The Chernobyl disaster (Ukrainian: Чорнобильська катастрофа, Tchornobylska katastrofa – Chernobyl Disaster; also known as Chernobyl accident) was a catastrophic nuclear accident that occurred between April 25 and 26, 1986 at nuclear reactor no. 4 of the Chernobyl Nuclear Power Plant near the town of Pripiat in northern Soviet Ukraine, near the border with Soviet Belarus.
The accident occurred during a safety test in the early hours of the morning that simulated a power outage at the station, during which emergency safety and power regulation systems were intentionally shut down. A combination of inherent failures in the reactor design, as well as the reactor operators who organized the core in a manner contrary to the checklist for testing, resulted in uncontrolled reaction conditions.
The overheated water was instantly turned into steam, causing a destructive steam explosion and a subsequent fire that threw graphite into the open air and produced considerable rising currents for about nine days. The fire was finally contained on May 4, 1986. The plumes of fission products thrown into the atmosphere by the fire rushed over parts of the Soviet Union and Western Europe. The estimated radioactive inventory that was released during the hottest phase of the fire was roughly equal in magnitude to the airborne fission products released in the initial explosion.
The total number of victims, including those killed as a disaster, remains a controversial and disputed issue. During the accident, the effects of the steam explosion caused two deaths inside the facility: one immediately after the explosion and one for a lethal dose of radiation. In the coming days and weeks, 134 military personnel were hospitalized with acute radiation syndrome (SAR), of which 28 firefighters and officials died in months.
In addition, about fourteen deaths from radiation-induced cancer among this group of 134 survivors occurred in the following ten years. Among the general population, a surplus of 15 infant deaths from thyroid cancer was documented in 2011. It will take more time and research to definitively determine the relatively high risk of cancer among surviving employees, those who were initially hospitalized with SAR, and the general population.
The Chernobyl disaster is considered the most disastrous nuclear accident in history, both in terms of cost and casualties. It is one of only two nuclear power accidents classified as a level 7 event (the maximum rating) on the International Nuclear Accident Scale, the other being the Fukushima I nuclear accident in Japan in 2011. The struggle to safeguard scenarios with the potential for a major catastrophe, along with subsequent decontamination efforts around the plant, involved more than 500,000 workers (so-called liquidators) and cost about 18 billion Soviet rubles.
The remains of reactor building number 4 were placed in a large penthouse called “Shelter Structure”, but known as “sarcophagus”. The purpose of the structure was to reduce the dispersion of dust debris and radioactive debris from the wreckage, thus limiting radioactive contamination and protection of the site against weather. The sarcophagus was completed in December 1986, at a time when what was left of the reactor was entering the cold shutdown phase.
The casing was not intended to be used as a radiation shield, but was quickly constructed as occupational safety for employees of the other undamaged reactors at the plant, such as number 3, which continued to produce electricity until the year 2000. An international team included reactor building number 4 and the original sarcophagus in a new, larger state-of-the-art cladding in 2017. The accident prompted improved safety in all RBMK reactors designed by the Soviet Union, the same type of Chernobyl, of which ten continued to power power electrical grids in 2019.
Context of the Chernobyl Disaster
The plant
The Vladimir Ilich Ulianov Power Plant, popularly known as the Chernobyl Nuclear Power Plant, located in Pripiat (Soviet Ukraine), began construction in August 1972 and was officially commissioned in September 1977, when its first reactor was installed, the second of which was put into activity the following year. Reactors 3 and 4 were installed in 1981 and 1983, respectively. The process of construction of reactors 5 and 6 was about to take place, but were suspended after the disaster in 1986. The Soviet Union envisioned building a total of twelve reactors, to be installed by 2010.
At the time of the disaster, the plant had four operational RBMK-1000 reactors capable of generating 1,000 megawatts (MW) of electricity (3,200 MW of thermal power) and accounted for 10% of Ukraine’s energy. Parallel to the construction of the plant, from 1970 the city of Pripiat was erected to serve as home to the plant workers and their families. Chernobyl was the first nuclear power plant on Ukrainian soil and the third with RBMK reactors, the other two being the Leningrad Nuclear Power Plant and the Kursk Power Plant.
Reactor cooling during power outage
In a normal operation, a significant fraction (more than 6%) of the power of a nuclear reactor is derived not from nuclear fission but from the radioactive decay (or radioactivity) of the accumulations of the fission process products. The heat generated from this decay continues even after the chain reaction has been stop (such as after an emergency stop, for example) and active cooling is necessary to prevent a nuclear meltdown from occurring. RBMK reactors, such as those present in Chernobyl, use water for cooling. Reactor #4 in Chernobyl had about 1,600 individual fuel channels, each of which required a cooling flow of 28 metric tons of water per hour.
Because cooling pumps also needed electricity and had to continue to operate for some time after an emergency shutdown in an event of power supply failure, each of the Chernobyl reactors had three diesel generators available as a backup. These generators could be activated within 15 seconds, but it took 60 to 75 seconds for them to reach full power and generate the 5.5 megawatts needed to operate one of the main pumps.
This one-minute delay represented a flaw in project security. It was theorized that the rotational inertia stored in the steam turbines and the residual pressure of the steam itself could be used to generate the electricity needed to fill this time gap. Analysis indicated that this could be enough to provide electricity for the operation of the cooling pumps for about 45 seconds, which did not completely fill the gap between an external power failure and the time it took for emergency generators to reach their full power.
Safety tests
This capability still needed to be confirmed experimentally and tests done before eventually failed. An initial test was carried out in 1982 and indicated that the energy generated in the turbines would not be enough, as it could not maintain the magnetic field after the turbine was fired. The system was then modified and the test was repeated in 1984, but again it was not successful. In 1985, a third test took place, also with negative results. A fourth test was then planned for April 1986 and was scheduled to coincide with a maintenance stop of reactor 4.
The test itself was not problematic, although its documentation of security measures no longer met modern standards. However, the developers of the test program were not aware of the unusual behavior of the RBMK-1000 reactor under the planned operating conditions. These checks were considered purely electrical tests and not a complex unit test, even if it involved critical system units. According to the regulations of the time, such a test did not require approval from the chief reactor designer (NIKIET), the scientific manager or the Soviet nuclear supervision regulator. The test also required the deactivation of some safety systems (in particular, the emergency central cooling system).
The experimental procedure is designed to happen as follows:
- The reactor should be running at low power, between 700 MW and 800 MW;
- The steam turbine generator should be running at full speed;
- When the required conditions were met, the steam supply for the turbine generator would be closed;
- The performance of the turbine generator should be recorded to determine whether it could provide the bridge power for the cooling pumps until the diesel generators were turned on and the power to the cooling pumps was automatically;
- After the emergency generators reached normal speed and voltage, the turbine generator could continue to turn down;
- The reactor maintenance stop should be completed.
Test delays and shift change
The test of April 25, 1986 would be conducted during the day shift, on the same date that reactor 4 was to be shut down for routine maintenance. Day shift employees were instructed in advance regarding the operational conditions of the reactor for testing, and in addition, a special team of electrical engineers would be present to conduct a one-minute test on the new voltage regulator system once the correct conditions had been met. As planned, a gradual reduction in the power production of the plant began at 1p. M. on April 25 and power reached 50% of the nominal level of 3,200 thermal MW at the beginning of the day shift.
The day shift performed many maintenance tasks unrelated to the test itself, which was supposed to begin around 2:15 p. M. with initial preparations, including the decommissioning of the emergency central refrigeration system. Meanwhile, another regional power station was unexpectedly shut down and, around 2 p. M., Kiev power grid controllers called for the reduction in Chernobyl’s power output to be delayed, as there was a huge energy demand in the early evening. The plant’s director, Viktor Bryukhanov, agreed and the test was postponed for ten hours.
