Chernobyl disaster

Steam turbine tests

In steady state operation, a significant fraction (over 6%) of the power from a nuclear reactor is derived not from fission but from the decay heat of its accumulated fission products. This heat continues for some time after the chain reaction is stopped (e.g., following an emergency SCRAM) and active cooling may be required to prevent core damage.^[32] RBMK reactors like those at Chernobyl use water as a coolant.^[33][34] Reactor 4 at Chernobyl consisted of about 1,600 individual fuel channels, each of which required coolant flow of 28 metric tons (28,000 litres or 7,400 US gallons) per hour.^[31]

Since cooling pumps require electricity to cool a reactor after a SCRAM, in the event of a power grid failure, Chernobyl's reactors had three backup diesel generators; these could start up in 15 seconds, but took 60–75 seconds^[31]:15 to attain full speed and reach the 5.5‑megawatt (MW) output required to run one main pump.^[31]:30

To solve this one-minute gap – considered an unacceptable safety risk – it had been theorized that rotational energy from the steam turbine (as it wound down under residual steam pressure) could be used to generate the required electrical power. Analysis indicated that this residual momentum and steam pressure might be sufficient to run the coolant pumps for 45 seconds,^[31]:16 bridging the gap between an external power failure and the full availability of the emergency generators.^[35]

This 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; it did not maintain the desired magnetic field after the turbine trip. The system was modified, and the test was repeated in 1984 but again proved unsuccessful. In 1985, the tests were attempted a third time but also yielded negative results. The test procedure would be repeated in 1986, and it was scheduled to take place during the maintenance shutdown of Reactor Four.^[35]

The test focused on the switching sequences of the electrical supplies for the reactor. The test procedure was expected to begin with an automatic emergency shutdown. No detrimental effect on the safety of the reactor was anticipated, so the test programme was not formally coordinated with either the chief designer of the reactor (NIKIET) or the scientific manager. Instead, it was approved only by the director of the plant (and even this approval was not consistent with established procedures).^[36]

According to the test parameters, the thermal output of the reactor should have been no lower than 700 MW at the start of the experiment. If test conditions had been as planned, the procedure would almost certainly have been carried out safely; the eventual disaster resulted from attempts to boost the reactor output once the experiment had been started, which was inconsistent with approved procedure.^[36]

The Chernobyl power plant had been in operation for two years without the capability to ride through the first 60–75 seconds of a total loss of electric power, and thus lacked an important safety feature. The station managers presumably wished to correct this at the first opportunity, which may explain why they continued the test even when serious problems arose, and why the requisite approval for the test had not been sought from the Soviet nuclear oversight regulator (even though there was a representative at the complex of 4 reactors).^{[notes 2]:18–20}

The experimental procedure was intended to run as follows:

1. The reactor was to be running at a low power level, between 700 MW and 800 MW.
2. The steam-turbine generator was to be run up to full speed.
3. When these conditions were achieved, the steam supply for the turbine generator was to be closed off.
4. Turbine generator performance was to be recorded to determine whether it could provide the bridging power for coolant pumps until the emergency diesel generators were sequenced to start and provide power to the cooling pumps automatically.
5. After the emergency generators reached normal operating speed and voltage, the turbine generator would be allowed to continue to freewheel down.

Conditions before the accident

The conditions to run the test were established before the day shift of 25 April 1986. The day-shift workers had been instructed in advance and were familiar with the established procedures. A special team of electrical engineers was present to test the new voltage regulating system.^[37] As planned, a gradual reduction in the output of the power unit was begun at 01:06 on 25 April, and the power level had reached 50% of its nominal 3200 MW thermal level by the beginning of the day shift.

At this point, another regional power station unexpectedly went offline, and the Kiev electrical grid controller requested that the further reduction of Chernobyl's output be postponed, as power was needed to satisfy the peak evening demand. The Chernobyl plant director agreed, and postponed the test. Despite this delay, preparations for the test not affecting the reactor's power were carried out, including the disabling of the emergency core cooling system or ECCS, a passive/active system of core cooling intended to provide water to the core in a loss-of-coolant accident. Given the other events that unfolded, the system would have been of limited use, but its disabling as a "routine" step of the test is an illustration of the inherent lack of attention to safety for this test.^[38] In addition, had the reactor been shut down for the day as planned, it is possible that more preparation would have been taken in advance of the test.

At 23:04, the Kiev grid controller allowed the reactor shutdown to resume. This delay had some serious consequences: the day shift had long since departed, the evening shift was also preparing to leave, and the night shift would not take over until midnight, well into the job. According to plan, the test should have been finished during the day shift, and the night shift would only have had to maintain decay heat cooling systems in an otherwise shut-down plant.^[31]:36–38

The night shift had very limited time to prepare for and carry out the experiment. A further rapid decrease in the power level from 50% was executed during the shift change-over. Alexander Akimov was chief of the night shift, and Leonid Toptunov was the operator responsible for the reactor's operational regimen, including the movement of the control rods. Toptunov was a young engineer who had worked independently as a senior engineer for approximately three months.^[31]:36–38

The test plan called for a gradual decrease in power output from reactor 4 to a thermal level of 700–1000 MW.^[39] An output of 700 MW was reached at 00:05 on 26 April. Due to the reactor's production of a fission byproduct, xenon-135, which is a reaction-inhibiting neutron absorber, core power continued to decrease without further operator action—a process known as reactor poisoning. This continuing decrease in power occurred because in steady state operation, xenon-135 is "burned off" as quickly as it is created from decaying iodine-135 by absorbing neutrons from the ongoing chain reaction to become highly stable xenon-136. When the reactor power was lowered, previously produced high quantities of iodine-135 decayed into the neutron-absorbing xenon-135 faster than the reduced neutron flux could burn it off. As the reactor power output dropped further, to approximately 500 MW, Toptunov mistakenly inserted the control rods too far—the exact circumstances leading to this are unknown because Akimov died in hospital on 10 May and Toptunov on 14 May. This combination of factors put the reactor into an unintended near-shutdown state, with a power output of 30 MW thermal or less.

The reactor was now producing 5 percent of the minimum initial power level established as safe for the test.^[36]:73 Control-room personnel decided to restore power by disabling the automatic system governing the control rods and manually extracting the majority of the reactor control rods to their upper limits.^[40] Several minutes elapsed between their extraction and the point that the power output began to increase and subsequently stabilize at 160–200 MW (thermal), a much smaller value than the planned 700 MW. The rapid reduction in the power during the initial shutdown, and the subsequent operation at a level of less than 200 MW led to increased poisoning of the reactor core by the accumulation of xenon-135.^[41][42] This restricted any further rise of reactor power, and made it necessary to extract additional control rods from the reactor core in order to counteract the poisoning.

The operation of the reactor at the low power level and high poisoning level was accompanied by unstable core temperature and coolant flow, and possibly by instability of neutron flux, which triggered alarms. The control room received repeated emergency signals regarding the levels in the steam/water separator drums, and large excursions or variations in the flow rate of feed water, as well as from relief valves opened to relieve excess steam into a turbine condenser, and from the neutron power controller. Between 00:35 and 00:45, emergency alarm signals concerning thermal-hydraulic parameters were ignored, apparently to preserve the reactor power level.^[43]

When the power level of 200 MW was achieved, preparation for the experiment continued. As part of the test plan, extra water pumps were activated at 01:05 on 26 April, increasing the water flow. The increased coolant flow rate through the reactor produced an increase in the inlet coolant temperature of the reactor core (the coolant no longer having sufficient time to release its heat in the turbine and cooling towers), which now more closely approached the nucleate boiling temperature of water, reducing the safety margin.

The flow exceeded the allowed limit at 01:19, triggering an alarm of low steam pressure in the steam separators. At the same time, the extra water flow lowered the overall core temperature and reduced the existing steam voids in the core and the steam separators.^[44] Since water weakly absorbs neutrons (and the higher density of liquid water makes it a better absorber than steam), turning on additional pumps decreased the reactor power further still. The crew responded by turning off two of the circulation pumps to reduce feedwater flow, in an effort to increase steam pressure, and by removing more manual control rods to maintain power.^[38][45]

All these actions led to an extremely unstable reactor configuration. Nearly all of the control rods were removed manually, including all but 18 of the "fail-safe" manually operated rods of the minimal 28 which were intended to remain fully inserted to control the reactor even in the event of a loss of coolant, out of a total 211 control rods.^[46] While the emergency SCRAM system that would insert all control rods to shut down the reactor could still be activated manually (through the "AZ-5" button), the automated system that could do the same had been disabled to maintain the power level, and many other automated and even passive safety features of the reactor had been bypassed. Further, the reactor coolant pumping had been reduced, which had limited margin so any power excursion would produce boiling, thereby reducing neutron absorption by the water. The reactor was in an unstable configuration that was outside the safe operating envelope established by the designers. If anything pushed it into supercriticality, it was unable to recover automatically.

