reports that the fuel melt had stopped, both the graphite cooling plate and the prior nitrogen-injection proposal were abandoned. It was later determined that the fuel had flowed down three floors, with a few cubic meters coming to rest at ground level. The precautionary underground channel, with its active cooling, was deemed redundant; instead, the excavation was filled with concrete to strengthen the foundation below the reactor. Site cleanup Debris removal In the months after the explosion, attention turned to removing the radioactive debris from the roof. While the worst of the debris had remained inside what was left of the reactor, an estimated 100 tonnes of debris on the roof had to be removed to enable the safe construction of the "sarcophagus"—a concrete structure that would entomb the reactor and inhibit release of radioactive dust. Many robots failed due to the difficult terrain, combined with the effect of high radiation fields on their batteries and electronic controls. Consequently, the most highly radioactive materials were shovelled by Chernobyl liquidators from the military, wearing protective gear. These soldiers could only spend a maximum of 40–90 seconds working on the rooftops of the surrounding buildings because of the extremely high radiation levels. Only 10% of the debris cleared from the roof was performed by robots; the other 90% was removed by 3,828 men who each received, on average, an estimated dose of 25 rem (250 mSv) of radiation. The construction workers had to be protected from radiation; techniques such as crane drivers working from lead-lined control cabins were employed. The construction work included erecting walls around the perimeter, clearing and surface-concreting the surrounding ground to remove sources of radiation and to allow access for large construction machinery, constructing a thick radiation-shielding wall to protect the workers in reactor no. 3, fabricating a high-rise buttress to strengthen parts of the old structure, constructing an overall roof, and provisioning a ventilation-extraction system to capture airborne contamination within the shelter. Investigations of the reactor condition From May 5, 1986, workers from NIKIET and the V. G. Khlopin Radium Institute collaborated under the name Field Integrated Scientific and Technical Brigade No. 9 (PKNTB-9) to work at the Chernobyl Nuclear Power Plant, led by G. S. Sinitsyna and S. S. Kovalenko. Their job was to conduct dosimetric surveys of both Unit 3, Unit 4, The Vent Block, and the Deaerator block, as well as assessing the damage and finding nuclear fuel. They ultimately did not get very far, however. They were only able to provide dosimetric measurements down to the floor on +12.5, where they were unable to go any further down due to extreme radiation levels. These radiation levels were later found to be caused by corium on the +9.0 level, as part of the horizontal flow. Their efforts would also be hampered by an electrical fire on May 23. Following several models by Oleksandr Borovoi and Valery Legasov, they determined that 90% of the nuclear fuel remained inside the building. Now, needing to locate where exactly, members of the Kurchatov Institute came to Chernobyl on June 6th, being led by Konstantin Checherov, began attempting to find routes into these high radiation areas. As previously assessed, they could not get to these lower levels from +12.5 due to extreme radiation levels coming from the staircase 257. On June 10th, Konstantin Checherov measured a radiation level of 11,400 roentgens per hour inside an open hatch within the Southern Main Circulation Pump Hall. This radiation would later be assessed to also be caused by Corium. Around June 15, Mikhail Kostyakov and Vladimir Kabanov were attempting to access the lower levels from the southern side. From corridor 017/2 on +0.0, they began to ascend a staircase leading to corridor 217/2 on +6.0. However, during this ascent, their dosimeter failed and broke, so they turned back. They did not know it but they had just been the first people to come across the Elephant's Foot. Following the start of the construction of the Sarcophagus, all expedition work had to be stopped, as concrete was being actively poured into the buildings. It would not be until after the Sarcophagus was complete - in November - that work could begin. In this time, Konstantin Checherov alongside several figures such as Oleksandr Borovoy formed the "Complex Expedition". Their job was to assess the condition of the building and also locate the fuel and determine if it could go critical again. It was a major concern among the Complex Expedition regarding the composition of the molten fuel, as if the molten fuel contained enough fuel and a moderator, in theory it could create more damage to the building, or even another explosion. The work began once again in late November 1986, as the Complex Expedition began work to assess the damage, while members of the Khoplin Radium Institute collected any fuel rods they found and took them back to their laboratory. In December 1986, after months of