The day shift was then replaced by the early evening shift. Despite the delays, the emergency central cooling system had remained disabled – it had been disconnected by a manual drawer valve, which in practice meant that two or three people spent the whole day having to turn the valves manually. This system would have no influence on the events that unfolded next, but allowing the reactor to operate for nearly eleven hours out of the test without emergency protection indicated a generally low level of Soviet safety culture.
At 11:04 p. M., Kiev power grid controllers allowed the reactor shutdown to proceed. This delay had very serious consequences: the day shift, which had been prepared for the test, had already left and the staff of the beginning of the night too, and the employees of the night shift had only begun to work around midnight, with several stages of the test already in progress. According to the plan, the test should have been completed before the end of the day shift and the night staff would only have to keep the thermal cooling systems by decay in a disconnected facility.
The night shift had limited time to prepare and run the experiment. Anatoly Diatlov, the deputy chief engineer of the Chernobyl Plant, was present to oversee the test and direct the experiment; since he was the employee with the highest rank among all those present at the plant, his orders and instructions were superior to that of the entire team in the control room. Serving under Diatlov were Aleksandr Akimov, head of the night shift, and Leonid Toptunov, the operator responsible for the reactor’s operations regime system, a function that included the movement of the control rods. Toptunov was a young engineer who had worked independently as a senior engineer for approximately three months.
Unexpected reactor energy drop
The test required a gradual reduction of the output energy from reactor No. 4 to a thermal level of 700–1 000 MW and a power of 720 MW was reached around 00:05 on April 26, 1986. Due to the production of a fission by-product in the reactor, Xenon-135, which is a neutron absorber that inhibits chain reaction, the core power continued to decrease in the absence of additional operator action, generating a process known as “reactor poisoning”. In standard procedure, this is avoided with the rapid burning of Xenon-135 until it becomes the highly stable Xenon-136. With the reactor’s power diminished, previously produced high amounts of iodine-135 were decaying at Xenon-135, for the neutron absorber, faster than the now reduced neutron flow could burn.
When the power had lowered to approximately 500 MW, the reactor control had been changed to a different mode to manually maintain the energy level. At this point, the power drops to an unexpected state of near shutdown of the reactor, with a power of 30 MW or less. The exact circumstances that caused the power outage are unknown because Aleksandr Akimov and Leonid Toptunov, nuclear engineers who, under the supervision of Anatoly Diatlov, were central parts in conducting the test, died of radioactive poisoning; initial reports placed the blame for the disaster on a Toptunov error, but this hypothesis was dismissed, taking into account mainly the technical problems at the plant.
The reactor was producing about 5% of the minimum initial power level prescribed for the test. This low reactivity inhibited the burning of Xenon-135 within the reactor core and prevented the increase in power. The employees in the control room had to raise the power by disconnecting most of the reactor control rods from the automatic control regulation system of the rods and manually extracted most of the rods to their upper limits in order to promote reactivity and neutralize the effect of reactor poisoning. Several minutes passed between the extraction of the rods and the point at which the power began to increase, until later stabilized at 160–200 MW.
Between 00:35 and 00:45, emergency alarm signals related to thermal hydraulic parameters were ignored, apparently to preserve the reactor’s low power. The reason for this disregard for security alarms is not known. Supposedly, Anatoly Diatlov, the plant’s deputy chief engineer, pressed his employees to proceed with the test despite apparent problems, even threatening to dismiss those who expressed some objection.
The flow exceeded the permitted limit at 1:19 a. M., triggering a low steam pressure alarm on the steam stoppers. At the same time, the extra flow of water reduced the overall core temperature and reduced the steam voids in the core and steam paraders. Because water is a weak neutron absorber (and the higher density of liquid water makes it a better absorber than steam), the activation of additional pumps has decreased the reactor power. The team responded to this by turning off the two circulation pumps to reduce the flow of feed water in an effort to increase steam pressure, and manually removing more control rods to maintain energy.
Accident
The disaster began during a test on April 26, 1986 at reactor 4 of the V. I. Lenin Nuclear Power Plant near Pripiat and near the administrative border with Belarus and the Dnieper River. There was a sudden and unexpected surge in energy. When operators attempted an emergency shutdown, there was a much greater increase in energy production.
This second peak led to a rupture of the reactor vessel and a series of steam explosions. These events exposed the reactor’s graphite moderator to the air, causing it to ignite. The following week, the resulting fire sent long plumes of highly radioactive dust into the atmosphere, causing radioactive precipitation in an extensive geographical area, including Pripiat. The plumes roamed large parts of the Soviet Union and Europe. According to official post-Soviet data, about 60% of them have reached Belarus.
Thirty-six hours after the accident, Soviet authorities established a 10-kilometer exclusion zone, which resulted in the rapid evacuation of 49,000 people, mainly from Pripiat, the nearest population center. During the accident the wind changed direction; the fact that the different reactor plumes had different proportions of radioisotopes indicates that the relative release rates of different elements of the site were changing.
As plumes and subsequent rainfall continued to be generated, the evacuation zone was increased from 10 km to 30 km about a week after the accident. Another 68,000 people were evacuated, including from the city of Chernobyl itself. The survey and detection of isolated precipitation points outside this area throughout the year resulted in the evacuation of another 135,000 people. Between 1986 and 2000 the total number of permanently resettled people from the most severely contaminated areas tripled to about 350,000.
The accident raised growing concerns about fission reactors around the world, and while the biggest concern was about those with similar designs, hundreds of different proposals for nuclear reactors, including those under construction in Chernobyl, reactors 5 and 6, were canceled. With the global issue being largely due to rising costs of new nuclear reactor safety standards and the legal and political costs of dealing with increasingly hostile and anxious public opinion, there was an abrupt drop in the rate of new openings after 1986.
The accident also raised concerns about the safety culture in Soviet nuclear power, slowing industry growth and forcing the Soviet government to become less secretive about its procedures. The cover-up of the Chernobyl disaster was a catalyst for glasnost, which “paved the way for the reforms that led to the Soviet collapse.”
Reactor shutdown and power excursion
At 01:23:40, as recorded by Skala’s centralized control system, a reactor scram (emergency shutdown) was initiated when the experiment was terminated. The shutdown probably occurred after the “AZ-5” button (which triggered the emergency shutdown of the nuclear reactor effected by the immediate closure of the fission reaction) was pressed.
This mechanism would even be used to routinely shut down the reactor after the maintenance experiment and the scram probably preceded the sharp increase in power. However, the exact reason why the AZ-5 button was pressed is uncertain, as only the late Akimov and Toptunov participated in this decision, although the atmosphere in the control room was calm at the time. Subsequently, rbmk designers stated that the button should have been pressed only after the reactor has already begun to self-destruct.
The insertion of the control rods in the reactor initially displaced water in the lower part of the reactor with neutron-moderating graphite. Thus, an emergency scram initially increased the chain reaction rate at the bottom of the nucleus. This behavior was discovered when the initial insertion of the control rods in another RBMK reactor at the Ignalina Nuclear Power Plant in 1983 induced a power peak. Procedural countermeasures were not implemented in response to the Ignalina incident; later, INSAG-7 stated, “Apparently, there was a general view that the conditions under which the positive effect of scram would be important would never occur. However, they appeared in almost every detail in the course of the actions that led to the accident.”
A few seconds later, a power spike occurred and the core overheated, causing some nuclear fuel rods to fracture and blocking the control rods, with graphite water shifters still at the bottom of the core. In three seconds, the reactor output and power rose above 530 MW. Subsequent events were not recorded by instruments, but reconstructed through mathematical simulation. By simulation, the peak energy would have caused an increase in fuel temperature and vapor buildup, leading to a rapid increase in steam pressure. This caused the fuel coating to fail, releasing the elements of this fuel into the coolant and breaking the channels in which those elements were located.