Experiment and explosion

At 1:23:04 a.m., the experiment began. Four of the main circulating pumps (MCP) were active; of the eight total, six are normally active during regular operation. 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. In the interim, the power for the MCPs was to be supplied by the turbine generator as it coasted down. 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 (bubbles) in the core.

Unlike western Light Water Reactors, the RBMK had a positive void coefficient of reactivity, meaning when water began to boil and produce voids in the coolant, the nuclear chain reaction increases instead of decreasing. With this feature at low reactor power levels, the no.4 RBMK reactor was now primed to embark on a positive feedback loop, in which the formation of steam voids reduced the ability of the liquid water coolant to absorb neutrons, which in turn increased the reactor's power output. This caused yet more water to flash into steam, giving a further power increase. During almost the entire period of the experiment the automatic control system successfully counteracted this positive feedback, inserting control rods into the reactor core to limit the power rise. This system had control of only 12 rods, and nearly all others had been manually retracted.

At 1:23:40, as recorded by the SKALA centralized control system, a SCRAM (emergency shutdown) of the reactor was initiated. The SCRAM was started when the EPS-5 button (also known as the AZ-5 button) of the reactor emergency protection system was pressed: this engaged the drive mechanism on all control rods to fully insert them, including the manual control rods that had been withdrawn earlier. The reason why the EPS-5 button was pressed is not known, whether it was done as an emergency measure in response to rising temperatures, or simply as a routine method of shutting down the reactor upon completion of the experiment.

There is a view that the SCRAM may have been ordered as a response to the unexpected rapid power increase, although there is no recorded data proving this. Some have suggested that the button was not manually pressed, that the SCRAM signal was automatically produced by the emergency protection system, but the SKALA registered a manual SCRAM signal. Despite this, the question as to when or even whether the EPS-5 button was pressed has been the subject of debate. There have been assertions that the manual SCRAM was initiated due to the initial rapid power acceleration. Others have suggested that the button was not pressed until the reactor began to self-destruct, while others believe that it happened earlier and in calm conditions.^[48]:578[49]

In any case, when the EPS-5 button was pressed, the insertion of control rods into the reactor core began. The control rod insertion mechanism moved the rods at 0.4 m/s, so that the rods took 18 to 20 seconds to travel the full height of the core, about 7 metres. A bigger problem was the design of the RBMK control rods, each of which had a graphite neutron moderator rod attached to the end to boost reactor output by displacing water when the control rod section had been fully withdrawn from the reactor. Thus, when a control rod was at maximum extraction, a neutron-moderating graphite extension was centered in the core with a 1.25 m column of water above and below it. Therefore, injecting a control rod downward into the reactor during a SCRAM initially displaced (neutron-absorbing) water in the lower portion of the reactor with (neutron-moderating) graphite on its way out of the core. As a result, an emergency SCRAM initially increased the reaction rate in the lower part of the core as the graphite section of rods moving out of the reactor displaced water coolant. This behaviour was revealed when the initial insertion of control rods in another RBMK reactor at Ignalina Nuclear Power Plant in 1983 induced a power spike, but since the subsequent SCRAM of that reactor was successful, the information was disseminated but deemed of little importance.

A few seconds into the SCRAM, a power spike occurred, and the core overheated, causing some of the fuel rods to fracture, blocking the control rod columns and jamming the control rods at one-third insertion, with the graphite displacers still in the lower part of the core. Within three seconds the reactor output rose above 530 MW.^[31]:31

The subsequent course of events was not registered by instruments; it is known only as a result of mathematical simulation. Apparently, the power spike caused an increase in fuel temperature and steam buildup, leading to a rapid increase in steam pressure. This caused the fuel cladding to fail, releasing the fuel elements into the coolant, and rupturing the channels in which these elements were located.^[50]

Then, according to some estimations, the reactor jumped to around 30,000 MW thermal, ten times the normal operational output. The last reading on the control panel was 33,000 MW. It was not possible to reconstruct the precise sequence of the processes that led to the destruction of the reactor and the power unit building, but a steam explosion, like the explosion of a steam boiler from excess vapour pressure, appears to have been the next event. There is a general understanding that it was 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, to which the entire reactor assembly is fastened, through the roof of the reactor building. This is believed to be the first explosion that many heard.^[51]:366 This explosion ruptured further fuel channels, as well as severing most of the coolant lines feeding the reactor chamber, and as a result the remaining coolant flashed to steam and escaped the reactor core. The total water loss in combination with a high positive void coefficient further increased the reactor's thermal power.

A second, more powerful explosion occurred about two or three seconds after the first; this explosion dispersed the damaged core and effectively terminated the nuclear chain reaction. This explosion also compromised more of the reactor containment vessel and ejected hot lumps of graphite moderator. The ejected graphite and the demolished channels still in the remains of the reactor vessel caught fire on exposure to air, greatly contributing to the spread of radioactive fallout and the contamination of outlying areas.^[38]

According to observers outside Unit 4, burning lumps of material and sparks shot into the air above the reactor. Some of them fell onto the roof of the machine hall and started a fire. About 25 percent of the red-hot graphite blocks and overheated material from the fuel channels was ejected. Parts of the graphite blocks and fuel channels were out of the reactor building. As a result of the damage to the building an airflow through the core was established by the high temperature of the core. The air ignited the hot graphite and started a graphite fire.^[31]:32

After the larger explosion, a number of employees at the power station went outside to get a clearer view of the extent of the damage. One such survivor, Alexander Yuvchenko, recounts that once he stepped outside and looked up towards the reactor hall, he saw a "very beautiful" LASER-like beam of light bluish light caused by the ionization of air that appeared to "flood up into infinity".^[52][53][54]

There were initially several hypotheses about the nature of the second explosion. One view was that the second explosion was caused by 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 Checherov, published in 1998, was that the second explosion was a thermal explosion of the reactor as a result of the uncontrollable escape of fast neutrons caused by the complete water loss in the reactor core.^[55] A third hypothesis was that the second explosion was another steam explosion. According to this version, the first explosion was a more minor steam explosion in the circulating loop, causing a loss of coolant flow and pressure that in turn caused the water still in the core to flash to steam. This second explosion then did the majority of the damage to the reactor and containment building.

The force of the second explosion and the ratio of xenon radioisotopes released after the accident (a vital tool in nuclear forensics) indicated to Yuri V. Dubasov in a 2009 publication (suggested before him by Checherov in 1998), that the second explosion could have been a nuclear power transient resulting from core material melting in the absence of its water coolant and moderator. Dubasov argues that the reactor did not simply undergo a runaway delayed-supercritical/exponential increase in power into the multi-gigawatt power range, which is somewhat similar to the conditions of a normal reactor coming up to its commercial power level (with the notable exception that Chernobyl's older RBMK reactor design had the largest positive void coefficient of reactivity of any reactor then operating commercially), permitting a dangerous "positive feedback"/runaway condition, given the lack of inherent safety stops when power levels began to increase above the commercial level. Although a positive-feedback power excursion that increased until the reactor disassembled itself by means of its internal energy and external steam explosions^[56] is the more accepted explanation for the cause of the explosions, Dubasov argues instead that a runaway prompt supercriticality occurred, with the internal physics being more similar to the explosion of a fizzled nuclear weapon, and that this failed/fizzle event produced the second explosion.^[57]

This nuclear fizzle hypothesis, then mostly defended by Dubasov, was examined further in 2017 by retired physicist Lars-Erik De Geer in an analysis that puts the hypothesized fizzle event as the more probable cause of the first explosion.^[58][59][60] The more energetic second explosion, which produced the majority of the damage, has been estimated by Dubasov in 2009 as equivalent to 40 billion joules of energy, the equivalent of about ten tons of TNT. Both the 2009 and 2017 analyses argue that the nuclear fizzle event, whether producing the second or first explosion, consisted of a prompt chain reaction (as opposed to the consensus delayed neutron mediated chain-reaction) that was limited to a small portion of the reactor core, since expected self-disassembly occurs rapidly in fizzle events.^[57][61][62]