investigation, Vasya Koryagin accidentally found a large object that appeared to be molten lead that had dropped into the reactor core and melted. The mass was called "The Elephant's Foot" for its wrinkled appearance. Only following samples being obtained were they able to assess that it was composed of melted sand, concrete, and a large amount of nuclear fuel that had escaped from the reactor. The concrete beneath the reactor was steaming hot, and was breached by now-solidified lava and spectacular unknown crystalline forms termed chernobylite. It was concluded that there was no further risk of explosion. Although a number of radioactive emergency vehicles were buried in trenches, many of the vehicles used by the liquidators still remained, as of 2018, parked in a field in the Chernobyl area. Scavengers have removed many functioning, but highly radioactive, parts. A unique "clean up" medal was given to the clean-up workers, known as "liquidators". Liquidators worked under deplorable conditions, poorly informed and with poor protection. Many, if not most, of them exceeded radiation safety limits. == Site remediation ==

Questions arose about the future of the plant and its fate. All work on the unfinished reactors no. 5 and no. 6 was halted three years later. The damaged reactor was sealed off and 200 cubic metres of concrete was placed between the disaster site and the operational buildings. The Ukrainian government allowed the three remaining reactors to continue operating because of an energy shortage. In October 1991, a fire occurred in the turbine building of reactor no. 2; the authorities subsequently declared the reactor damaged beyond repair, and it was taken offline. Reactor no. 1 was decommissioned in November 1996 as part of a deal between the Ukrainian government and international organizations such as the IAEA to end operations at the plant. On 15 December 2000, then-President Leonid Kuchma personally turned off reactor no. 3 in an official ceremony, shutting down the entire site. No. 4 reactor confinement in 2017 The concrete sarcophagus which had been built in the months after the accident was never intended to last very long, with an expected lifespan of only 30 years. On 12 February 2013, a 600 square metre section of the roof of the turbine-building collapsed, adjacent to the sarcophagus, causing a new release of radioactivity. At first, it was assumed that this was due to the weight of snow; however, the quantity of snow was not exceptional, and the report of a Ukrainian fact-finding panel concluded that the collapse was the result of sloppy repair work and aging of the structure. Experts warned the sarcophagus itself was on the verge of collapse. In 1997, the international Chernobyl Shelter Fund was founded to design and build a more permanent cover for the unstable sarcophagus. It received €864 million from international donors in 2011 and was managed by the European Bank for Reconstruction and Development. The new shelter was named the New Safe Confinement (NSC), and construction began in 2010. It consisted of a metal arch high and spanning , built on rails adjacent to the reactor no. 4 building so that it could be slid over the top of the existing sarcophagus. The shelter was completed in 2016 and was moved into place over the sarcophagus on 29November. In February 2025, a Russian drone struck the shelter causing a fire, damaging the outer and inner protective covering of the NSC, and insulating layers inside. In December 2025 the IAEA said that the drone strike meant that the structure could no longer perform its main safety function. IAEA Director General Rafael Grossi stated that an IAEA mission in late November "confirmed that the [protective structure] had lost its primary safety functions, including the confinement capability, but also found that there was no permanent damage to its load-bearing structures or monitoring systems". A need was recognised for larger, longer-term radioactive waste management at the site, and a new facility designated ISF-2 was designed. This serves as dry storage for used fuel assemblies from units 1–3 and other operational wastes, as well as material from decommissioning units 1–3. A contract was signed in 1999 with Areva NP (Framatome) for construction of ISF-2. In 2003, after a significant part of the storage structures had been built, technical deficiencies in the design concept became apparent. In 2007, Areva withdrew and Holtec International was contracted for a new design and construction of ISF-2. The new design was approved in 2010, work started in 2011, and construction was completed in August 2017. ISF-2 is the world's largest nuclear fuel storage facility, expected to hold more than 21,000fuel assemblies for at least 100 years. The project includes a processing facility able to cut the fuel assemblies and to place the material in canisters, to be filled with inert gas and welded shut. The canisters are then to be transported to dry storage vaults, where the fuel containers will be enclosed for up to 100years. Expected processing capacity is 2,500fuel assemblies per