Chernobyl disaster

The nuclear reactor after the disaster. Reactor 4 (center). Turbine building (lower left). Reactor 3 (center right).

The Chernobyl disaster (Ukrainian: Чорнобильська катастрофа, Chornobylska Katastrofa – Chornobyl Catastrophe) was a catastrophic nuclear accident that occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in Ukraine (then officially the Ukrainian SSR), which was under the direct jurisdiction of the central authorities of the Soviet Union. An explosion and fire released large quantities of radioactive particles into the atmosphere, which spread over much of the western USSR and Europe.

The Chernobyl disaster is widely considered to have been the worst nuclear power plant accident in history, and is one of only two classified as a level 7 event on the International Nuclear Event Scale (the other being the Fukushima Daiichi nuclear disaster in 2011). The battle to contain the contamination and avert a greater catastrophe ultimately involved over 500,000 workers and cost an estimated 18 billion rubles.  The official Soviet casualty count of 31 deaths has been disputed, and long-term effects such as cancers and deformities are still being accounted for.

Overview

The disaster began during a systems test on Saturday, 26 April 1986 at reactor number four of the Chernobyl plant, which is near the city of Pripyat and in proximity to the administrative border with Belarus and the Dnieper river. There was a sudden and unexpected power surge, and when an emergency shutdown was attempted, an exponentially larger spike in power output occurred, which led to a reactor vessel rupture and a series of steam explosions. These events exposed the graphite moderator of the reactor to air, causing it to ignite. The resulting fire sent a plume of highly radioactive fallout into the atmosphere and over an extensive geographical area, including Pripyat. The plume drifted over large parts of the western Soviet Union and Europe. From 1986 to 2000, 350,400 people were evacuated and resettled from the most severely contaminated areas of Belarus, Russia, and Ukraine. According to official post-Soviet data, about 60% of the fallout landed in Belarus.

The accident raised concerns about the safety of the Soviet nuclear power industry, as well as nuclear power in general, slowing its expansion for a number of years and forcing the Soviet government to become less secretive about its procedures. The government coverup of the Chernobyl disaster was a "catalyst" for glasnost, which "paved the way for reforms leading to the Soviet collapse".

Russia, Ukraine, and Belarus have been burdened with the continuing and substantial decontamination and health care costs of the Chernobyl accident. A report by the International Atomic Energy Agency examines the environmental consequences of the accident. Another UN agency, UNSCEAR, 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 local recovery workers who averaged an effective dose equivalent to an extra 50 years of typical natural background radiation exposure each. 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.

Thirty one deaths are directly attributed to the accident, all among the reactor staff and emergency workers. An UNSCEAR report places the total confirmed deaths from radiation at 64 as of 2008. The Chernobyl Forum predicts 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, nine children who have died of thyroid cancer and a future predicted total of 3940 deaths from radiation-induced cancer and leukemia.

In a peer reviewed publication in the International Journal of Cancer in 2006, the authors of which, following a different conclusion methodology to the Chernobyl forum study, which arrived at the total predicted, 4000, death toll after cancer survival rates were factored in, the paper stated, without entering into a discussion on deaths, that in terms of total excess cancers attributed to the accident: The risk projections suggest that by now Chernobyl may have caused about 1,000 cases of thyroid cancer and 4,000 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. Also based upon extrapolations from the Linear no-threshold model of radiation induced damage, down to zero, the Union of Concerned Scientists estimates that, among the hundreds of millions of people living in broader geographical areas, there will be 50,000 excess cancer cases resulting in 25,000 excess cancer deaths.

For this broader group, the 2006 TORCH report, commissioned by the European Greens political party, predicts 30,000 to 60,000 excess cancer deaths.