Contrary to safety regulations, bitumen, a combustible material, had been used in the construction of the roof of the reactor building and the turbine hall. Ejected material ignited at least five fires on the roof of the adjacent reactor 3, which was still operating. It was imperative to put those fires out and protect the cooling systems of reactor 3.^[31]:42 Inside reactor 3, the chief of the night shift, Yuri Bagdasarov, wanted to shut down the reactor immediately, but chief engineer Nikolai Fomin would not allow this. The operators were given respirators and potassium iodide tablets and told to continue working. At 05:00, Bagdasarov made his own decision to shut down the reactor, leaving only those operators there who had to work the emergency cooling systems.^[31]:44

Immediate crisis management

Fire containment

Shortly after the accident, firefighters arrived to try to extinguish the fires. First on the scene was a Chernobyl Power Station firefighter brigade under the command of Lieutenant Volodymyr Pravik, who died on 9 May 1986 of acute radiation sickness. They were not told how dangerously radioactive the smoke and the debris were, and may not even have known that the accident was anything more than a regular electrical fire: "We didn't know it was the reactor. No one had told us."^[65]

Grigorii Khmel, the driver of one of the fire engines, later described what happened:

We arrived there at 10 or 15 minutes to two in the morning.... We saw graphite scattered about. Misha asked: "Is that graphite?" I kicked it away. But one of the fighters on the other truck picked it up. "It's hot," he said. The pieces of graphite were of different sizes, some big, some small, enough to pick them up...

We didn't know much about radiation. Even those who worked there had no idea. There was no water left in the trucks. Misha filled a cistern and we aimed the water at the top. Then those boys who died went up to the roof – Vashchik, Kolya and others, and Volodya Pravik.... They went up the ladder ... and I never saw them again.^[66]:54

Anatoli Zakharov, a fireman stationed in Chernobyl since 1980, offers 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."^[67]

He also said:

Of course we knew! If we'd followed regulations, we would never have gone near the reactor. But it was a moral obligation – our duty. We were like kamikaze.^[67]

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 and keep its core cooling systems intact. The fires were extinguished by 5:00, but many firefighters received high doses of radiation. The fire inside reactor 4 continued to burn until 10 May 1986; it is possible that well over half of the graphite burned out.^[31]:73

The fire was extinguished by a combined effort of helicopters dropping over 5000 metric tons of sand, lead, clay, and neutron-absorbing boron onto the burning reactor and injection of liquid nitrogen. It is now known that virtually none of the neutron absorbers reached the core.^[68]

From eyewitness accounts of the firefighters involved before they died (as reported on the CBC television series Witness), one described his experience of the radiation as "tasting like metal", and feeling a sensation similar to that of pins and needles all over his face. (This is similar to the description given by Louis Slotin, a Manhattan Project physicist who died days after a fatal radiation overdose from a criticality accident.)^[69]

The explosion and fire threw hot particles of the nuclear fuel and also far more dangerous fission products, radioactive isotopes such as caesium-137, iodine-131, strontium-90 and other radionuclides, into the air: the residents of the surrounding area observed the radioactive cloud on the night of the explosion.

Equipment assembled included remote-controlled bulldozers and robot-carts that could detect radioactivity and carry hot debris. Valery Legasov (first deputy director of the Kurchatov Institute of Atomic 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 quit working."^[70]

Timeline

• 1:26:03 – fire alarm activated
• 1:28 – arrival of local firefighters, Pravik's guard
• 1:35 – arrival of firefighters from Pripyat, Kibenok's guard
• 1:40 – arrival of Telyatnikov
• 2:10 – turbine hall roof fire extinguished
• 2:30 – main reactor hall roof fires suppressed
• 3:30 – arrival of Kiev firefighters^[71]
• 4:50 – fires mostly localized
• 6:35 – all fires extinguished^‡[72]

^‡With the exception of the "fire" contained inside Reactor 4, which continued to burn for many days.^[31]:73

Evacuation

	Pripyat evacuation broadcast Russian language announcement
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The nearby city of Pripyat was not immediately evacuated. The townspeople, in the early hours of the morning, at 01:23 AM local time, went about their usual business, completely oblivious to what had just happened. However, within a few hours of the explosion, dozens of people fell ill. Later, they reported severe headaches and metallic tastes in their mouths, along with uncontrollable fits of coughing and vomiting.^[73]

As the plant was run by authorities in Moscow, the government of Ukraine did not receive prompt information on the accident.^[74]

Valentyna Shevchenko, then Chairman of the Presidium of Verkhovna Rada Supreme Soviet of the Ukrainian SSR, recalls that Ukraine's acting Minister of Internal Affairs Vasyl Durdynets phoned her at work at 9 am to report current affairs; only at the end of the conversation did he add that there had been a fire at the Chernobyl nuclear power plant, but 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".^[74] Shevchenko then spoke over the phone to Volodymyr Shcherbytsky, Head of the Central Committee of the CPU and de facto head of state, who said he anticipated a delegation of the state commission headed by the deputy chairman of the Council of Ministers of USSR.^[74]

A commission was set up the same day (26 April) to investigate the accident. It was headed by Valery Legasov, First Deputy Director of the Kurchatov Institute of Atomic Energy, and included leading nuclear specialist Evgeny Velikhov, hydro-meteorologist Yuri Izrael, radiologist Leonid Ilyin and others. They flew to Boryspil International Airport and arrived at the power plant in the evening of 26 April.^[74] By that time two people had already died and 52 were hospitalized. The delegation soon had ample evidence that the reactor was destroyed and extremely high levels of radiation had caused a number of cases of radiation exposure. In the early daylight hours of 27 April, approximately 36 hours after the initial blast, they ordered the evacuation of Pripyat. Initially it was decided to evacuate the population for three days; later this was made permanent.^[74]

By 11:00 on 27 April, buses had arrived in Pripyat to start the evacuation.^[74] The evacuation began at 14:00. A translated excerpt of the evacuation announcement follows:^[75]

For the attention of the residents of Pripyat! The City Council informs you that due to the accident at Chernobyl Power Station in the city of Pripyat the radioactive conditions in the vicinity are deteriorating. The Communist Party, its officials and the armed forces are taking necessary steps to combat this. Nevertheless, with the view to keep people as safe and healthy as possible, the children being top priority, we need to temporarily evacuate the citizens in the nearest towns of Kiev region. For these reasons, starting from 27 April 1986 2 pm each apartment block will be able to have a bus at its disposal, supervised by the police and the city officials. It is highly advisable to take your documents, some vital personal belongings and a certain amount of food, just in case, with you. The senior executives of public and industrial facilities of the city has decided on the list of employees needed to stay in Pripyat to maintain these facilities in a good working order. All the houses will be guarded by the police during the evacuation period. Comrades, leaving your residences temporarily please make sure you have turned off the lights, electrical equipment and water and shut the windows. Please keep calm and orderly in the process of this short-term evacuation.

— Evacuation announcement in Pripyat, 27 April 1986 (14:00)

To expedite the evacuation, residents were told to bring only what was necessary, and that they would remain evacuated for approximately three days. As a result, most personal belongings were left behind, and remain there today. By 15:00, 53,000 people were evacuated to various villages of the Kiev region.^[74] The next day, talks began for evacuating people from the 10 km zone.^[74] Ten days after the accident, the evacuation area was expanded to 30 km (19 mi).^{[76]:115,120–1} This "exclusion zone" has remained ever since, although its shape has changed and its size has been expanded.

Steam explosion risk

Two floors of bubbler pools beneath the reactor served as a large water reservoir for the emergency cooling pumps and as a pressure suppression system capable of condensing steam in case of a small broken steam pipe; the third floor above them, below the reactor, served as a steam tunnel. The steam released by a broken pipe was supposed to enter the steam tunnel and be led into the pools to bubble through a layer of water. After the disaster, the pools and the basement were flooded because of ruptured cooling water pipes and accumulated firefighting water, and constituted a serious steam explosion risk.