year. According to a published paper, degradation of the lava is likely to be a slow, gradual process. The same paper states that the loss of uranium from the wrecked reactor is only 10 kg per year; this low rate of uranium leaching suggests that the lava is resisting its environment. Prior to the completion of the New Safe Confinement building, rainwater acted as a neutron moderator, triggering increased fission in the remaining materials, risking criticality. Gadolinium nitrate solution was used to quench neutrons to slow the fission. Even after completion of the building, fission reactions may have increased. While neutron activity has declined across most of the destroyed fuel, from 2017 until late 2020 a doubling in neutron density was recorded in the sub-reactor space, before levelling off in early 2021. This indicated increasing levels of fission as water levels dropped, the opposite of what had been expected, and atypical compared to other fuel-containing areas. The fluctuations have led to fears that a self-sustaining reaction could be created, which would likely spread more radioactive dust and debris throughout the New Safe Confinement, making future cleanup even more difficult. Potential solutions include using a robot to drill into the fuel and insert boron carbide control rods. In early 2021, a ChNPP press release stated that the observed increase in neutron densities had levelled off since the beginning of that year. Exclusion zone around Chernobyl The Exclusion Zone was originally an area with a radius of 30 km in all directions from the plant, but was subsequently greatly enlarged to include an area measuring approximately 2600 square km, officially called the "zone of alienation". The area has largely reverted to forest and was overrun by wildlife due to the lack of human competition for space and resources. Mass media sources have provided generalized estimates for when the Zone could be considered habitable again. These informal estimates have ranged to multiples of 20,000 years, referring to the half-life of Plutonium-239 which contaminates the central portion of the Zone. In the years following the disaster, residents known as samosely illegally returned to their abandoned homes. Most people are retired and survive mainly from farming and packages delivered by visitors. , 187 locals had returned to the zone and were living permanently there. Forest fire concerns During the dry season, forest fires are a perennial concern in areas contaminated by radioactive material. Dry conditions and build-up of debris make the forests a ripe breeding ground for wildfires. Depending on prevailing atmospheric conditions, smoke from wildfires could potentially spread more radioactive material outside the exclusion zone. In Belarus, the Bellesrad organization is tasked with overseeing food cultivation and forestry management in the area. In April 2020, forest fires spread through 20,000 ha of the exclusion zone, causing increased radiation from the release of caesium-137 and strontium-90 from the ground and biomass. The increase in radioactivity was detectable by the monitoring network but did not pose a threat to human health. The average radiation dose that Kyiv residents received as a result of the fires was estimated to be 1 nSv. Recovery projects The Chernobyl Trust Fund was created in 1991 by the United Nations to help victims of the Chernobyl accident. It is administered by the United Nations Office for the Coordination of Humanitarian Affairs, which also manages strategy formulation, resource mobilization, and advocacy efforts. Beginning in 2002, under the United Nations Development Programme, the fund shifted its focus from emergency assistance to long-term development. The programme was initiated in February 2002 based on the recommendations in the report on Human Consequences of the Chernobyl Nuclear Accident. The main goal of the CRDP was to support the Government of Ukraine in mitigating the long-term social, economic, and ecological consequences of the Chernobyl catastrophe. CRDP works in the four most affected Ukrainian areas: Kyivska, Zhytomyrska, Chernihivska and Rivnenska. More than 18,000 Ukrainian children affected by the disaster have been treated in the resort town of Tarará, Cuba, since 1990. The International Project on the Health Effects of the Chernobyl Accident was created and received US$20 million, mainly from Japan, in the hope of discovering the main cause of health problems due to iodine-131 radiation. These funds were divided among Ukraine, Belarus, and Russia for the investigation of health effects. As there was significant corruption in former Soviet republics, most foreign aid was given to Russia, and no results from the funding were demonstrated. Tourism The first limited guided tours began in 2002. The 2007 release of the video game S.T.A.L.K.E.R. increased the site popularity and tour operators estimated that 40,000 tourists visited the site between 2007 and 2017. Between 2017 and 2022, over 350,000 tourists visited the site, hitting the maximum peak of almost 125,000 visitors in 