Steam explosions
When the scram was started, the reactor’s power output jumped to about 30,000 thermal MW, the last reading indicated on the power meter on the control panel and 10 times its normal operating output. Some, however, estimate that the peak energy may have been 10 times higher than this. It was not possible to reconstruct the precise sequence of the processes that led to the destruction of the reactor and the building of the power unit, but an explosion of steam, such as the explosion of a boiler by excess pressure, seems to have been the next event.
There is a general understanding that it was the explosive vapor pressure from the damaged fuel channels that escaped into the reactor’s external cooling structure and caused the explosion that destroyed its cladding, ripping off and exploding the top plate called the “upper biological shield”, where the entire reactor assembly is trapped, through the roof of the building. This is believed to be the first explosion many have heard.
A second, more powerful explosion occurred about two or three seconds after the first; this explosion dispersed the damaged core and effectively shut down the nuclear chain reaction. This explosion also further compromised the reactor containment vessel and ejected hot pieces of the moderating graffiti. The canals demolished in the reactor debris and graphite caught fire with exposure to air, greatly contributing to the spread of radioactive precipitation and contamination of peripheral areas.
According to observers outside Unit 4, pieces of burning material and sparks were thrown into the air above the reactor. Some of them fell on the roof of the engine room and started a fire. About 25% of the hot graphite blocks and overheated material from the fuel channels were ejected. Parts of the graphite blocks and fuel channels were outside the reactor building. As a result of damage to the building, an airflow through the core was established by the high core temperature. The air ignited the hot graffiti and started a fire.
After the larger explosion, several plant employees left to get a clearer view of the extent of the damage. One of these survivors, Alexander Yuvchenko, says that as soon as he went out and looked into the reactor room he saw a “very beautiful” blue beam of light, similar to laser, caused by the brightness of the ionized air that seemed to “flood infinity”.
Initially, there were several hypotheses about the nature of the second explosion. One version was that the second explosion was caused by hydrogen combustion, produced by the reaction of superheated zirconium vapor or by the reaction of incandescent graphite with vapor that produced hydrogen and carbon monoxide. Another hypothesis, by Checherov, published in 1998, was that the second explosion was a thermal explosion of the reactor as a result of the uncontrollable leakage of neutrons caused by the complete loss of water in the reactor core.
Nuclear explosion hypothesis
The strength of the second explosion and the proportion of Xenon radioisotopes released after the accident (a vital tool in nuclear forensic investigation) indicated to Iuri V. Dubasov, in a 2009 publication, that the second explosion could have been a nuclear energy transient resulting from the melting of core material in the absence of its refrigerant and water moderator, this allowed for a dangerous “positive feedback” leak condition, given the lack of passive safety stops, such as Doppler flare, when energy levels begin to increase above the commercial level.
Evidence for this hypothesis originates in Tcherepovets, Vologda Oblast, Russia, 1,000 km northeast of Chernobyl. Physicists at the V. G. Khlopin Radiological Institute in Leningrad measured anomalous levels of Xenon-135 – a half-life isotope – at Tcherepovets four days after the explosion, even when the general distribution was spreading radiation north in Scandinavia. It is thought that a nuclear event in the reactor may have increased Xenon to higher levels in the atmosphere, which moved Xenon to that location.
Although this power outage with positive feedback, which increased until the reactor dismounted through its internal energy and external steam explosions, is the most accepted explanation for the cause of the explosions, Dubasov argues that an immediate critical point occurred, with internal physics being more akin to the explosion of a failed nuclear weapon, which would have produced the second explosion.
This hypothesis of nuclear failure, mostly advocated by Dubasov, was further examined in 2017 by retired physicist Lars-Erik De Geer in an analysis that places the hypothetical failure event as the most likely cause of the first explosion, not the second.
The second explosion, more energetic and which produced most of the damage, was estimated by Dubasov in 2009 as equivalent to 40 billion joules of energy, equivalent to about 10 tons of TNT. The 2009 and 2017 analyses argue that the nuclear failure event, which would have produced the second or first explosion, consisted of a rapid chain reaction (as opposed to the neutron-mediated chain reaction consensus) that was limited to a small portion of the core reactor, since the expected self-disassembly occurs rapidly in disintegration events.
Lars-Eric De Geer commented:
“We believe that nuclear explosions mediated by thermal neutrons at the bottom of several fuel channels in the reactor caused a jet of debris to fire upward through the refueling pipes. This jet hit the 350 kg plugs of the pipes, continued through the ceiling and traveled in the atmosphere, at altitudes of 2.5 to 3 km, where weather conditions provided a route to Tcherepovets. The steam explosion that ruptured the reactor compartment occurred about 2.7 seconds later.”
Crisis management during the Chernobyl Disaster
Fire fighting
Contrary to the safety regulation, bitumen, a combustible material, was used in the construction of the roof of the reactor building and in the turbine hall. The ejected material ignited at least five fires on the roof of the adjacent reactor No. 3, which was still in operation. It was imperative to put out these fires and protect the cooling system from reactor number three. Inside this reactor, the head of the night shift, Yuri Bagdasarov, wanted to shut it down immediately, but chief engineer Nikolai Fomin did not allow it. Plant operators received iodized potassium respirators and tablets and were then instructed to continue working. At 05:00, Bagdasarov decided to shut down Reactor 3, leaving behind only the workers responsible for the emergency cooling system.
About twenty minutes after the accident, around 1:45 a. M., firefighters began arriving at the plant complex. The first firefighters to arrive were from the unit led by Lieutenant Volodymyr Pravik. They were not told how dangerous the radioactivity of the site was, in the smoke and debris, and many thought it was just a common surface fire, perhaps from an electrical fault. Lieutenant Pravik died on May 9, 1986, thirteen days after the accident, as a result of radiation poisoning. Grigori Khmel, one of the fire truck drivers, described what happened:
We arrived about 10 or 15 minutes before 2 am… We saw graffiti strewn across the floor. Misha (a fireman) asked, “Is this graffiti?” I kicked [the object] to the side. But one of the firefighters from another truck took the graffiti by hand. “It’s hot,” he said. The graphite pieces were of different sizes, some large, others small, enough to pick them up […] We didn’t know much about radiation. Even those who worked there had no idea. There was no more water in the trucks. Misha filled a cistern and we aimed the water at the ceiling. And then the boys who died went to the ceiling — Vashchik, Kolya and others, and Volodya Pravik […] They went up the stairs… and I never saw them again.
Anatoly Zakharov, a firefighter stationed in Chernobyl since 1980, offered a different description in 2008: “I remember playing with others, ‘There must be an incredible amount of radiation here. We’ll be lucky if we’re still alive in the morning.’”He also stated, “Of course we knew! If we had followed the regulations, we never would have gotten close to the reactor. But it was a moral obligation —our duty. We were like kamikaze.”
The immediate priority was to extinguish the fire outbreaks on the roof of the station and in the area around the building containing reactor 4 (which exploded) to protect reactor 3 and keep its cooling system intact. The surface fires had already been extinguished around 5:00 a. M., but by then many of the firefighters had already received lethal doses of radiation. The fire inside reactor 4 continued until May 10, 1986; it is possible that half of the reactor graphite burned to ashes.
At the time, the reactor fire was believed to have been extinguished by a combined effect of helicopters that threw more than 5,000 metric tons of neutral sand, lead, clay and absorbing boron into the burning reactor. It is now known that nothing that absorbed the neutrons could penetrate the nucleus. Historians estimate that at least 600 Soviet pilots had to fly over the reactor, risking receiving high doses of radiation, making hundreds of flights to cover reactor 4.