The smoldering graphite, fuel and other material above, at more than 1200 °C,^[84] started to burn through the reactor floor and mixed with molten concrete from the reactor lining, creating corium, a radioactive semi-liquid material comparable to lava.^[83][85] If this mixture had melted through the floor into the pool of water, it was feared it could have created a serious steam explosion that would have ejected more radioactive material from the reactor. It became necessary to drain the pool.^[86]

The bubbler pool could be drained by opening its sluice gates. However, the valves controlling it were underwater, located in a flooded corridor in the basement. So volunteers in wetsuits and respirators (for protection against radioactive aerosols) and equipped with dosimeters, entered the knee-deep radioactive water and managed to open the valves.^[87][88] These were the engineers Alexei Ananenko and Valeri Bezpalov (who knew where the valves were), accompanied by the shift supervisor Boris Baranov.^[89][90] Upon succeeding and emerging from the water, according to many English language news articles, books and the prominent BBC docudrama Surviving Disaster – Chernobyl Nuclear, the three knew it was a suicide-mission and began suffering from radiation sickness and died soon after.^[91] Some sources also incorrectly claimed that they died there in the plant.^[90] However, research by Andrew Leatherbarrow, author of the 2016 book Chernobyl 01:23:40,^[87] determined that the frequently recounted story is a gross exaggeration. Alexei Ananenko continues to work in the nuclear energy industry, and rebuffs the growth of the Chernobyl media sensationalism surrounding him.^[92] While Valeri Bezpalov was found to still be alive by Leatherbarrow, the 65-year-old Baranov had lived until 2005 and had died of heart failure.^[93]

Once the bubbler pool gates were opened by the Ananenko team, fire brigade pumps were then used to drain the basement. The operation was not completed until 8 May, after 20,000 metric tons of highly radioactive water were pumped out.

With the bubbler pool gone, a meltdown was less likely to produce a powerful steam explosion. To do so, the molten core would now have to reach the water table below the reactor. To reduce the likelihood of this, it was decided to freeze the earth beneath the reactor, which would also stabilize the foundations. Using oil drilling equipment, the injection of liquid nitrogen began on 4 May. It was estimated that 25 metric tons of liquid nitrogen per day would be required to keep the soil frozen at −100 °C.^[31]:59 This idea^[94] was soon scrapped and the bottom room where the cooling system would have been installed was filled with concrete.

It is likely that intense alpha radiation hydrolysed the water, generating a low-pH hydrogen peroxide (H₂O₂) solution akin to an oxidizing acid.^[95] Conversion of bubbler pool water to H₂O₂ is confirmed by the presence in the Chernobyl lavas of studtite and metastudtite,^[96][97] the only minerals that contain peroxide.^[98]

Debris removal

The worst of the radioactive debris was collected inside what was left of the reactor. During clean-up operations, remotely controlled machinery were used to try to move the most highly radioactive debris, but these failed in the harsh conditions. Consequently, the most highly radioactive materials were shoveled by Chernobyl liquidators from the military wearing heavy protective gear (dubbed "bio-robots" by the military); these soldiers could only spend a maximum of 40 seconds working on the rooftops of the surrounding buildings because of the extremely high doses of radiation given off by the blocks of graphite and other debris. Though the soldiers were only supposed to perform the role of the "bio-robot" a maximum of once, some soldiers reported having done this task five or six times. The reactor itself was covered with bags of sand, lead and boric acid dropped from helicopters: some 5000 metric tons of material were dropped during the week that followed the accident. Historians estimate that about 600 Soviet pilots risked dangerous levels of radiation to fly the thousands of flights needed to cover reactor No. 4 in this attempt to seal off radiation.^[99]

At the time there was still fear that the reactor could re-enter a self-sustaining nuclear chain-reaction and explode again, and a new containment structure was planned to prevent rain entering and triggering such an explosion, and to prevent further release of radioactive material. This was the largest civil engineering task in history, involving a quarter of a million construction workers who all reached their official lifetime limits of radiation.^[68] The Ukrainian filmmaker Vladimir Shevchenko captured film footage of an Mi-8 helicopter as its main rotor collided with a nearby construction crane cable, causing the helicopter to fall near the damaged reactor building and killing its four-man crew on 2nd October 1986.^[100] By December 1986, a large concrete sarcophagus had been erected to seal off the reactor and its contents.^[101] A unique "clean up" medal was given to the workers.^[102]

Although many of the radioactive emergency vehicles were buried in trenches, many of the vehicles used by the liquidators, including the helicopters, still remain parked in a field in the Chernobyl area. Scavengers have since removed many functioning, but highly radioactive, parts. ^[103]

During the construction of the sarcophagus, a scientific team re-entered the reactor as part of an investigation dubbed "Complex Expedition", to locate and contain nuclear fuel in a way that could not lead to another explosion. These scientists manually collected cold fuel rods, but great heat was still emanating from the core. Rates of radiation in different parts of the building were monitored by drilling holes into the reactor and inserting long metal detector tubes. The scientists were exposed to high levels of radiation and radioactive dust.^[68]

After six months of investigation, in December 1986, they discovered with the help of a remote camera an intensely radioactive mass in the basement of Unit Four, more than two metres wide, which they called "the elephant's foot" for its wrinkled appearance.^[104] The mass 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.^[68]

Liquidators worked under deplorable conditions, poorly informed and with poor protection. Many, if not most of them, exceeded radiation safety limits.^{[76]:177–183[105]:2} Some exceeded limits by over 100 times—leading to rapid death.^[76]:187

The official contaminated zones became stage to a massive clean-up effort lasting seven months.^{[76]:177–183} The official reason for such early (and dangerous) decontamination efforts, rather than allowing time for natural decay, was that the land must be re-peopled and brought back into cultivation. Indeed, within fifteen months 75% of the land was under cultivation, even though only a third of the evacuated villages were resettled. Defence forces must have done much of the work. Yet this land was of marginal agricultural value. According to historian David Marples, the administration had a psychological purpose for the clean-up: they wished to forestall panic regarding nuclear energy, and even to restart the Chernobyl power station.^{[76]:78–9,87,192–3}

INSAG-1 Report (1986)

The first official explanation of the accident, later acknowledged to be erroneous, was published in August 1986. It effectively placed the blame on the power plant operators. To investigate the causes of the accident the IAEA created a group known as the International Nuclear Safety Advisory Group (INSAG), which in its report of 1986, INSAG-1, on the whole also supported this view, based on the data provided by the Soviets and the oral statements of specialists.^[106] In this view, the catastrophic accident was caused by gross violations of operating rules and regulations. "During preparation and testing of the turbine generator under run-down conditions using the auxiliary load, personnel disconnected a series of technical protection systems and breached the most important operational safety provisions for conducting a technical exercise."^[107]:311

The operator error was probably due to their lack of knowledge of nuclear reactor physics and engineering, as well as lack of experience and training. According to these allegations, at the time of the accident the reactor was being operated with many key safety systems turned off, most notably the Emergency Core Cooling System (ECCS), LAR (Local Automatic control system), and AZ (emergency power reduction system). Personnel had an insufficiently detailed understanding of technical procedures involved with the nuclear reactor, and knowingly ignored regulations to speed test completion.^[107]

The developers of the reactor plant considered this combination of events to be impossible and therefore did not allow for the creation of emergency protection systems capable of preventing the combination of events that led to the crisis, namely the intentional disabling of emergency protection equipment plus the violation of operating procedures. Thus the primary cause of the accident was the extremely improbable combination of rule infringement plus the operational routine allowed by the power station staff.^[107]:312

In this analysis of the causes of the accident, deficiencies in the reactor design and in the operating regulations that made the accident possible were set aside and mentioned only casually. Serious critical observations covered only general questions and did not address the specific reasons for the accident.

The following general picture arose from these observations, and several procedural irregularities also helped to make the accident possible, one of which was insufficient communication between the safety officers and the operators in charge of the experiment being run that night.

The reactor operators disabled safety systems down to the generators, which the test was really about. The main process computer, SKALA, was running in such a way that the main control computer could not shut down the reactor or even reduce power. Normally the computer would have started to insert all of the control rods. The computer would have also started the "Emergency Core Protection System" that introduces 24 control rods into the active zone within 2.5 seconds, which is still slow by 1986 standards. All control was transferred from the process computer to the human operators.

On the subject of the disconnection of safety systems, Valery Legasov said, in 1987, that "[i]t was like airplane pilots experimenting with the engines in flight".^[70]

This view is reflected in numerous publications and also artistic works on the theme of the Chernobyl accident that appeared immediately after the accident,^[31] and for a long time remained dominant in the public consciousness and in popular publications.