2019, coinciding with the release of HBO's mini-series about the disaster. After its release in July 2019, Ukrainian president Volodymyr Zelenskyy announced that the Chernobyl site would become an official tourist attraction. Zelenskyy said, "We must give this territory of Ukraine a new life." Dr. T. Steen, a microbiology and immunology teacher at Georgetown's School of Medicine, recommends that tourists wear clothes and shoes they are comfortable throwing away and to avoid plant life. The zone remains closed to tourists as of April 2026. A parallel "stalker" subculture developed of illegal visitors roaming the area for prolonged periods, with some hiking into the zone over 100 times, often without taking appropriate precautions against radiation. ==Long-term effects==

Release and spread of radioactive materials Although it is difficult to compare the Chernobyl accident with a deliberate air burst nuclear detonation, it is estimated that Chernobyl released about 400 times more radioactive material than the combined atomic bombings of Hiroshima and Nagasaki. However, the Chernobyl disaster released only about one-hundredth to one-thousandth of the total radioactivity released during nuclear weapons testing at the height of the Cold War, due to varying isotope abundances. Approximately 100,000 square km of land was significantly contaminated, with the worst-affected areas in Belarus, Ukraine, and Russia. Lower contamination levels were detected across Europe, except for the Iberian Peninsula. On 28 April, workers at the Forsmark Nuclear Power Plant, 1100 km from Chernobyl, were found with radioactive particles on their clothing. Sweden's elevated radioactivity levels, detected at noon on 28 April, were traced back to the western Soviet Union. Meanwhile, in Finland, the Finnish Meteorological Institute's measuring instruments located in Nurmijärvi detected rising radiation levels on 27 April, but a civil service strike delayed the response and publication. Contamination from the Chernobyl accident was scattered irregularly depending on weather conditions, much of it deposited on mountainous regions such as the Alps, the Welsh mountains and the Scottish Highlands, where adiabatic cooling caused radioactive rainfall. The resulting patches of contamination were often highly localized, and localized water-flows contributed to large variations in radioactivity over small areas. Sweden and Norway also received heavy fallout when the contaminated air collided with a cold front, bringing rain. There was also groundwater contamination. Rain was deliberately seeded over 10,000 square km of Belarus by the Soviet Air Force to remove radioactive particles from clouds heading toward highly populated areas. Heavy, black-coloured rain fell on the city of Gomel. Reports from Soviet and Western scientists indicate that the Belarusian SSR received about 60% of the contamination that fell on the former Soviet Union. However, the 2006 TORCH report stated that up to half of the volatile particles had actually landed outside the former USSR area currently making up Ukraine, Belarus, and Russia. An unconnected large area in Russian SFSR south of Bryansk was also contaminated, as were parts of northwestern Ukrainian SSR. Studies in surrounding countries indicate that more than one million people could have been affected by radiation. 2016 data from a long-term monitoring program showed a decrease in internal radiation exposure of the inhabitants of a region in Belarus close to Gomel. In Western Europe, precautionary measures taken in response to the radiation included banning the importation of certain foods. A 2006 study found contamination was "relatively limited, diminishing from west to east", such that a hunter consuming 40 kilograms of contaminated wild boar in 1997 would be exposed to about one millisievert. Relative isotopic abundances The Chernobyl release was characterized by the physical and chemical properties of the radio-isotopes in the core. Particularly dangerous were the highly radioactive fission products, those with high nuclear decay rates that accumulate in the food chain, such as some of the isotopes of iodine, caesium and strontium. Iodine-131 was and caesium-137 remains the two most responsible for the radiation exposure received by the general population. At different times after the accident, different isotopes were responsible for the majority of the external dose. The remaining quantity of any radioisotope, and therefore the activity of that isotope, after 7 decay half-lives have passed, is less than 1% of its initial magnitude, and it continues to reduce beyond 0.78% after 7 half-lives to 0.10% remaining after 10 half-lives have passed and so on. Some radionuclides have decay products that are likewise radioactive, which is not accounted for here. The release of radioisotopes from the nuclear fuel was largely controlled by their boiling points, and the majority of the radioactivity present in the core was retained in the reactor. • All of the noble gases, including krypton and xenon, contained within the reactor were released