Eyewitnesses from firefighters heard before they died (as reported by CBC television series Witness) reported their descriptions of the tragedy, with one firefighter describing his experience with radiation as “having a taste of metal” and also feeling something alike to having pins and needles piercing the skin of his face. These reports were similar to that described by Louis Slotin, a Manhattan Project physicist who died after receiving a lethal dose of radiation during a critical accident.
The explosion and fire threw into the air nuclear fuel particles and other more dangerous fission products, as well as radioactive isotopes such as Cesium-137, iodine-131, strontium-90 and other radionuclides. Residents of the surrounding areas observed the radioactive cloud on the night of the explosion.
To deal with various issues such as debris and other contaminated products, the Soviets initially used several remotely controlled excavators and robot cars to detect radiation and throw away radioactive debris, but these initiatives mostly failed. Valery Legasov, first assistant director of the Kurchatov Institute of Nuclear Energy in Moscow, said in 1987: “But we learned that robots are not the great remedy for everything. Where there was very high radiation, the robot ceased to be a robot —the electronics stopped working.”
Radiation levels
Ionizing radiation levels in the most affected areas in the reactor building were estimated at 5.6 roentgens per second (R/s), equivalent to 20,000 roentgens per hour. A lethal dose of radiation is equivalent to 500 roentgens (or ~5 Gray (Gy) in modern units) for a period of five hours.
Many plant workers received nearly five times the lethal dose of radiation, absorbing it fully in less than a minute. One dosimeter capable of measuring up to 1,000 R/s was buried under the rubble of the collapsed building and another failed to be turned on. All other dosimeters only had a range of 0.001 R/s and showed only the reading of “measurement beyond scale”. Thus, workers in the reactor area were aware of contamination of only 3.6 R/h, although in many areas the radiation was extremely higher than that.
Due to the inaccurate low readings, Aleksandr Akimov, the head of reactor workers, assumed that reactor 4 was probably intact. Evidence of graphite and nuclear fuel scattered throughout the building was ignored, and new measurements, which began arriving around 4:30 a. M., showing higher radiation levels were also ignored because it was assumed that the equipment was defective, thus explaining the discrepancy in the measurements.
Akimov remained with his companions in the control room in the reactor building until morning, sending subordinates to try to continue sending water to the reactor, possibly doing so on the orders of Anatoly Diatlov, the deputy chief engineer of the Chernobyl plant. None of the control room workers used any protective equipment. Most of them, including Akimov, died within three weeks as a result of radiation poisoning. Diatlov, who oversaw the test that led to the accident, was initially in denial that the reactor exploded. He later said that this was a widespread opinion among the team in the control room.
Upon leaving the building, Diatlov went directly to Nikolai Fomin (the plant’s chief engineer) and Viktor Bryukhanov (plant director), where he stressed to the two that the reactor core was intact. Bryukhanov met party representatives in Pripiat and informed the Central Committee of the incident, trying to avoid guilt and stating that the disaster was not as bad as it seemed. He was removed from his post less than 24 hours after the accident, along with Fomin, while Diatlov was sent to the hospital with symptoms of acute radiation syndrome. The three survived and would be held responsible for the Chernobyl accident.
Evacuation
The town of Pripiat, adjacent to the power plant, was not immediately evacuated. The local population, on the night of the accident, continued with their lives normally, completely oblivious to what was happening. However, in the hours after the explosion, some people began to get sick. It was reported that many people in Pripiat began to suffer from severe headaches and to feel metallic taste in the mouth, along with severe coughs and vomiting. As the plant was run by Moscow staff, the Ukrainian government was not promptly notified of the accident.
Valentyna Shevtchenko, the President of the Verkhovna Rada, the Supreme Soviet of Soviet Ukraine, recalled that the acting Minister of the Ukrainian Interior, Vasyl Durdynets, phoned her at 9:00 a. M. to inform her of the current events; only at the end of the conversation did he inform her of the fire at the Chernobyl Power Plant, but stated that it had been controlled and that everything had returned to normal. When Shevtchenko asked “How are the people?”, he replied that there was nothing to worry about: “Some are celebrating weddings, others taking care of the garden and others fishing on the Pripiat River.
“Shevchenko then spoke on the phone with Volodymyr Shtcherbytsky, head of the Central Committee of the Ukrainian Communist Party and the de facto head of state of Soviet Ukraine, when he said he hoped that a commission, under the command of Boris Shcherbina, vice-president of the Council of Ministers of the Soviet Union, would be formed and sent to Pripiat.
The crisis management committee was established on the afternoon of the day of the accident to investigate what happened. The commission was formed by Boris Shcherbina, under the instructions of Premier Mikhail Gorbatchov; the group of scientists was headed by Valeri Legasov, assistant director of the Kurchatov Institute, and included prominent scientists such as nuclear specialist Evgeny Velikhov, hydro-meteorologist Iuri Izrael, radiologist Leonid Ilin, among others.
They flew to Boryspil International Airport and arrived at the plant on the night of April 26. By then, two people had already died and 52 others had been hospitalized with radioactive poisoning. The delegation soon obtained ample evidence that the reactor had been destroyed and that high levels of radiation were being dumped into the atmosphere, leading to countless contaminations. In the early morning hours of April 27, approximately 36 hours after the explosion, the evacuation of Pripiat was ordered. It was initially planned that the city would be uninhabited for only three days; later this decision became permanent.
At 11:00 a. M. on April 27, bus fleets began arriving in Pripiat for evacuation. The citizens began to be removed at 14:00 and were informed that they had only a few minutes to evacuate and could carry very few belongings per person. The translated ad looked like this:
To the attention of the residents of Pripiat! The City Council reports that due to the accident at the Chernobyl Power Plant in the city of Pripiat, radioactive conditions nearby are deteriorating. The Communist Party, its authorities and the armed forces are taking the necessary steps to combat this. Even so, in order to keep the people safe and healthy as possible, children being top priority, we need to evacuate citizens to the cities closest to the Kiev region.
For these reasons, starting on April 27, 1986, 2:00 p. M., each apartment block will have a bus at its disposal, supervised by police and city authorities. It is highly recommended to take with you your documents, some vital personal belongings and a certain amount of food as a precaution. Senior executives at the city’s public and industrial facilities have set out a list of employees who are needed to stay in Pripiat to keep the facilities in good working order. All houses will be guarded by police during the evacuation period. Comrades, leaving your homes temporarily, please make sure you have turned off your lights, electrical equipment and water and closed all the windows. Please remain calm and order in the process of this short-term evacuation.
To expedite the evacuation, residents were instructed to take only what was needed, telling them they would be away for only three days. As a result, most people have left behind much of their personal property, many of which remain there to the present day. By 3 p. M., some 53,000 people had already been evacuated to nearby villages in the Kiev region. The next day, plans began to expand the evacuations in an area of more than 10 km. Ten days after the accident, the evacuation zone was expanded again to encompass 30 km. The Chernobyl Exclusion Zone was then implemented, which remains to this day, although its shape and size have been expanded over time.
In all, some 135,000 people were permanently evacuated from Pripiat and surrounding areas. Between 1986 and 2000, permanent re-escalation affected some 350,000 people inside Kiev Oblast due to the Chernobyl disaster.
Late announcement
The evacuation of Pripiat began before the Soviet Union formally acknowledged the accident. On the morning of April 28, radiation levels became so high that they were detected at Sweden’s Forsmark nuclear power plant, more than 1,000 kilometers away from Chernobyl. Forsmark workers reported the case to the Swedish Radiological Safety Authority, which determined that the radiation originated elsewhere. On the same day, the Swedish government contacted the Soviet political leadership in Moscow asking if there was a nuclear accident on the territory of the Soviet Union. The Soviets initially denied any incident, but when the Swedes suggested they would register an official alert with the International Atomic Energy Agency, the Soviet government admitted to the world the accident that happened in Chernobyl.