INSAG-7 Report (1992)

Ukraine has declassified a number of KGB documents from the period between 1971 and 1988 related to the Chernobyl plant, mentioning for example previous reports of structural damages caused by negligence during construction of the plant (such as splitting of concrete layers) that were never acted upon. They document over 29 emergency situations in the plant during this period, 8 of which were caused by negligence or poor competence on the part of personnel.^[110]

In 1991 a Commission of the USSR State Committee for the Supervision of Safety in Industry and Nuclear Power reassessed the causes and circumstances of the Chernobyl accident and came to new insights and conclusions. Based on it, in 1992 the IAEA Nuclear Safety Advisory Group (INSAG) published an additional report, INSAG-7,^[36] which reviewed "that part of the INSAG-1 report in which primary attention is given to the reasons for the accident," and was included the USSR State Commission report as Appendix I.^[36]

In this INSAG report, most of the earlier accusations against staff for breach of regulations were acknowledged to be either erroneous, based on incorrect information obtained in August 1986, or less relevant. This report reflected a different view of the main reasons for the accident, presented in Appendix I. According to this account, the operators' actions in turning off the Emergency Core Cooling System, interfering with the settings on the protection equipment, and blocking the level and pressure in the separator drum did not contribute to the original cause of the accident and its magnitude, although they may have been a breach of regulations. In fact, turning off the emergency system designed to prevent the two turbine generators from stopping was not a violation of regulations.^[36]

Human factors, however, contributed to the conditions that led to the disaster. These included operating the reactor at a low power level—less than 700 MW—a level documented in the run-down test programme, and operating with a small operational reactivity margin (ORM). The 1986 assertions of Soviet experts notwithstanding, regulations did not prohibit operating the reactor at this low power level.^[36]:18

However, regulations did forbid operating the reactor with a small margin of reactivity. Yet "post-accident studies have shown that the way in which the real role of the ORM is reflected in the Operating Procedures and design documentation for the RBMK-1000 is extremely contradictory", and furthermore, "ORM was not treated as an operational safety limit, violation of which could lead to an accident".^[36]:34–25

According to the INSAG-7 Report, the chief reasons for the accident lie in the peculiarities of physics and in the construction of the reactor. There are two such reasons:^[36]:18

• The reactor had a dangerously large positive void coefficient of reactivity. The void coefficient is a measurement of how a reactor responds to increased steam formation in the water coolant. Most other reactor designs have a negative coefficient, i.e. the nuclear reaction rate slows when steam bubbles form in the coolant, since as the vapour phase in the reactor increases, fewer neutrons are slowed down. Faster neutrons are less likely to split uranium atoms, so the reactor produces less power (a negative feedback). Chernobyl's RBMK reactor, however, used solid graphite as a neutron moderator to slow down the neutrons, and the water in it, on the contrary, acts like a harmful neutron absorber. Thus neutrons are slowed down even if steam bubbles form in the water. Furthermore, because steam absorbs neutrons much less readily than water, increasing the intensity of vaporization means that more neutrons are able to split uranium atoms, increasing the reactor's power output. This makes the RBMK design very unstable at low power levels, and prone to suddenly increasing energy production to a dangerous level. This behaviour is counter-intuitive, and this property of the reactor was unknown to the crew.
• A more significant flaw was in the design of the control rods that are inserted into the reactor to slow down the reaction. In the RBMK reactor design, the lower part of each control rod was made of graphite and was 1.3 metres shorter than necessary, and in the space beneath the rods were hollow channels filled with water. The upper part of the rod, the truly functional part that absorbs the neutrons and thereby halts the reaction, was made of boron carbide. With this design, when the rods are inserted into the reactor from the uppermost position, the graphite parts initially displace some water (which absorbs neutrons, as mentioned above), effectively causing fewer neutrons to be absorbed initially. Thus for the first few seconds of control rod activation, reactor power output is increased, rather than reduced as desired. This behaviour is counter-intuitive and was not known to the reactor operators.
• Other deficiencies besides these were noted in the RBMK-1000 reactor design, as were its non-compliance with accepted standards and with the requirements of nuclear reactor safety.

While INSAG-1 and INSAG-7 reports both identified operator error as an issue of concern, the INSAG-7 identified that there were numerous other issues that were contributing factors that led to the incident. These contributing factors include:^[111]

• The plant was not designed to safety standards in effect and incorporated unsafe features
• "Inadequate safety analysis" was performed
• There was "insufficient attention to independent safety review"
• "Operating procedures not founded satisfactorily in safety analysis"
• Safety information not adequately and effectively communicated between operators, and between operators and designers
• The operators did not adequately understand safety aspects of the plant
• Operators did not sufficiently respect formal requirements of operational and test procedures
• The regulatory regime was insufficient to effectively counter pressures for production
• There was a "general lack of safety culture in nuclear matters at the national level as well as locally"

Analysis of views

Both views were heavily lobbied by different groups, including the reactor's designers, power plant personnel, and the Soviet and Ukrainian governments. According to the IAEA's 1986 analysis, the main cause of the accident was the operators' actions. But according to the IAEA's 1993 revised analysis the main cause was the reactor's design.^[112] One reason there were such contradictory viewpoints and so much debate about the causes of the Chernobyl accident was that the primary data covering the disaster, as registered by the instruments and sensors, were not completely published in the official sources.

Once again, the human factor had to be considered as a major element in causing the accident. INSAG notes that both the operating regulations and staff handled the disabling of the reactor protection easily enough: witness the length of time for which the ECCS was out of service while the reactor was operated at half power. INSAG's view is that it was the operating crew's deviation from the test programme that was mostly to blame. "Most reprehensibly, unapproved changes in the test procedure were deliberately made on the spot, although the plant was known to be in a very different condition from that intended for the test."^[36]:24

As in the previously released report INSAG-1, close attention is paid in report INSAG-7 to the inadequate (at the moment of the accident) "culture of safety" at all levels. Deficiency in the safety culture was inherent not only at the operational stage but also, and to no lesser extent, during activities at other stages in the lifetime of nuclear power plants (including design, engineering, construction, manufacture, and regulation). The poor quality of operating procedures and instructions, and their conflicting character, put a heavy burden on the operating crew, including the chief engineer. "The accident can be said to have flowed from a deficient safety culture, not only at the Chernobyl plant, but throughout the Soviet design, operating and regulatory organizations for nuclear power that existed at that time."^[36]:24

National and international spread of radioactive substances

Although no informing comparisons can be made between the accident and a strictly air burst-fuzed nuclear detonation, it has still been approximated that about four hundred times more radioactive material was released from Chernobyl than by the atomic bombing of Hiroshima and Nagasaki. By 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, 1950 – 1960s, with the 1/100 to 1/1000 variance due to trying to make comparisons with different spectrums of isotopes released.^[113] Approximately 100,000 km² of land was significantly contaminated with fallout, with the worst hit regions being in Belarus, Ukraine and Russia.^[114] Slighter levels of contamination were detected over all of Europe except for the Iberian Peninsula.^{[115][116][117]}

The initial evidence that a major release of radioactive material was affecting other countries came not from Soviet sources, but from Sweden. On the morning of 28 April^[118] workers at the Forsmark Nuclear Power Plant (approximately 1,100 km (680 mi) from the Chernobyl site) were found to have radioactive particles on their clothes.^[119]

It was Sweden's search for the source of radioactivity, after they had determined there was no leak at the Swedish plant, that at noon on 28 April led to the first hint of a serious nuclear problem in the western Soviet Union. Hence the evacuation of Pripyat on 27 April 36 hours after the initial explosions, was silently completed before the disaster became known outside the Soviet Union. The rise in radiation levels had at that time already been measured in Finland, but a civil service strike delayed the response and publication.^[120]

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 water-flows across the ground contributed further 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.^{[122]:43–44, 78}

Rain was purposely seeded over 10,000 km² of the Belorussian SSR 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.^[123] Reports from Soviet and Western scientists indicate that Belarus received about 60% of the contamination that fell on the former Soviet Union. However, the 2006 TORCH report stated that half of the volatile particles had landed outside Ukraine, Belarus, and Russia. A large area in Russia south of Bryansk was also contaminated, as were parts of northwestern Ukraine. Studies in surrounding countries indicate that over one million people could have been affected by radiation.^[124]

Recently published data from a long-term monitoring program (The Korma Report II)^[125] shows a decrease in internal radiation exposure of the inhabitants of a region in Belarus close to Gomel. Resettlement may even be possible in prohibited areas provided that people comply with appropriate dietary rules.