In terms of non-scientific publications, two affiliated with the anti-nuclear advocacy group Greenpeace, have been released, one of which reports the figure at 200,000 or more. The Russian founder of that regions chapter of Greenpeace, also authored a book titled Chernobyl:Consequences of the Catastrophe..., which concludes that among the billions of people worldwide who were exposed to radioactive contamination from the disaster, nearly a million premature cancer deaths occurred between 1986 and 2004.

The book however has failed the peer review process, five reviews were published in the academic press, with four of them considering the book severely flawed and contradictory, and one praising it while noting some shortcomings. The review by M. I. Balonov published by the New York Academy of Sciences concludes that the value of the report is negative, because it has very little scientific merit while being highly misleading to the lay reader. It also characterized the estimate of nearly a million deaths as more in the realm of science fiction than science.

Accident

A model of the Chernobyl reactor after the lid of the reactor chamber blew off. Ukrainian National Chornobyl Museum, Kiev, Ukraine.

On 26 April 1986, at 01:23 (UTC+3), reactor four suffered a catastrophic power increase, leading to explosions in its core. This dispersed large quantities of radioactive fuel and core materials into the atmosphere and ignited the combustible graphite moderator. The burning graphite moderator increased the emission of radioactive particles, carried by the smoke, as the reactor had not been encased by any kind of hard containment vessel. The accident occurred during an experiment scheduled to test a potential safety emergency core cooling feature, which took place during a normal shutdown procedure.

Steam turbine tests

An inactive nuclear reactor continues to generate a significant amount of residual decay heat. In an initial shut-down state (for example, following an emergency SCRAM) the reactor produces around 7 percent of its total thermal output and requires cooling to avoid core damage. RBMK reactors, like those at Chernobyl, use water as a coolant. Reactor 4 at Chernobyl consisted of about 1,600 individual fuel channels; each required a coolant flow of 28 metric tons (28,000 liters or 7,400 US gallons) per hour.

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 to attain full speed and reach the 5.5‑megawatt (MW) output required to run one main pump.

To solve this one-minute gap, considered an unacceptable safety risk, it had been theorised 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, bridging the gap between an external power failure and the full availability of the emergency generators.

This capability still needed to be confirmed experimentally, and previous tests had ended unsuccessfully. An initial test carried out in 1982 showed 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 was to be repeated again in 1986, and it was scheduled to take place during the maintenance shutdown of Reactor Four.

The test focused on the switching sequences of the electrical supplies for the reactor. The test procedure was to begin with an automatic emergency shutdown. No detrimental effect on the safety of the reactor was anticipated, so the test program 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).

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.

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).

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 freewheel down.

Conditions prior to the accident

A schematic diagram of the reactor.

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. 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 postponement, 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. In addition, had the reactor been shutdown 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 shut-down 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.

The night shift had very limited time to prepare for and carry out the experiment. A further rapid reduction 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.

The test plan called for a gradual reduction in power output from reactor 4 to a thermal level of 700–1000 MW. An output of 700 MW was reached at 00:05 on 26 April. However, due to the natural production of xenon-135, a neutron absorber, core power continued to decrease without further operator action—a process known as reactor poisoning. 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 and Toptunov died in the hospital on May 10 and 14, respectively. This combination of factors rendered the reactor in an unintended near-shutdown state, with a power output of 30 MW thermal or less.

The reactor was now only producing around 5 percent of the minimum initial power level established as safe for the test. Control-room personnel consequently made the decision 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. 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. 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.

Chernobyl plant model on display at Kiev Ukrainian National Chernobyl Museum.

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. Various alarms started going off at this point. 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. In the period between 00:35 and 00:45, emergency alarm signals concerning thermal-hydraulic parameters were ignored, apparently to preserve the reactor power level.

After a while, a more or less stable state at a power level of 200 MW was achieved, and 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. Since water also 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 also to remove more manual control rods to maintain power.