At first, the Soviets claimed that the accident had been “small”, but after they had evacuated 100,000 people from the region, the international community finally became aware of the magnitude of the situation. At 9:02 p. M. on April 28, the Soviet government issued its first official statement on the disaster on national television. The late announcement lasted approximately 20 seconds and was read on the TV show Vremya: “There was an accident at the Chernobyl Power Plant. One of the nuclear reactors was damaged.
The effects of the accident are being remedied. Assistance has been provided to those affected. An investigative committee has been set up.”That was the whole message. The TASS news agency then discussed the Three Mile Island Crash and other nuclear disasters on U. S. soil, a common example of Soviet tactics known as whataboutism (a fallacy version of the Tu quoque). However, the announcement that a crisis management committee had been set up indicated, for outside observers, the seriousness of the accident, and subsequent messages were replaced by classical music, a common method to prepare the public for the announcement of a tragedy.
At the same time, ABC News released its own report on the disaster. U. S. news agency UPI initially said 2,000 people had died, citing a source inside Pripiat. The Soviet government denied and claimed that only two people had died in the first twenty-four hours of the accident. Both sides of the Cold War, following the “game theory”, tried to paint the other side in the worst possible way.
Valentyna Shevchenko was the first Ukrainian official to visit the disaster site on April 28. She then talked to the medical staff and people in the city, who were calm and hoped to resume normal life in their homes. Shevchenko returned home at midnight, stopping at a radiological checkpoint in Vilcha, one of the first of its kind installed in the region after the disaster.
There was a notification from Moscow that there was no reason to postpone the International Labor Day celebrations on May 1 in Kiev (including the parades), but on April 30 there was a meeting of the Central Committee of the Ukrainian Communist Party to discuss plans for the Labor Day parade. Scientists reported that the radiological level measured in Kiev was normal.
At the meeting, which ended at 6 p. M., it was decided that the celebrations would be shortened from four hours to less than two. Several buildings in Pripiat were kept open for use by plant workers. Among these buildings were the Jupiter factory, officially closed in 1996, and the Azure Pool complex, which closed in 1998, both of which were used by the Chernobyl Liquidators.
Risk of more explosions
Two floors of bubbling pools underneath the reactor served as a large water reservoir for emergency cooling pumps and as a pressure suppression system capable of condensing steam in the event of a small broken steam pipe; the third floor above them, below the reactor, served as a radiation tunnel. The steam released by a broken pipe should have entered the steam tunnel and guided up the bubbling pools through a layer of water. After the disaster, swimming pools and out pools were flooded because of the rupture of cooling water pipes and accumulated firefighting water and constituted a serious risk of steam explosion.
Steaming graphite, fuel and other nuclear materials, recording a temperature above 1 200 °C, began to burn the reactor floor and blend with the molten concrete of the reactor coating, creating a corium, a semi-liquid radioactive material compared to lava (which in the case of Chernobyl became known as “Elephant’s Foot”). If this mixture had melted down the floor in the water pool, it was feared that it could create a series of steam explosions serious enough to eject back into the atmosphere huge amounts of radioactive materials from the reactor. To avoid this, it would be necessary to drain the pool formed under the reactor.
The bubbling pool could be drained by opening its lock gates. However, the valves controlling this were underwater, located in a flooded corridor in the basement of the complex. Volunteers in wetsuits and respirators (for protection against radioactive aerosols) and equipped with dosimeters, entered the deep radioactive water and managed to open the floodgates. The men who achieved this feat were engineers Alexei Ananenko and Valeri Bezpalov (who knew where the valves were), accompanied by the shift supervisor, Boris Baranov.
With the success of this operation, the risk of further steam explosions has been eliminated. Initially, it was believed that the three men would have died due to radioactive poisoning in the days following the incident, but they actually survived. In May 2018, they received the Order for Courage medal from President Petro Poroshenko. In fact, Alexei Ananenko continues to work in the nuclear industry and often gets annoyed by the journalistic sensationalism that surrounds Chernobyl. Bezpalov was alive until 2005 when he died of heart failure at the age of 65. When the water gates were opened by Ananenko’s group, firemen’s water pumps were used to drain the basement. The emptying was only completed on May 8, after 20,000 metric tons of water were drained.
Even without the bubbling pool in the plant’s canes, there was still a possibility that a steam explosion would happen if the molten core had reached the groundwater below the reactor. To reduce the possibility of this happening, it was decided to freeze the land under the reactor, which would also stabilize the foundations below the plant. Using oil well drill rig, liquid nitrogen injection began on May 4. It was estimated that 25 metric tons of liquid nitrogen were used per day to keep the soil frozen at −100 °C. That idea was soon abandoned.
As an alternative to nitrogen, coal mine workers were used to tunnel under the reactor to make room for a cooling system. The final improvised design for the cooling system was to incorporate a spiral formation of water-cooled pipes covered at the top with a thin layer of thermally conductive graphite. The graphite layer as a refractory material would quickly cool the possible molten uranium oxide without burning.
This graphite cooling plate layer should be encapsulated between two layers of concrete, each one meter thick for stabilization. This system was designed by Leonid Bolshov, the director of the Institute of Security and Nuclear Development, formed in 1988. Bolshov’s graphite-concrete “sandwich” would be similar in concept with the corium reclaimer that is now part of many nuclear reactor projects.
Bolshov’s graphite cooling plate, along with the previous nitrogen injection proposal, were not used after the drop in air temperatures and reports indicating that the fuel melt had slowed. It was later determined that the fuel had passed through three floors before resting in one of the several basement rooms. The underground precautionary channel with its active cooling system was then considered redundant, as the fuel was self-refrigerant. The excavation was then simply filled with concrete to strengthen the foundations below the reactor.
Removal of debris
In the months following the explosion, as the cleanup effectively followed the regions surrounding Chernobyl, the authorities’ attention turned to the removal of radioactive debris from the roof of the plant.
The worst radioactive debris was collected inside what was left of the reactor; however, it was estimated that there was approximately 100 tons of debris on that roof as a result of the explosion and that it had to be removed to ensure the safe construction of the “Sarcophagus” – a concrete structure that would bury the reactor and reduce the radioactive dust that the open core dumped into the atmosphere. The initial plan was to use robots to clean up the debris on the roof.
The Soviets used at least 60 remotely controlled robots, but most of them were lost, as the high levels of radiation in the area destroyed the machines’ electronic circuits, rendering them useless.
Consequently, the most radioactive materials had to be removed with shovels by liquidators of the Soviet army (called ‘bio-robots’ by the authorities). The liquidators used their shovels and removed the radioactive debris and got rid of most of them by throwing them back into the open reactor, staying on the roofs for approximately 40 to 90 seconds at a time, as the high levels of radiation in the reactor building 4 prevented continuous work, causing any exposure greater than 90 seconds to be fatal.
Among the most radioactive materials were pieces of graphite coming from the reactor core. Initial orders were given so that groups of military personnel cleaning the ceiling should do only one shift, to avoid overexposure to radiation, but many soldiers did so five or six times. Only 10% of the debris on the roof of the reactor building was removed by robots, with the rest being cleaned by approximately 5,000 men who, on average, absorbed at least 25 rem (250 mSv) of radiation.
At that time there was still fear that the reactor might explode a second time and a containment structure was planned to prevent rain from entering the core and could trigger the onset of an explosive chain reaction. There was also a need to contain the radioactive material that leaked from the reactor and went up into the atmosphere.