In Western Europe, precautionary measures taken in response to the radiation included seemingly arbitrary regulations banning the importation of certain foods but not others. In France some officials stated that the Chernobyl accident had no adverse effects.^[126] Official figures in southern Bavaria in Germany indicated that some wild plant species contained substantial levels of caesium, which were believed to have been passed onto them during their consumption by wild boars, a significant number of which already contained radioactive particles above the allowed level.^[127]

Radioactive release

Like many other releases of radioactivity into the environment, the Chernobyl release was controlled by the physical and chemical properties of the radioactive elements in the core. Particularly dangerous are 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 and caesium-137 are responsible for most of the radiation exposure received by the general population.^[3]

Detailed reports on the release of radioisotopes from the site were published in 1989^[128] and 1995,^[129] with the latter report updated in 2002.^[3]

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,^[130] and it continues to reduce beyond 0.78% after 7 half-lives to 0.098% remaining after 10 half-lives have passed and so on.^[131][132] (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.^[3] The atmospheric release of xenon-133, with a half-life of 5 days, is estimated at 5200 PBq.^[3]
• 50 to 60% of all core radioiodine in the reactor, about 1760 PBq (7018176000000000000♠1760×10¹⁵ becquerels), or about 0.4 kg, was released, as a mixture of sublimed vapour, solid particles, and organic iodine compounds. Iodine-131 has a half-life of 8 days.^[3]
• 20 to 40% of all core caesium-137 was released, 85 PBq in all.^[3][133] 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.^[134] 7016850000000000000♠8.5×10¹⁶ Bq equals 24 kilograms of caesium-137.^[134] Cs-137 has a half-life of 30 years.^[3]
• Tellurium-132, half-life 78 hours, an estimated 1150 PBq was released.^[3]
• An early estimate for total nuclear fuel material released to the environment was 7000300000000000000♠3±1.5%; this was later revised to 7000350000000000000♠3.5±0.5%. This corresponds to the atmospheric emission of 6 t of fragmented fuel.^[129]

Two sizes of particles were released: small particles of 0.3 to 1.5 micrometres, each an individually unrecognizable small dust or smog sized particulate matter and larger settling dust sized particles that therefore were quicker to fall-out of the air, of 10 micrometres in diameter. These larger particles contained about 80% to 90% of the released high boiling point or non-volatile radioisotopes; zirconium-95, niobium-95, lanthanum-140, cerium-144 and the transuranic elements, including neptunium, plutonium and the minor actinides, embedded in a uranium oxide matrix.

The dose that was calculated is the relative external gamma dose rate for a person standing in the open. The exact dose to a person in the real world who would spend most of their time sleeping indoors in a shelter and then venturing out to consume an internal dose from the inhalation or ingestion of a radioisotope, requires a personnel specific radiation dose reconstruction analysis.

Residual radioactivity in the environment

Rivers, lakes and reservoirs

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.^[135]:60 The radioactive contamination of aquatic systems therefore became a major problem in the immediate aftermath of the accident.^[136] In the most affected areas of Ukraine, levels of radioactivity (particularly from radionuclides ¹³¹I, ¹³⁷Cs and ⁹⁰Sr) in drinking water caused concern during the weeks and months after the accident,^[136] though officially it was stated that all contaminants had settled to the bottom "in an insoluble phase" and would not dissolve for 800–1000 years.^[135]:64 Guidelines for levels of radioiodine in drinking water were temporarily raised to 3,700 Bq/L, allowing most water to be reported as safe,^[136] and a year after the accident it was announced that even the water of the Chernobyl plant's cooling pond was within acceptable norms. Despite this, two months after the disaster the Kiev water supply was abruptly switched from the Dnieper to the Desna River.^[135]:64–5 Meanwhile, massive silt traps were constructed, along with an enormous 30m-deep underground barrier to prevent groundwater from the destroyed reactor entering the Pripyat River.^{[135]:65–67}

Bio-accumulation of radioactivity in fish^[137] resulted in concentrations (both in western Europe and in the former Soviet Union) that in many cases were significantly above guideline maximum levels for consumption.^[136] Guideline maximum levels for radiocaesium in fish vary from country to country but are approximately 1000 Bq/kg in the European Union.^[138] In the Kiev Reservoir in Ukraine, concentrations in fish were several thousand Bq/kg during the years after the accident.^[137]

In small "closed" lakes in Belarus and the Bryansk region of Russia, concentrations in a number of fish species varied from 100 to 60,000 Bq/kg during the period 1990–92.^[139] The contamination of fish caused short-term concern in parts of the UK and Germany and in the long term (years rather than months) in the affected areas of Ukraine, Belarus, and Russia as well as in parts of Scandinavia.^[136]

Flora and fauna

After the disaster, four square kilometres of pine forest directly downwind of the reactor turned reddish-brown and died, earning the name of the "Red Forest".^[141] Some animals in the worst-hit areas also died or stopped reproducing. Most domestic animals were removed from the exclusion zone, but horses left on an island in the Pripyat River 6 km (4 mi) from the power plant died when their thyroid glands were destroyed by radiation doses of 150–200 Sv.^[142] Some cattle on the same island died and those that survived were stunted because of thyroid damage. The next generation appeared to be normal.^[142]

A robot sent into the reactor itself has returned with samples of black, melanin-rich radiotrophic fungi that are growing on the reactor's walls.^[145]

Of the 440,350 wild boar killed in the 2010 hunting season in Germany, approximately one thousand were found to be contaminated with levels of radiation above the permitted limit of 600 becquerels of Cesium per kilogram, due to residual radioactivity from Chernobyl.^[146] While all animal meat contains a natural level of potassium 40 at a similar level of activity, with both wild and farm animals in Italy containing "415 ± 56 becquerels kg−1 dw" of that naturally occuring gamma emitter.^[147] The cesium contamination issue has historically reached some uniquely isolated and high levels approaching 20,000 Becquerels of Cesium per kilogram in some specific tests, however as it has not been observed in the wild boar population of Fukushima after the 2011 accident.^[148] Evidence exists to suggest that the wild German and Ukranian boar population are in a unique location were they have subsisted on a diet high in plant or fungi sources that biomagnifies or concentrates radio-cesium, with the most well known food source the consumption of the outer shell or wall of the "deer-truffle"/Elaphomyces which along with magnifying radio-cesium also magnifies or concentrates natural soil concentrations of Arsenic.^[149]

The Norwegian Agricultural Authority reported that in 2009 a total of 18,000 livestock in Norway needed to be given uncontaminated feed for a period of time before slaughter in order to ensure that their meat was safe for human consumption. This was due to residual radioactivity from Chernobyl in the plants they graze on in the wild during the summer. 1,914 sheep needed to be given uncontaminated feed for a period of time before slaughter during 2012, and these sheep were located in just 18 of Norway's municipalities, a decrease of 17 from the 35 municipalities affected animals were located in during 2011 (117 municipalities were affected during 1986).^[150]

The after-effects of Chernobyl were expected to be seen for a further 100 years, although the severity of the effects would decline over that period.^[151] Scientists report this is due to radioactive caesium-137 isotopes being taken up by fungi such as Cortinarius caperatus which is in turn eaten by sheep whilst grazing.^[150]

The United Kingdom was forced to restrict 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 in 1986, a total of 4,225,000 sheep had their movement restricted across a total of 9,700 farms, in order to prevent contaminated meat entering the human food chain.^[152] The number of sheep and the number of 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.^[152] The restrictions applying in Scotland were lifted in 2010, whilst those applying to Wales and Cumbria were lifted during 2012, meaning no farms in the UK remain restricted because of Chernobyl fallout.^[153][154]

The legislation used to control sheep movement and compensate farmers (farmers were latterly compensated per animal to cover additional costs in holding animals prior to radiation monitoring) was revoked during October and November 2012 by the relevant authorities in the UK.^[155]

Assessment difficulties and disputes

By the year 2000, the number of Ukrainians claiming to be radiation 'sufferers' (poterpili) and receiving state benefits had jumped to 3.5 million, or 5% of the population. Many of these are populations resettled from contaminated zones, or former or current Chernobyl plant workers.^[105]:4–5 According to IAEA-affiliated scientific bodies, these apparent increases of ill health result partly from economic strains on these countries and poor health-care and nutrition; also, they suggest that increased medical vigilance following the accident has meant that many cases that would previously have gone unnoticed (especially of cancer) are now being registered.^[114]

The World Health Organization states, "children conceived before or after their father's exposure showed no statistically significant differences in mutation frequencies".^[165] This statistically insignificant increase was also seen by independent researchers analyzing the children of the Chernobyl liquidators.^[166]