All these actions led to an extremely unstable reactor configuration. Nearly all of the control rods were removed manually, including all but 9 of the "fail-safe" manually-operated rods, which were intended to remain fully inserted to control the reaction even in the event of a loss of coolant. While the emergency SCRAM system that would insert all control rods to shut down the reactor could still be activated manually, the automated system that could do the same had been disabled to maintain power, and many other automated and even passive safety features of the reactor had been bypassed. Further, the reactor coolant had reduced boiling, reducing steam voids, but had limited margin to boiling, so any power excursion would produce boiling, reducing neutron absorption by the water. The reactor was in an unstable configuration that was clearly outside the safe operating envelope established by the designers.

Experiment and explosion

Aerial view of the damaged core on 3 May 1986. Roof of the turbine hall is damaged (image center). Roof of the adjacent reactor 3 (image lower left) shows minor fire damage.

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 generator 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, however, 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.

Because of the positive void coefficient of the RBMK reactor at low reactor power levels, it 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 yet a further power increase. During almost the entire period of the experiment the automatic control system successfully counteracted this positive feedback, continuously inserting control rods into the reactor core to limit the power rise. However, 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, an emergency shutdown of the reactor, which inadvertently triggered the explosion, 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 incautiously 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 conclusively proving this. Some have suggested that the button was not pressed, and instead the signal was automatically produced by the emergency protection system; however, the SKALA clearly registered a manual SCRAM signal. In spite of this, the question as to when or even whether the EPS-5 button was pressed has been the subject of debate. There are assertions that the pressure was caused by the rapid power acceleration at the start, and allegations that the button was not pressed until the reactor began to self-destruct but others assert that it happened earlier and in calm conditions.

After 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 meters. A bigger problem was a flawed graphite-tip control rod design, which initially displaced neutron-absorbing coolant with moderating graphite before introducing replacement neutron-absorbing boron material to slow the reaction. As a result, the SCRAM actually increased the reaction rate in the upper half of the core as the tips displaced water. This behavior was known after a shutdown of another RBMK reactor induced an initial power spike, but as the SCRAM of that reactor was successful, the information was not widely disseminated.

A few seconds after the start of the SCRAM, the graphite rod tips entered the fuel pile. A massive 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 tips in the middle of the core. Within three seconds the reactor output rose above 530 MW.

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 massive 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.

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 vapor pressure, appears to have been the next event. There is a general understanding that it was steam from the wrecked fuel channels escaping into the reactor's exterior cooling structure that caused the destruction of the reactor casing, tearing off and lifting the 2,000-ton upper plate, to which the entire reactor assembly is fastened, sending it through the roof of the reactor building. Apparently, this was the first explosion that many heard. This explosion ruptured further fuel channels, as well as severing most of the coolant lines feeding the reactor chamber, 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. However, this explosion also compromised more of the reactor containment vessel and ejected superheated 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. According to observers outside Unit 4, burning lumps of material and sparks shot into the air above the reactor. Some of them fell on to 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.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 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. A third hypothesis was that the explosion was a second 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.

However, the sheer force of the second explosion, and the ratio of xenon radioisotopes released during the event, indicate that the second explosion could have been a nuclear power transient; the result of the melting core material, in the absence of its cladding, water coolant and moderator, undergoing runaway prompt criticality similar to the explosion of a fizzled nuclear weapon. This nuclear excursion released 40 billion joules of energy, the equivalent of about ten tons of TNT. The analysis indicates that the nuclear excursion was limited to a small portion of the core.

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. 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, however, Bagdasarov made his own decision to shut down the reactor, leaving only those operators there who had to work the emergency cooling systems.