immediately into the atmosphere by the first steam explosion. Caesium was released in aerosol form; caesium-137, along with isotopes of strontium, are the two primary elements preventing the Chernobyl exclusion zone being re-inhabited. equals 24 kilograms of caesium-137. Environmental impact Water bodies The Chernobyl nuclear power plant is located next to the Pripyat River, which feeds into the Dnieper reservoir system, one of the largest surface water systems in Europe, which at the time supplied water to Kiev's 2.4 million residents, and was still in spring flood when the accident occurred. In the most affected areas of Ukraine, levels of radioactivity in drinking water caused concern during the weeks and months after the accident. resulted in concentrations significantly above guideline maximum levels for consumption. In the Kiev Reservoir in Ukraine, concentrations in fish were in the range of 3000 Bq/kg during the early years after the accident. The contamination of fish caused short-term concern in parts of the UK and Germany and in the long term in the affected areas of Ukraine, Belarus, and Russia as well as Scandinavia. Some cattle on the same island died and those that survived were stunted. The next generation appeared to be normal. On farms in Narodychi Raion of Ukraine it is claimed that from 1986 to 1990 nearly 350 animals were born with gross deformities; in comparison, only three abnormal births had been registered in the five years prior. Subsequent research on microorganisms, while limited, suggests that in the aftermath of the disaster, bacterial and viral specimens exposed to the radiation underwent rapid changes. Activations of soil micromycetes have been reported. Cladosporium sphaerospermum, an extremophile species of fungus which has thrived in the Chernobyl contaminated area, has been investigated for the purpose of using the fungus' particular melanin to protect against high-radiation environments, particularly space travel. It remains under intensive study, being radioresistant to such an extent, and indeed using ionizing radiation, that it has reached the rooms inside the exploded reactor building. The disaster has been described by lawyers, academics and journalists as an example of ecocide. Human food chain With radiocaesium binding less with humic acid, peaty soils than the known binding "fixation" that occurs on kaolinite-rich clay soils, many marshy areas of Ukraine had the highest soil to dairy-milk transfer coefficients, of soil activity in ~ 200 kBq/m2 to dairy milk activity in Bq/L, that had ever been reported, with the transfer, from initial land activity into milk activity, ranging from 0.3−2 to 20−2 times that which was on the soil. This photograph was taken years later, in March 2009, after the forest began to grow again, with the lack of foliage at the time of the photograph merely due to the local winter at the time. In a 2007 paper, a robot sent into the no.4 reactor returned with samples of black, melanin-rich radiotrophic fungi that grow on the reactor's walls. Of the 440,350 wild boar killed in the 2010 hunting season in Germany, approximately one thousand were contaminated with levels of radiation above the permitted limit of 600 becquerels of caesium per kilogram, of dry weight, due to residual radioactivity from Chernobyl. Because Elaphomyces fungal species bioaccumulate radiocaesium, boars of the Bavarian Forest that consume these "deer truffles" are contaminated at higher levels than their environment's soil. Given that nuclear weapons release a higher 135Cs/137Cs ratio than nuclear reactors, the high 135Cs content in these boars suggests that their radiological contamination can be largely attributed to the Soviet Union's nuclear weapons testing in Ukraine, which peaked during the late 1950s and early 1960s. In 2015, long-term empirical data showed no evidence of a negative influence of radiation on mammal abundance. Precipitation on distant high ground On high ground, such as mountain ranges, there is increased precipitation due to adiabatic cooling. This resulted in localized concentrations of contaminants in distant areas; higher in Bq/m2 values to many lowland areas much closer to the source of the plume. The Norwegian Agricultural Authority reported that in 2009, a total of 18,000 livestock in Norway required uncontaminated feed for a period before slaughter, to ensure that their meat had an activity below the government permitted value of caesium per kilogram deemed suitable for human consumption. This contamination was due to residual radioactivity from Chernobyl in the mountain plants they grazed on in the wild during the summer. 