On farms in Narodychi Raion of Ukraine it is claimed that in the first four years of the disaster nearly 350 animals were born with gross deformities such as missing or extra limbs, missing eyes, heads or ribs, or deformed skulls; in comparison, only three abnormal births had been registered in the five years prior.^{[167][168][169][170][171][172]} The two primary individuals involved with the attempt to suggest that the mutation rate amongst animals was, and continues to be, higher in the Chernobyl zone, are the Anders Moller and Timothy Mousseau group.^{[173][174][175][164]} Apart from continuing to publish experimentally unrepeatable and discredited papers, Mousseau routinely gives talks at the Helen Caldicott organized symposiums for "Physicians for Social Responsibility", an anti-nuclear advocacy group, devoted to bring about a "nuclear free planet".^[176] Moreover, in years past Moller was previously caught and reprimanded for publishing papers that crossed the scientific "misconduct"/"fraud" line.^[177] The duo have more recently attempted to publish meta-analyses in which the primary references they weigh-up, analyze and draw their conclusions from is their own prior papers along with the discredited book Chernobyl: Consequences of the Catastrophe for People and the Environment.^[178]

In 1996, geneticist colleagues Ronald Chesser and Robert Baker published a paper on the thriving vole population within the exclusion zone, in which the central conclusion of their work 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.^[179] These alarming conclusions led the paper to appear on the front cover of the prestigious journal Nature, however not long after publication Chesser & Baker discovered a fundamental error in their research in which they had incorrectly classified the species of vole, and therefore were comparing the genetics of two entirely different vole species to start with.^[164][180]

Сancer assessments

A report by the International Atomic Energy Agency examines the environmental consequences of the accident.^[16] The United Nations Scientific Committee on the Effects of Atomic Radiation has estimated a global collective dose of radiation exposure from the accident "equivalent on average to 21 additional days of world exposure to natural background radiation"; individual doses were far higher than the global mean among those most exposed, including 530,000 primarily male recovery workers (the Chernobyl liquidators) who averaged an effective dose equivalent to an extra 50 years of typical natural background radiation exposure each.^{[199][200][201]}

Estimates of the number of deaths that will eventually result from the accident vary enormously; disparities reflect both the lack of solid scientific data and the different methodologies used to quantify mortality—whether the discussion is confined to specific geographical areas or extends worldwide, and whether the deaths are immediate, short term, or long term.

In 1994, thirty-one deaths were directly attributed to the accident, all among the reactor staff and emergency workers.^[156] As of the 2008 report by the United Nations Scientific Committee on the Effects of Atomic Radiation, and the total number of confirmed deaths from radiation was 64 and was expected to continue to rise.

The Chernobyl Forum predicts that the eventual death toll could reach 4,000 among those exposed to the highest levels of radiation (200,000 emergency workers, 116,000 evacuees and 270,000 residents of the most contaminated areas); this figure is a total causal death toll prediction, combining the deaths of approximately 50 emergency workers who died soon after the accident from acute radiation syndrome, 15 children who have died of thyroid cancer and a future predicted total of 3935 deaths from radiation-induced cancer and leukaemia.^[202]

In a peer-reviewed paper in the International Journal of Cancer in 2006, the authors expanded the discussion on those exposed to all of Europe (but following a different conclusion methodology to the Chernobyl Forum study, which arrived at the total predicted death toll of 4,000 after cancer survival rates were factored in) they stated, without entering into a discussion on deaths, that in terms of total excess cancers attributed to the accident:^[203]

The risk projections suggest that by now [2006] Chernobyl may have caused about 1000 cases of thyroid cancer and 4000 cases of other cancers in Europe, representing about 0.01% of all incident cancers since the accident. Models predict that by 2065 about 16,000 cases of thyroid cancer and 25,000 cases of other cancers may be expected due to radiation from the accident, whereas several hundred million cancer cases are expected from other causes.

Two anti-nuclear advocacy groups have publicized non-peer-reviewed estimates that include mortality estimates for those who were exposed to even smaller amounts of radiation. The Union of Concerned Scientists (UCS) calculated that, among the hundreds of millions of people exposed worldwide, there will be an eventual 50,000 excess cancer cases, resulting in 25,000 excess cancer deaths, excluding thyroid cancer.^[204] However, these calculations are based on a simple linear no-threshold model multiplication and the misapplication of the collective dose, which the International Commission on Radiological Protection (ICRP) states "should not be done" as using the collective dose is "inappropriate to use in risk projections".^[205]

Along similar lines to the UCS approach, the 2006 TORCH report, commissioned by the European Greens political party, likewise simplistically calculates an eventual 30,000 to 60,000 excess cancer deaths in total, around the globe.^[115]

Due in largest part from the ingestion of contaminated dairy products along with the inhalation of the short-lived and therefore highly radioactive isotope, Iodine-131, the 2005 UN collaborative Chernobyl Forum revealed thyroid cancer among children to be one of the main health impacts from the Chernobyl accident. In that publication more than 4000 cases were reported, and that there was no evidence of an increase in solid cancers or leukemia. It said that there was an increase in psychological problems among the affected population.^[158] Dr Michael Repacholi, manager of WHO's Radiation Program reported that the 4000 cases of thyroid cancer resulted in nine deaths.^[6]

According to the United Nations Scientific Committee on the Effects of Atomic Radiation, up to the year 2005, an excess of over 6000 cases of thyroid cancer have been reported. That is, over the estimated pre-accident baseline thyroid cancer rate, more than 6000 casual cases of thyroid cancer have been reported in children and adolescents exposed at the time of the accident, a number that is expected to increase. They concluded that there is no other evidence of major health impacts from the radiation exposure.^[206]

Well-differentiated thyroid cancers are generally treatable,^[207] and when treated the five-year survival rate of thyroid cancer is 96%, and 92% after 30 years.^[208] the United Nations Scientific Committee on the Effects of Atomic Radiation had reported 15 deaths from thyroid cancer in 2011.^[209] The International Atomic Energy Agency (IAEA) also states that there has been no increase in the rate of birth defects or abnormalities, or solid cancers (such as lung cancer) corroborating the assessments by the UN committee.^[210] UNSCEAR raised the possibility of long term genetic defects, pointing to a doubling of radiation-induced minisatellite mutations among children born in 1994.^[211] However, the risk of thyroid cancer associated with the Chernobyl accident is still high according to published studies.^[212][213]

The German affiliate of the ultra-anti-nuclear energy organization, the International Physicians for the Prevention of Nuclear War suggest that 10,000 people are affected by thyroid cancer as of 2006 and that 50,000 cases are expected in the future.^[214]

Other health disorders

Fred Mettler, a radiation expert at the University of New Mexico, puts the number of worldwide cancer deaths outside the highly contaminated zone at "perhaps" 5000, for a total of 9000 Chernobyl-associated fatal cancers, saying "the number is small (representing a few percent) relative to the normal spontaneous risk of cancer, but the numbers are large in absolute terms".^[215] The same report outlined studies based in data found in the Russian Registry from 1991 to 1998 that suggested that "of 61,000 Russian workers exposed to an average dose of 107 mSv about 5% of all fatalities that occurred may have been due to radiation exposure."^[158]

The report went into depth about the risks to mental health of exaggerated fears about the effects of radiation.^[158] According to the IAEA the "designation of the affected population as "victims" rather than "survivors" has led them to perceive themselves as helpless, weak and lacking control over their future". The IAEA says that this may have led to behaviour that has caused further health effects.^[216]

Fred Mettler commented that 20 years later: "The population remains largely unsure of what the effects of radiation actually are and retain a sense of foreboding. A number of adolescents and young adults who have been exposed to modest or small amounts of radiation feel that they are somehow fatally flawed and there is no downside to using illicit drugs or having unprotected sex. To reverse such attitudes and behaviours will likely take years although some youth groups have begun programs that have promise."^[217] In addition, disadvantaged children around Chernobyl suffer from health problems that are attributable not only to the Chernobyl accident, but also to the poor state of post-Soviet health systems.^[210]

The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), part of the Chernobyl Forum, have produced their own assessments of the radiation effects.^[218] UNSCEAR was set up as a collaboration between various United Nation bodies, including the World Health Organization, after the atomic bomb attacks on Hiroshima and Nagasaki, to assess the long-term effects of radiation on human health.^[219]

Deaths due to radiation exposure

The number of potential deaths arising from the Chernobyl disaster is heavily debated. The WHO's prediction of 4000 future cancer deaths in surrounding countries^[220] is based on the Linear no-threshold model (LNT), which assumes that the damage inflicted by radiation at low doses is directly proportional to the dose.^[221] Radiation epidemiologist Roy Shore contends that estimating health effects in a population from the LNT model "is not wise because of the uncertainties".^[222]