The construction of the “Sarcophagus” (as it became known) was one of the largest civil engineering tasks in history, involving 250,000 workers who took to work to avoid being exposed to too much radiation. Ukrainian filmmaker Vladimir Shevchenko, who was filming reactor 4 cleanup efforts, captured on video the crash of a Mi-8 helicopter on October 2, 1986, when the aircraft’s propellers tangled with a crane handle. The helicopter crashed near the damaged reactor and its four crew members died in the crash.
On December 21, 1986, the concrete structure of the “Chernobyl Sarcophagus” began to be erected in the Reactor 4 building to seal the core and all the radioactive debris leaking from it. Meanwhile, the liquidators continued with the cleaning of the entire region around Chernobyl, cleaning the buildings and streets with water and a substance called “Bourda”, a sticky polymerization fluid created to engage radioactive dust and when dry could be removed and compacted in configurations. A “cleaning medal” was given to the workers.
Although many of the emergency vehicles used during crisis management have been buried in trenches, most vehicles, such as those of liquidators, including trucks and helicopters, still remain abandoned in open fields in the Chernobyl area.
Since then, scavengers and onlookers have invaded the site and removed several parts of these vehicles, even though many of them are still radioactive. The liquidators worked under deplorable conditions, not receiving much information about the magnitude of the danger they faced and several did not have adequate protections. Many, or perhaps most, were exposed to radiation levels higher than considered safe. In the decades after the disaster, many of the liquidators had to battle in court to get the right to gain government health care to cope with the consequences of radiation on their health.
During the construction of the sarcophagus, a group of scientists reentered the reactor room as part of an investigation known as the “Complex Expedition”, to locate and contain nuclear fuel to prevent a second explosion. These scientists manually collected several cold fuel rods, but immense heat still emanated from the core. Radiation rates in different parts of the building were monitored by drilling holes in the reactor and inserting long metal detector tubes. The area was still incredibly dangerous and most scientists were exposed to high levels of radiation and contaminated dust.
After six months of research in December 1986, with the help of a remote camera, scientists discovered an intensely radioactive mass more than two meters wide in the basement of Unit Four, which they called “Elephant’s Foot” for its wrinkled appearance. Later, in 1996, a photo of the substance was taken by Artur Korneyev and sent to the United States for analysis.
The caption of the photo said that the man who photographed the substance died a short time later, indicating that, even after a decade, the “Elephant’s Foot” remained highly radioactive. The mass of this substance was composed of melted sand, concrete and a huge amount of nuclear fuel that escaped from the reactor. The concrete under the reactor was steaming hot, and was destroyed by now solidified lava and unknown crystalline forms called chernobilite. It was then concluded, in this investigation, that a second reactor explosion was no longer possible.
Clean-up efforts within the contaminated areas around Chernobyl and nearby towns and villages lasted more than six months and was a mammoth task. The official reason that the dangerous decontamination efforts started so early, rather than waiting some time for radiation levels to fall naturally, was that the government intended to repopularly repopular that area and reuse its soil for cultivation. Within fifteen months, close to 75% of the land around the Chernobyl area was already suitable for re-cultivation, although less than a third of abandoned villages were repopulated.
The city of Pripiat remains abandoned and the Ukrainian government does not allow its original occupants to return to the area to claim their former homes, although the region is open for tourist visitation (with many areas remaining prohibited due to the risk of contamination). In the fields, the land that was cultivated again came to have little value and agriculture in the region as a whole became marginal. In several areas, especially in Belarus, the soil remains highly contaminated. According to historian David Marples, the Soviet government had a greater psychological purpose with cleanup: they wanted to avoid panic over nuclear power and even keep the Chernobyl Power Plant active.
According to the WHO, about 240,000 workers were engaged in cleanup activities in the Chernobyl area between 1986 and 1987. In total, more than 600,000 people worked as liquidators.
Causes of the Chernobyl Disaster
INSAG-1 Report (1986)
The first official explanation of the accident, later considered erroneous, was published in August 1986. He effectively blamed the power plant operators. To investigate the causes of the accident, the International Atomic Energy Agency (IAEA) has created a group known as the International Nuclear Safety Advisory Group (INSAG), which in its 1986 report, INSAG-1, also supported this view, based on data provided by the Soviets and oral statements from experts. In this view, the catastrophic accident was caused by serious violations of operational rules and regulations. “During the preparation and testing of the turbine generator under operating conditions using auxiliary load, employees disconnected a number of technical protection systems and violated the most important operational safety provisions for conducting a technical exercise.”
INSAG-7 Report (1992)
Ukraine disqualified several KGB documents from the period between 1971 and 1988 related to the Chernobyl plant, mentioning, for example, previous reports of structural damage caused by negligence during the construction of the plant (such as the division of concrete layers) that were never put into practice. They documented more than 29 emergency situations at the plant during this period, 8 of which were caused by negligence or poor competence on the part of staff.
In 1991, a Commission of the USSR State Committee for The Supervision of Safety in Industry and Nuclear Energy reassessed the causes and circumstances of the Chernobyl accident and came to new perspectives and conclusions. Based on this, in 1992 the IAEA Nuclear Safety Advisory Group (INSAG) published an additional report, INSAG-7, which reviewed “that part of the INSAG-1 report in which primary care is given to the reasons for the accident” and included the USSR State Commission report as Appendix I.
According to the INSAG-7 report, the main reasons for the accident are the peculiarities of physics and the construction of the reactor. There are two reasons:
- The reactor had a dangerously high fraction of positive void. Put simply, this means that if vapor bubbles form in the cooling water, the nuclear reaction accelerates, leading to overspeed if there is no intervention. Worse, with low load, this empty coefficient was not compensated by other factors, which made the reactor unstable and dangerous. The operators were unaware of this danger and this was not intuitive for an untrained operator;
- A more significant reactor defect was the design of the control rods. In a nuclear reactor, control rods are inserted into the reactor to decrease the reaction. However, in the RBMK reactor design, the tips of the control rods were made of graphite and the extensors (the final areas of the control rods above the tips, measuring one meter in length) were hollow and full of water, while the rest of the rod – the really functional part that absorbs the neutrons and therefore for the reaction – was made of carbon-boron. With this design, when the rods were inserted into the reactor, the graphite tips displaced a quantity of the cooler (water). This increases the rate of nuclear fission, since graphite is a more potent neutron moderator. Then in the first few seconds after the activation of the control rods, the power of the reactor increases, instead of decreasing, as desired. This behavior of the equipment is not intuitive (on the contrary, the expected would be that the power would start to drop immediately), and, mainly, it was not known to the operators.
Impact of the Chernobyl Disaster
Environmental
Although no informative comparison can be made between the accident and a strictly air-blown nuclear detonation, it has still been approaching that about four hundred times more radioactive material was released from Chernobyl than by the bombings of Hiroshima and Nagasaki in Japan during World War II.
In contrast, the Chernobyl accident released about one hundredth to one-thousandth of the total amount of radioactivity released during the era of nuclear weapons testing at the height of the Cold War between the years 1950 and 1960, ranging from 1/100 to 1/1,000 due to attempts to make comparisons with different spectrums of released isotopes. Approximately 100,000 square kilometers of land were significantly contaminated with nuclear ash, with the most affected regions in Belarus, Ukraine and Russia. Lower levels of contamination have been detected throughout Europe, except in the Iberian Peninsula.