Radiation levels

Approximate radiation levels at different locations shortly after the explosion were as follows:

Location	Radiation (Roentgens per hour)	Sieverts per hour (SI Unit)
Vicinity of the reactor core	30,000	300
Fuel fragments	15,000–20,000	150–200
Debris heap at the place of circulation pumps	10,000	100
Debris near the electrolyzers	5,000–15,000	50–150
Water in the Level +25 feedwater room	5,000	50
Level 0 of the turbine hall	500–15,000	5–150
Area of the affected unit	1,000–1,500	10–15
Water in Room 712	1,000	10
Control room	3–5	0.03–0.05
Gidroelektromontazh depot	30	0.3
Nearby concrete mixing unit	10–15	0.10–0.15

Plant layout

Based on the image of the plant

Level	Objects
Metres	Levels are distances above (or below for minus values) ground level at the site.
49.6	Roof of the reactor building, gallery of the refueling mechanism
39.9	Roof of the deaerator gallery
35.5	Floor of the main reactor hall
31.6	Upper side of the upper biological shield, floor of the space for pipes to steam separators
28.3	Lower side of the turbine hall roof
24.0	Deaerator floor, measurement and control instruments room
16.4	Floor of the pipe aisle in the deaerator gallery
12.0	Main floor of the turbine hall, floor of the main circulation pump motor compartments
10.0	Control room, floor under the reactor lower biological shield, main circulation pumps
6.0	Steam distribution corridor
2.2	Upper pressure suppression pool
0.0	Ground level; house switchgear, turbine hall level
−0.5	Lower pressure suppression pool
−5.2, −4.2	Other turbine hall levels
−6.5	Basement floor of the turbine hall

Immediate crisis management

Radiation levels

The radiation levels in the worst-hit areas of the reactor building have been estimated to be 5.6 roentgens per second (R/s) (1.4 milliamperes per kilogram), equivalent to more than 20,000 roentgens per hour. A lethal dose is around 500 roentgens (5Gy, 0.13 coulombs per kilogram) over 5 hours, so in some areas, unprotected workers received fatal doses in less than a minute. However, a dosimeter capable of measuring up to 1,000 R/s (0.3 A/kg) was buried in the rubble of a collapsed part of the building, and another one failed when turned on. All remaining dosimeters had limits of 0.001 R/s (0.3 µA/kg) and therefore read "off scale". Thus, the reactor crew could ascertain only that the radiation levels were somewhere above 0.001 R/s (3.6 R/h, or 0.3 µA/kg), while the true levels were much higher in some areas.

Because of the inaccurate low readings, the reactor crew chief Alexander Akimov assumed that the reactor was intact. The evidence of pieces of graphite and reactor fuel lying around the building was ignored, and the readings of another dosimeter brought in by 04:30 were dismissed under the assumption that the new dosimeter must have been defective. Akimov stayed with his crew in the reactor building until morning, sending members of his crew to try to pump water into the reactor. None of them wore any protective gear. Most, including Akimov, died from radiation exposure within three weeks.

Fire containment

Firefighter Leonid Telyatnikov, being decorated for bravery.

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."

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. However, Anatoli Zakharov, a fireman stationed in Chernobyl since 1980, offers a different description: 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." Twenty years after the disaster, he said the firefighters from the Fire Station No. 2 were aware of the risks. 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. 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.

The fire was extinguished by a combined effort of helicopters dropping over 5,000 metric tons of sand, lead, clay, and neutron absorbing boron onto the burning reactor and injection of liquid nitrogen. 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. It is now known that virtually none of the neutron absorbers reached the core.

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.)

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."

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
• 4:50 – fires mostly localized
• 6:35 – all fires extinguished

^‡With the exception of the fire contained inside Reactor 4, which continued to burn for many days.

Evacuation developments

The nearby city of Pripyat was not immediately evacuated after the incident. The townspeople 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.

The general population of the Soviet Union was first informed of the disaster on 28 April, two days after the explosion, with a 20 second announcement in the TV news program Vremya. At that time ABC released its report about the disaster. During that time, all radio broadcasts run by the state were replaced with classical music, which was a common method of preparing the public for an announcement of a tragedy that had taken place. Scientist teams were armed and placed on alert as instructions were awaited.