According to the Union of Concerned Scientists the number of excess cancer deaths worldwide (including all contaminated areas) is approximately 27,000 based on the same LNT.^[223]

Another study critical of the Chernobyl Forum report was commissioned by Greenpeace, which asserted that the most recently published figures indicate that in Belarus, Russia and Ukraine the accident could have resulted in 10,000–200,000 additional deaths in the period between 1990 and 2004.^[224] The Scientific Secretary of the Chernobyl Forum criticized the report's reliance on non-peer-reviewed locally produced studies. Although most of the study's sources were from peer-reviewed journals, including many Western medical journals, the higher mortality estimates were from non-peer-reviewed sources,^[224] while Gregory Härtl (spokesman for the WHO) suggested that the conclusions were motivated by ideology.^[225]

Chernobyl: Consequences of the Catastrophe for People and the Environment is a 2007 Russian publication that concludes that there were 985,000 premature deaths as a result of the radioactivity released.^[226] The results were criticized by M. I. Balonov of the Institute of Radiation Hygiene in St. Petersburg, who described them as biased, drawing from sources which were difficult to independently verify and lacking a proper scientific base. Balanov expressed his opinion that "the authors unfortunately did not appropriately analyze the content of the Russian-language publications, for example, to separate them into those that contain scientific evidence and those based on hasty impressions and ignorant conclusions".^[227]

Decommissioning

In October 1991, a fire broke out in the turbine building of reactor 2;^[235] the authorities subsequently declared the reactor damaged beyond repair, and it was taken offline. Reactor 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 3 in an official ceremony, shutting down the entire site.^[236]

Radioactive waste management

Containment of the reactor

The Chernobyl reactor is now enclosed in a large concrete sarcophagus, which was built quickly to allow continuing operation of the other reactors at the plant.^[11]

A New Safe Confinement was to have been built by the end of 2005; however, it has suffered ongoing delays and as of 2010, when construction finally began, was expected to be completed in 2013. This was delayed again to 2016, the end of the 30-year lifespan of the sarcophagus. The structure was built adjacent to the existing shelter and in November 2016 was slid into place on rails. It is a metal arch 105 metres (344 ft) high and spanning 257 metres (843 ft), to cover both unit 4 and the hastily built 1986 structure. The Chernobyl Shelter Fund, set up in 1997, has received €810 million from international donors and projects to cover this project and previous work. It and the Nuclear Safety Account, also applied to Chernobyl decommissioning, are managed by the European Bank for Reconstruction and Development (EBRD).

As of 29 November 2016, Reactor No. 4 has been covered by the New Safe Confinement that covers the reactor and the unstable “sarcophagus”.^[237] The huge steel arch was moved into place over several weeks, and the completion of this procedure was celebrated with a ceremony at the site, attended by the Ukrainian president, Petro Poroshenko, diplomats and site workers.^[238] Unlike the original sarcophagus, the New Safe Confinement is designed to allow the reactor to be safely dismantled using remotely operated equipment.

By 2002, roughly 15,000 Ukrainian workers were still working within the Zone of Exclusion, maintaining the plant and performing other containment- and research-related tasks, often in dangerous conditions.^[233]:2 A handful of Ukrainian scientists work inside the sarcophagus, but outsiders are rarely granted access. In 2006 an Australian 60 Minutes team led by reporter Richard Carleton and producer Stephen Rice were allowed to enter the sarcophagus for 15 minutes and film inside the control room.^[239]

On 12 February 2013, a 600 m² (6,500 sq ft) section of the roof of the turbine-building, adjacent to the sarcophagus, collapsed. At first it was assumed that the roof collapsed because of the weight of snow on it. However the amount of snow was not exceptional, and the report of a Ukrainian fact-finding panel concluded that the part collapse of the turbine-building was the result of sloppy repair work and aging of the structure. The report mentioned the possibility that the repaired part of the turbine-building added a larger strain on the total structure than expected, and the braces in the roof were damaged by corrosion and sloppy welding. Experts such as Valentin Kupny, former deputy director of the nuclear plant, did warn that the complex was on the verge of a collapse, leaving the building in an extremely dangerous condition. A proposed reinforcement in 2005 was cancelled by a superior official. After the 12 February incident, radioactivity levels were up to 19 becquerels per cubic meter of air: 12 times normal. The report assumed radioactive materials from inside the structure spread to the surroundings after the roof collapsed. All 225 workers employed by the Chernobyl complex and the French company that is building the new shelter were evacuated shortly after the collapse. According to the managers of the complex, radiation levels around the plant were at normal levels (between 5 and 6 µSv/h) and should not affect workers' health. According to Kupny the situation was underestimated by the Chernobyl nuclear complex managers, and information was kept secret.^[240][241]

The Exclusion Zone

An area originally extending 30 kilometres (19 mi) in all directions from the plant is officially called the "zone of alienation". It is largely uninhabited, except for about 300 residents who have refused to leave. The area has largely reverted to forest, and has been overrun by wildlife because of a lack of competition with humans for space and resources. Even today, radiation levels are so high that the workers responsible for rebuilding the sarcophagus are only allowed to work five hours a day for one month before taking 15 days of rest. Ukrainian officials estimated the area would not be safe for human life again for another 20,000 years^[73] (although by 2016, 187 local Ukrainians had returned and were living permanently in the zone^[245]).

In 2011 Ukraine opened up the sealed zone around the Chernobyl reactor to tourists who wish to learn more about the tragedy that occurred in 1986.^{[246][247][248]} Sergii Mirnyi, a radiation reconnaissance officer at the time of the accident, and now an academic at National University of Kyiv-Mohyla Academy in Kiev, Ukraine, has written about the psychological and physical effects on survivors and visitors, and worked as an advisor to Chernobyl tourism groups.^[248][249]

Forest fires

If the forests that have been contaminated by radioactive material catch on fire, they will spread the radioactive material further outwards in the smoke.^[250][251]

The Chernobyl Shelter Fund

The Chernobyl Shelter Fund was established in 1997 at the Denver 23rd G8 summit to finance the Shelter Implementation Plan (SIP). The plan calls for transforming the site into an ecologically safe condition by means of stabilization of the sarcophagus followed by construction of a New Safe Confinement (NSC). While the original cost estimate for the SIP was US$768 million, the 2006 estimate was $1.2 billion. The SIP is being managed by a consortium of Bechtel, Battelle, and Électricité de France, and conceptual design for the NSC consists of a movable arch, constructed away from the shelter to avoid high radiation, to be slid over the sarcophagus. The NSC was moved into position in November 2016 and is expected to be completed in late 2017,^[252] and is the largest movable structure ever built.

Dimensions:

• Span: 270 m (886 ft)
• Height: 100 m (330 ft)
• Length: 150 m (492 ft)

The United Nations Development Programme

The United Nations Development Programme has launched in 2003 a specific project called the Chernobyl Recovery and Development Programme (CRDP) for the recovery of the affected areas.^[253] 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's activities is supporting the Government of Ukraine in mitigating long-term social, economic, and ecological consequences of the Chernobyl catastrophe. CRDP works in the four most Chernobyl-affected areas in Ukraine: Kyivska, Zhytomyrska, Chernihivska and Rivnenska.

The International Project on the Health Effects of the Chernobyl Accident

The International Project on the Health Effects of the Chernobyl Accident (IPEHCA) was created and received US $20 million, mainly from Japan, in hopes of discovering the main cause of health problems due to ¹³¹I radiation. These funds were divided among Ukraine, Belarus, and Russia, the three main affected countries, for further investigation of health effects. As there was significant corruption in former Soviet countries, most of the foreign aid was given to Russia, and no positive outcome from this money has been demonstrated.

Chernobyl Children International

Chernobyl Children International (CCI) is a United Nations-accredited, non-profit, international development, medical, and humanitarian organization that works with children, families and communities that continue to be affected by the economic outcome of the Chernobyl accident. The organization's founder and chief executive is Adi Roche. The CCI was founded in 1991 in response to an appeal from Ukrainian and Belarusian doctors for aid. Roche then began organizing 'rest and recuperation' holidays for a few Chernobyl children. Recruiting Irish families who would welcome and care for them, CCI expanded into the United States in 2001.^[254][255]

It works closely with the Belarusian government, the United Nations, and many thousands of volunteers worldwide to deliver a broad range of economic supports to the children and the wider community. It also acts as an advocate for the rights of those affected by the Chernobyl explosion, and engages in research and outreach activities to encourage the rest of the world to remember the victims and understand the long-term impact on their lives.^[254][255]