Initial evidence that a large release of radioactive material was affecting other countries came not from Soviet sources, but from Sweden. On the morning of April 28, workers at the Forsmark Nuclear Power Plant (approximately 1,100 km (680 mi) from the Chernobyl site) had radioactive particles on their clothes. It was Sweden’s search for the source of radioactivity, after they determined that there was no leak at the Swedish plant, which at noon on April 28 led to the first indication of a serious nuclear problem in the Western Soviet Union. Thus, the evacuation of Pripiat on April 27, 36 hours after the initial explosions, was quietly completed before the disaster became known outside the Soviet Union. Rising radiation levels had already been measured in Finland, but a public service strike delayed response and publication.
The contamination of the Chernobyl accident was spread irregularly, depending on the weather conditions. A lot of radioactive material has been deposited in mountainous regions such as the Alps, welsh mountains and the Scottish Highlands, where adiabatic cooling caused radioactive rain. The stains resulting from contamination were often localized and the water flows in the soil contributed even more to large variations in radioactivity in small areas. Sweden and Norway also experienced heavy rainfall when contaminated air collided with a cold front, which caused rain.
Like many other radioactivity releases in the environment, the release of Chernobyl was controlled by the physical and chemical properties of the radioactive elements in the nucleus. Particularly dangerous are highly radioactive fission products, those with high rates of nuclear decay that accumulate in the food chain, such as some of the isotopes of iodine, cesium and strontium. iodine-131 and Cesium-137 account for most of the radiation exposure received by the general population.
The Chernobyl nuclear power plant is located next to the Pripiat River, which powers the Dnieper reservoir system, one of Europe’s largest surface water systems, which at the time supplied Kiev’s 2.4 million inhabitants and was still flooded when the accident occurred. Radioactive contamination of aquatic systems, therefore, became a major problem immediately after the accident. In the most affected areas of Ukraine, radioactivity levels (particularly radionuclides I, Cs and Sr) in drinking water caused concern during the weeks and months after the accident, although it was officially declared that all contaminants had settled at the bottom site “in an insoluble phase” and would not dissolve for 800-1,000 years.
Guidelines for radioiodine levels in drinking water were temporarily increased to 3,700 Bq/L, allowing most of the water to be reported as safe, and a year after the accident it was announced that even water from the Chernobyl plant’s cooling pond was within acceptable standards. Despite this, two months after the disaster, Kiev’s water supply was abruptly moved from the Dnieper River to the Desna River. Meanwhile, huge sediment traps have been built, along with a huge 30-m-deep underground barrier of the destroyed reactor that enters the Pripiat River.
The bioaccumulation of radioactivity in fish resulted in concentrations (both in Western Europe and in the former Soviet Union) which, in many cases, were significantly above the maximum levels of orientation for consumption. The maximum levels of guidance for radiocalysium in fish vary from country to country, but are approximately Bq 1 000/kg in the European Union. In the Kiev reservoir in Ukraine, fish concentrations were several thousand Bq/kg during the years after the accident.
In small “closed” lakes in Belarus and the Briansk region in Russia, concentrations in various fish species ranged from 100 to 60,000 Bq/kg during the period 1990-92. Fish contamination has also caused short-term concern in parts of the UK and Germany and long-term (years instead of months) in the affected areas of Ukraine, Belarus and Russia, as well as in parts of Scandinavia.
After the disaster, four square kilometers of pine forest, directly in the direction of the reactor, became reddish-brown and died, earning the name “Red Forest”. Some animals in the hardest-hit areas also died or stopped breeding. Most of the domestic animals were removed from the exclusion zone, but the horses left on an island on the Pripiat River, 6 km from the plant, died when their thyroid glands were destroyed by radiation doses of 150-200 Sv. Some cattle on the same island died and those who survived suffered from rickets because of thyroid damage. The next generation seemed normal.
The aftereffects of Chernobyl were expected to be seen for another hundred years, although the severity of the effects decreased during this period. Scientists report that this is due to radioactive Cesium-137 isotopes being absorbed by fungi such as Cortinarius Caperatus, which in turn is eaten by sheep while grazing. A robot sent to the reactor returned with samples of black radiotrophic fungi, rich in melanin, that grow on the reactor walls.
Human
After the accident, 237 people suffered from acute radiation syndrome (SAR), of which 31 died in the first three months. In 2005, the Chernobyl Forum, composed of the International Atomic Energy Agency (IAEA), other UNITED NATIONS organizations and the governments of Belarus, Russia and Ukraine, published a report on the environmental radiological and health consequences of the Chernobyl accident.
On the death toll from the accident, the report states that 28 emergency workers (“liquidators”) died of acute radiation syndrome, including beta burns, and 15 patients died of thyroid cancer in the following years. However, it is estimated that about 4,000 of the five million people living in the contaminated areas may have developed cancer because of the accident. The report projects cancer mortality to “increase by less than one percent” (~0.3 percent) over an 80-year period, warning that this estimate is “speculative” as only a few cancer deaths are linked to the Chernobyl disaster.
Of all 66,000 Belarusian emergency workers in the mid-1990s, only 150 (about 0.2%) were notified by their government as having died. In contrast, 5,722 victims were reported among Ukrainian cleaning workers by 1995 by the National Committee for the Radiological Protection of the Ukrainian Population.
The four most harmful radionuclides disseminated from Chernobyl were iodine-131, cesium-134, Cesium-137 and strontium-90, with half-lives of 8.02 days, 2.07 years, 30.2 years and 28.8 years, respectively. iodine was initially seen with less alarm than the other isotopes due to its short half-life, but is highly volatile and may have traveled further and caused the most serious short-term health problems.
Strontium, on the other hand, is the least volatile of the four, and of greatest concern in areas near Chernobyl. Iodine tends to focus on the thyroid and mammary glands, leading, among other things, to an increased incidence of thyroid cancers. Cesium tends to accumulate in vital organs such as the heart, while strontium accumulates in the bones and can be a risk to bone marrow and lymphocytes. Radiation is more harmful to cells that are actively dividing. In adult mammals, cell division is slow, except in hair follicles, skin, bone marrow and gastrointestinal tract, which is why vomiting and hair loss are common symptoms of acute radiation syndrome.
In 2000, the number of Ukrainians claiming to be “sufferers” of radiation (poterpili) and receiving state benefits had jumped to 3.5 million, or 5% of the population. Many of these are resettled populations from contaminated areas or former workers of Chernobyl factories. According to scientific bodies affiliated with the IAEA, these apparent increases in health problems result in part from economic tensions in these countries and health and nutrition problems; furthermore, they suggest that increased medical surveillance after the accident has meant that many previously gone unnoticed cases (especially of cancer) are being recorded.
The World Health Organization states that “children conceived before or after their father’s exposure did not show statistically significant differences in mutation frequencies.”This statistically insignificant increase was also observed by independent researchers who analyzed the children of Chernobyl liquidators.
An IAEA report examines the environmental consequences of the accident. The United Nations Scientific Committee on the Effects of Atomic Radiation estimated a global collective dose of radiation exposure from the accident “equivalent, on average, to an additional 21 days of worldwide exposure to natural background radiation”; individual doses were much higher than the global average among the most exposed, including 530,000 decontamination workers, mainly male (Chernobyl liquidators), who calculated an effective dose equivalent to an extra 50 years of natural radiation exposure.
In 2004, the Chernobyl Forum revealed that thyroid cancer among children is one of the main impacts of the disaster on health. This is due to the ingestion of contaminated dairy products, along with inhalation of the highly radioactive short-lived isotope, iodine-131. In this publication, more than 4,000 cases of childhood thyroid cancer were reported. It is important to note that there was no evidence of increased solid cancers or leukemia. The document states that there has been an increase in psychological problems among the affected population. The WHO Radiation Program reported that the 4,000 cases of thyroid cancer resulted in nine deaths.