1,914 sheep required uncontaminated feed for a time before slaughter during 2012, with these sheep located in only 18 of Norway's municipalities, a decrease from the 35 municipalities in 2011 and the 117 municipalities affected during 1986. The after-effects of Chernobyl on the mountain lamb industry in Norway were expected to be seen for a further 100 years, although the severity of the effects would decline over that period. The United Kingdom restricted the movement of sheep from upland areas when radioactive caesium-137 fell across parts of Northern Ireland, Wales, Scotland, and northern England. In the immediate aftermath of the disaster, the movement of a total of 4,225,000 sheep was restricted across a total of 9,700 farms, to prevent contaminated meat entering the human food chain. The number of sheep and farms affected has decreased since 1986. Northern Ireland was released from all restrictions in 2000, and by 2009, 369 farms containing around 190,000 sheep remained under the restrictions in Wales, Cumbria, and northern Scotland. The legislation used to control sheep movement and compensate farmers was revoked in 2012. Human impact lies abandoned with the Chernobyl facility visible in the distance. Acute radiation effects and immediate aftermath The only known causal deaths from the accident involved plant workers and firefighters. The reactor explosion killed two engineers, and 28 others died within three months from acute radiation syndrome (ARS). due to poorly substantiated reports of an individual who died during the evacuation of Pripyat from coronary thrombosis attributed to stress. Most serious ARS cases were treated with the assistance of American specialist Robert Peter Gale, who supervised bone marrow transplant procedures, although these were unsuccessful. The fatalities were largely due to wearing dusty, soaked uniforms causing beta burns over large areas of skin, and due to bacterial infections of the gastrointestinal tract. Long-term impact In the 10 years following the accident, 14 more people who had been initially hospitalized died, mostly from causes unrelated to radiation exposure, with only two deaths resulting from myelodysplastic syndrome. However, childhood thyroid cancer increased, with about 4,000 new cases reported by 2002 in contaminated areas of Belarus, Russia, and Ukraine, largely due to high levels of radioactive iodine. The recovery rate is ~99%, with 15 terminal cases reported. The iodine was initially viewed with less alarm than the other isotopes, because of its short half-life, but it is highly volatile and appears to have travelled furthest and caused the most severe health problems. Similarly in dose reconstruction, for those evacuated at different times and from various towns, the inhalation dose was dominated by iodine (40%), along with airborne tellurium (20%) and oxides of rubidium (20%) both as equally secondary, appreciable contributors. Long term hazards such as caesium tends to accumulate in vital organs such as the heart, while strontium accumulates in bones and may be a risk to bone-marrow and lymphocytes. Disputed investigation The mutation rates among animals in the Chernobyl zone have been a topic of ongoing scientific debate, notably regarding the research conducted by Anders Moller and Timothy Mousseau. Their research, which suggests higher mutation rates among wildlife in the Chernobyl zone, has been met with criticism over the reproducibility of their findings and the methodologies used. Withdrawn investigation In 1996, geneticist Ronald Chesser and Robert Baker published a paper on the thriving vole population within the exclusion zone, in which the central conclusion was essentially that "The mutation rate in these animals is hundreds and probably thousands of times greater than normal". This claim occurred after they had done a comparison of the mitochondrial DNA of the "Chernobyl voles" with that of a control group of voles from outside the region. The authors discovered they had incorrectly classified the species of vole and were genetically comparing two different vole species. They issued a retraction in 1997. Abortions Following the accident, journalists encouraged public mistrust of medical professionals. This media-driven framing led to an increase in induced abortions across Europe out of fear of radiation. An estimated 150,000 elective abortions were performed worldwide due to radiophobia. The statistical data exclude Soviet–Ukraine–Belarus abortion rates, which are unavailable. However, in Denmark, about 400 additional abortions were recorded, and in Greece, an increase of 2,500 terminations occurred despite the low radiation dose. Larger studies, such as the EUROCAT database, assessed nearly a million births and found no impacts from Chernobyl. Researchers concluded that widespread fear about the effects on unborn fetuses was not justified. The only robust evidence of negative pregnancy outcomes linked to the accident were the elective abortion effects due to anxiety. Studies on regions of Ukraine and Belarus suggest that around 50 children exposed in utero during weeks 8 to 25 of gestation may have experienced an increased rate of intellectual disability and lower verbal IQ. The Chernobyl liquidators fathered children without an increase in developmental anomalies or a significant rise in germline mutations. A 2021 study based on whole-genome sequencing of children of liquidators indicated no trans-generational genetic effects. Cancer assessments A report by the International