According to the United Nations Scientific Committee on the Effects of Atomic Radiation, by the year 2005, an excess of more than 6,000 cases of thyroid cancer was reported. That is, above the estimate of the baseline index of pre-accident thyroid cancer, more than 6,000 casual cases of thyroid cancer were reported in children and adolescents exposed at the time of the accident, a number that should increase. They concluded that there is no other evidence of important health impacts from radiation exposure.
Fred Mettler, a radiation expert at the University of New Mexico, puts the number of cancer deaths worldwide outside the highly contaminated zone at “maybe” 5,000, for a total of 9,000 fatal cancers associated with Chernobyl, saying that “the number is small (representing a small percentage) compared to the normal spontaneous risk of cancer, but the numbers are great in absolute terms.”The same report outlined studies based on data found in the Russian Registry from 1991 to 1998, which suggested that “of 61,000 Russian workers exposed to an average dose of 107 mSv, about 5% of all fatalities occurred may have been due to radiation exposure.”
Political, economic and social
It is difficult to establish the total economic cost of the disaster. According to Mikhail Gorbachev, the Soviet Union spent 18 billion Soviet rubles (the equivalent of 18 billion dollars at the time, or 41.1 billion dollars in current values) in the process of containment and decontamination, which virtually bankrupted the country. In 2005, the total cost in 30 years for Belarus alone was estimated at $235 billion, or about $301 billion in today’s dollars, given inflation rates.
The ongoing costs are well known; in its 2003-2005 report, the Chernobyl Forum stated that between 5% and 7% of government spending in Ukraine are still related to Chernobyl, while in Belarus it is estimated that more than 13 billion were spent between 1991 and 2003, with 22% of the national budget targeted at the effects of the disaster in 1991, falling to 6% in 2002. In 2018, Ukraine spent 5-7% of its national budget on recovery activities related to the nuclear accident. The global economic loss is estimated at US$235 billion in Belarus. Much of the current cost is related to the payment of Chernobyl-related social benefits to about 7 million people in the three countries.
A significant economic impact at the time was the removal of 784,320 hectares of agricultural land and 694,200 hectares of forest. Although much of this was returned to use, agricultural production costs increased due to the need for special cultivation techniques, fertilizers and additives.
Politically, the accident gave great significance to the new Soviet glasnost policy and helped forge closer Soviet-American relations at the end of the Cold War through bioscientific cooperation. The disaster also became a key factor in the eventual dissolution of the Soviet Union in 1991 and a major influence on the formation of the new Eastern Europe.
Both Ukraine and Belarus, in their first months of independence, reduced the legal radiation thresholds from the previous thresholds raised by the Soviet Union (from 35 rems per life throughout the USSR to 7 rems per life in Ukraine and 0.1 rems per year in Belarus).
Consequences
Plant decommissioning
After the accident, doubts arose about the future of the plant and its eventual destination. All work on unfinished reactors 5 and 6 was stopped three years later. However, the problem at the Chernobyl plant did not end with the disaster at reactor 4. The damaged reactor was sealed off and 200 cubic meters of concrete were placed between the disaster site and operating buildings. The work was administered by Grigori Mikhailovitch Naginski, the deputy chief engineer of the Installation and Construction Directorate. The Ukrainian government continued to let the remaining three reactors operate because of a power shortage in the country.
In October 1991, however, a fire occurred in the turbine building of reactor 2; authorities later declared that the reactor was damaged and it was shut down. Reactor 1 was shut down in November 1996 as part of an agreement between the Ukrainian government and international organizations, such as the IAEA, to shut down operations at the plant. On December 15, 2000, then-President Leonid Kutchma personally shut down reactor 3 in an official ceremony, closing the entire site.
Confinement
Shortly after the accident, the reactor building was quickly enveloped by a gigantic concrete sarcophagus in a remarkable feat of construction under severe conditions. Crane operators worked blindly from inside lead-coated cabins, receiving instructions from distant radio observers, while gigantic pieces of concrete were moved to the site in custom-made vehicles. The purpose of the sarcophagus was to prevent any further release of radioactive particles into the atmosphere, mitigate damage if the core was critical and exploded, and provide safety for the continued operations of adjacent reactors 1, 2 and 3.
The concrete sarcophagus never intended to last long, lasting only 30 years. On February 12, 2013, a 600 m² section of the roof of the turbine building collapsed, adjacent to the sarcophagus, causing a new release of radioactivity and the temporary evacuation of the area. Initially, it was assumed that the roof collapsed due to the weight of the snow, but the amount of snow was not exceptional and the report of a Ukrainian panel of inquiry concluded that the collapse was the result of sloppy repair work and aging of the structure. Experts warned that the sarcophagus itself was on the verge of collapse.
In 1997, the International Chernobyl Protection Fund was created to design and build a more permanent cover for the unstable, short-lived sarcophagus. It received more than €810 million and was managed by the European Bank for Reconstruction and Development (EBRD). The new shelter began construction in 2010. It consists of a metal arch 105 meters high and 257 meters long that was built on rails adjacent to the building of reactor 4, so that it could be shifted over the existing sarcophagus. The new shelter was completed in 2016 and slid to the top of the sarcophagus on November 29. The huge steel arch was put in place over several weeks.
Exclusion zone
An area that originally spans 30 kilometers in all directions of the plant is officially called an “exclusion zone”. It is largely uninhabited, with the exception of about 300 residents who refused to leave. The area was widely reverted to the forest and was taken over by wildlife because of the lack of competition with humans for space and resources. Even today, radiation levels are so high that the workers responsible for rebuilding the sarcophagus were only able to work five hours a day for a month before taking 15 days’ rest. Ukrainian officials estimated that the area would not be safe for human life for another 20,000 years. However, in 2016, 187 local Ukrainians returned and began to live permanently in the area.
In 2011, Ukraine opened the sealed zone around the Chernobyl reactor to tourists wishing to learn more about the tragedy that occurred in 1986. Sergii Mirnyi, a radiation reconnaissance officer at the time of the accident, and now an academic at the Kyiv-Mohyla National Academy University in Kiev, Ukraine, wrote about the psychological and physical effects on survivors and visitors and worked as a consultant for Chernobyl tourism groups.
Judgment of those responsible
Between July 7 and 30, 1987, a trial in an impromptu court was made at the Chernobyl House of Culture in Soviet Ukraine. Five plant workers (Deputy Chief Engineer Anatoly Diatlov, plant director Viktor P. Bryukhanov, chief engineer Nikolai M. Fomin, shift director of Reactor 4 Boris V. Rogozhin, and reactor head 4 Aleksandr P. Kovalenko) and Gosatomenergonadzor (State Committee for Supervision of The Safe Conduct of Soviet Atomic Energy Work) Inspector Yuri A. Laushkin were sentenced to 10, 10, 10, 5, 3 and 2 years of forced services, respectively, in labor camps as a consequence of the disaster investigations.
Anatoly Diatlov ended up becoming one of the faces of the disaster, as it was under his supervision that the safety test failed, leading to the accident. He was convicted of “criminal mismanagement of potentially explosive enterprises” and sentenced to ten years in prison —but he served only three. Diatlov, however, denied responsibility for the accident. He said he was haunted by what happened and blamed what happened on mechanical and design failures of the RBMK reactors and not on human failure. For experts, the cause of the Chernobyl disaster was a combination of the two, both mechanical and human failure, but Diatlov’s supervised test was deemed “incompetent” and himself negligent in the face of what happened.
Concern about forest fires
During the dry seasons, a perennial concern and that forests that have been contaminated by radioactive material catch fire. Dry conditions and the accumulation of debris make forests a breeding ground for forest fires. Depending on the prevailing weather conditions, fires could spread the radioactive material further out of the exclusion zone through the smoke.
References (sources)
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