Atomic Energy Agency examines the environmental consequences of the accident. The United Nations Scientific Committee on the Effects of Atomic Radiation estimated a global collective dose from the accident equivalent to "21 additional days of world exposure to natural background radiation"; doses were far higher among 530,000 recovery workers, who averaged an extra 50 years of typical natural background radiation exposure. Estimates of deaths resulting from the accident vary greatly due to differing methodologies and data. In 1994, thirty-one deaths were directly attributed to the accident, all among reactor staff and emergency workers. incidence in children and adolescents in BelarusWhile widely regarded as having a cause-and-effect relationship, the causality of Chernobyl with the increase in recorded rates of thyroid cancer is disputed. The Chernobyl Forum predicts an eventual death toll of up to 4,000 among those exposed to the highest radiation levels (200,000 emergency workers, 116,000 evacuees, and 270,000 residents of the most contaminated areas), including around 50 emergency workers who died shortly after the accident, 15 children who died of thyroid cancer, and a predicted 3,935 deaths from radiation-induced cancer and leukemia. A 2006 paper in the International Journal of Cancer estimated that Chernobyl may have caused about 1,000 cases of thyroid cancer and 4,000 cases of other cancers in Europe by 2006. By 2065, models predict 16,000 cases of thyroid cancer and 25,000 cases of other cancers due to the accident. Anti-nuclear groups, such as the Union of Concerned Scientists (UCS), have publicized estimates suggesting an eventual 50,000 excess cancer cases, resulting in 25,000 cancer deaths worldwide, excluding thyroid cancer. These figures are based on a linear no-threshold model, which the International Commission on Radiological Protection (ICRP) advises against using for risk projections. The 2006 TORCH report estimated 30,000 to 60,000 excess cancer deaths worldwide. The Chernobyl Forum revealed in 2004 that thyroid cancer among children was one of the main health impacts of the Chernobyl accident, due to ingestion of contaminated dairy products and inhalation of Iodine-131. More than 4,000 cases of childhood thyroid cancer were reported, but there was no evidence of increased solid cancers or leukemia. The WHO's Radiation Program reported nine deaths out of the 4,000 thyroid cancer cases. By 2005, UNSCEAR reported an excess of over 6,000 thyroid cancer cases among those exposed as children or adolescents. Well-differentiated thyroid cancers are generally treatable, with a five-year survival rate of 96% and 92% after 30 years. By 2011, UNSCEAR reported 15 deaths from thyroid cancer. However, the risk of thyroid cancer associated with the Chernobyl accident remains high according to published studies. The German affiliate of the International Physicians for the Prevention of Nuclear War suggests that 10,000 people have been affected by thyroid cancer as of 2006, with 50,000 cases expected in the future. Other disorders Fred Mettler, a radiation expert, estimated 9,000 Chernobyl-related cancer deaths worldwide, noting that while small relative to normal cancer risks, the numbers are large in absolute terms. The report highlighted the risks to mental health from exaggerated radiation fears, noting that labeling the affected population as "victims" contributed to a sense of helplessness. Possibly due to the Chernobyl disaster, an unusually high number of cases of Down syndrome were reported in Belarus in January 1987, but there was no subsequent upward trend. Long-term radiation deaths The potential deaths from the Chernobyl disaster are heavily debated. The World Health Organization predicted 4,000 future cancer deaths in surrounding countries, The Union of Concerned Scientists estimated approximately 27,000 excess cancer deaths worldwide, using the same LNT model. A study by Greenpeace estimated 10,000–200,000 additional deaths in Belarus, Russia, and Ukraine from 1990 to 2004. The report was criticized for relying on non-peer-reviewed studies, while Gregory Härtl, a WHO spokesman, suggested its conclusions were ideologically motivated. The publication Chernobyl: Consequences of the Catastrophe for People and the Environment claimed 985,000 premature deaths, but was criticized for bias and using unverifiable sources. Socio-economic impact It is difficult to establish the total economic cost of the disaster. According to Mikhail Gorbachev, the Soviet Union spent 18 billion Rbls ($ in today's dollars) on containment and decontamination, virtually bankrupting itself. In 2005, the total cost over 30 years for Belarus was estimated at US$235 billion. Ongoing costs remain significant; in their 2003–2005 report, the Chernobyl Forum stated that between five and seven percent of government spending in Ukraine is still related to Chernobyl, while in Belarus, over $13 billion was spent between 1991 and 2003. The economic loss is estimated at $235 billion in Belarus. and helped forge closer USSR–US relations at the end of the Cold War. have argued that the Chernobyl disaster was more likely to occur in a communist country than in a capitalist one. Soviet power plant administrators were reportedly not empowered to make crucial decisions during the crisis. == Significance ==

Nuclear debate , 1986 in West Berlin Because of the distrust many had in the Soviet authorities, who engaged in a cover-up, a great deal of debate about the situation occurred in the First World during the early days of the event. Journalists mistrusted many professionals, and in turn encouraged the public to mistrust them as well. , 2011 in 1986 dedicated to Chernobyl disaster. The accident also raised concerns about the cavalier safety culture in the Soviet nuclear power industry, slowing industry growth and forcing the Soviet government to become less secretive about its operating procedures. The government cover-up of the Chernobyl disaster was a catalyst for glasnost, which "paved the way for reforms leading to the Soviet collapse." Numerous structural and construction quality issues, as well as deviations from the original plant design, had been known to the KGB since at least 1973 and passed on to the Central Committee, which took no action and classified the information. In Italy, political fallout from the Chernobyl accident was reflected in the outcome of the 1987 nuclear power referendum. As a result, Italy began phasing out its nuclear power plants in 1988, a decision that was effectively reversed in 2008. A 2011 referendum reiterated Italians' objections to nuclear power, thus abrogating the government's 2008 decision. In Germany, the Chernobyl accident led to the creation of a federal environment ministry. The German environmental minister was given authority over reactor safety as well, a responsibility the minister still holds today. The Chernobyl disaster is also credited with strengthening the anti-nuclear movement in Germany, which culminated in the decision to end the use of nuclear power made by the 1998–2005 Schröder government. A temporary reversal of this policy ended with the Fukushima nuclear disaster. In direct response to the Chernobyl disaster, a conference to create a Convention on Early Notification of a Nuclear Accident was called in 1986 by the International Atomic Energy Agency. The resulting treaty has bound members to provide notification of any nuclear and radiation accidents that occur that could affect other states, along with the Convention on Assistance in the Case of a Nuclear Accident or Radiological Emergency. Chernobyl has been used as a case study in research concerning the root causes of such disasters, such as sleep deprivation and mismanagement. The State Archives of Ukraine hold an archival collection of around 1,000 declassified documents relating to the construction of the power station, the disaster, and its aftermath extending to the early 2000s. This archive was added by UNESCO to its Memory of the World International Register in 2017, recognising it as documentary heritage of global importance. In popular culture The Chernobyl tragedy has inspired many artists across the world to create works of art, animation, video games, theatre and cinema about the disaster. The HBO series Chernobyl and the book Voices from Chernobyl by the Ukrainian-Belarusian writer Svetlana Alexievich are two well-known works. The Ukrainian artist Roman Gumanyuk created a series of artworks called "Pripyat Lights, or Chernobyl shadows" that includes 30 oil paintings about the Chernobyl accident, exhibited in 2012–2013. The video game S.T.A.L.K.E.R.: Shadows of Chernobyl, developed by GSC Game World and released by THQ in 2007, is a first-person shooter game set in the Exclusion zone. A prequel called S.T.A.L.K.E.R.: Clear Sky was released in 2008 following with a sequel S.T.A.L.K.E.R.: Call of Pripyat released in 2010. Finally, the horror film Chernobyl Diaries released in 2012 is about six tourists that hire a tour guide to take them to the abandoned city of Pripyat where they discover they are not alone. Filmmakers have created documentaries that examine the aftermath of the disaster over the years. Documentaries like the Oscar-winning Chernobyl Heart released in 2003, explore how radiation affected people living in the area and information about the long-term side effects of radiation exposure. The Babushkas of Chernobyl (2015) is a documentary about three old women ("babushkas") who decided to return to the exclusion zone after the disaster. In the documentary, the women show the polluted water, their food from radioactive gardens, and explain how they manage to survive in this exclusion zone despite the radioactive levels. The documentary The Battle of Chernobyl (2006) shows rare original footage a day before the disaster in the city of Pripyat, then through different methods goes in depth on the chronological events that led to the explosion of reactor no.4 and the disaster response. The critically acclaimed 2019 historical drama television miniseries Chernobyl revolves around the disaster and the cleanup efforts that followed. == See also ==