Only after radiation levels set off alarms at the Forsmark Nuclear Power Plant in Sweden, over one thousand kilometers from the Chernobyl Plant, did the Soviet Union admit that an accident had occurred. Nevertheless, authorities attempted to conceal the scale of the disaster. For example, after evacuating the city of Pripyat, the following warning message was read on the state TV: There has been an accident at the Chernobyl Nuclear Power Plant. One of the nuclear reactors was damaged. The effects of the accident are being remedied. Assistance has been provided for any affected people. An investigative commission has been set up.—Vremya, 28 April 1986 (21:00)A state commission was set up the same day (26 April) and tasked with investigating the accident. It was headed by Valery Legasov, who arrived at Chernobyl in the evening of 26 April. By the time Legasov arrived, two people had already died and 52 were receiving medical attention in hospital. By the night of 26–27 April – more than 24 hours after the explosion – Legasov's committee had ample evidence that extremely high levels of radiation had caused a number of cases of radiation exposure. Based on the evidence at hand, Legasov's committee acknowledged the destruction of the reactor and ordered the evacuation of Pripyat.

The evacuation began at 14:00 on 27 April. An excerpt of the evacuation announcement was translated into English in the program Seconds From Disaster on the National Geographic Channel in 2004. A translation of the rest of the audio follows. 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 Oblast. For these reasons, starting from April 27, 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)In order to expedite the evacuation, the residents were told to bring only what was necessary, as the authorities had said it would only last approximately three days. As a result, most of the residents left their personal belongings, which are still there today. An exclusion zone of 30 km (19 mi) remains in place today, although its shape has changed and its size has been expanded.

As the plant was run by authorities in Moscow, the government of Ukraine did not receive prompt information on the situation at the site, according to the former chairman of Presidium of Verkhovna Rada of Ukrainian SSR, Valentyna Shevchenko. In her recollections she stated that she was at work when at 09:00 Vasyl Durdynets who performed duties of the Minister of Internal Affairs at the time (as the First Deputy Minister) called in with a report on the recent situation, adding at the end that there was a fire at the Chernobyl AES (AES – an abbreviation for a nuclear power plant), which was extinguished and everything was fine. When Shevchenko asked "How are the people?", he replied that there was nothing to be concerned with: "some are celebrating a wedding, others are gardening, and others are fishing in the Pripyat River".

On 25 April 2011 the President of Ukraine Viktor Yanukovych awarded Durdynets the "Distinguished Juror of Ukraine" as an advisor of the Ministry of Internal Affairs, a participant in the liquidation of consequences of Chernobyl disaster, and a general of Internal Service of Ukraine. After the report Shevchenko called in to Volodymyr Shcherbytsky (Head of the Central Committee of CP(b)U, de facto – a head of state). Shcherbytsky stated that he anticipated a delegation of the state commission headed by the deputy chairman of the Council of Ministers of USSR.

Among the delegation's officials were academic Evgeny Velikhov, a leading nuclear specialist in the Soviet Union; a head of Hydro-Meteorologic Service of USSR Yuriy Izrael; a chief radiologist of the country Leonid Ilyin; and others. From the Boryspil International Airport the delegation drove to the power plant, realised the seriousness of the situation that night, and decided to evacuate the residents of Pripyat. On 26 April 2011 Velikhov was awarded Order of Merit of the III degree from the President of Ukraine Viktor Yanukovych for his contributions in the liquidation of consequences of the Chernobyl disaster.

By the morning of 27 April, buses arrived in Pripyat to start the evacuation at 11:00. By 15:00, 53,000 people were evacuated to various villages of Kiev region. At first it was decided to evacuate the population temporarily for three days, however later it was postponed permanently. Many took only the most necessary items and their documents leaving all the rest behind. The next day, talks began for evacuating people from the 10 km zone.

Shevchenko was the first of the Ukrainian state top officials to arrive at the disaster site early on 28 April. There she spoke with members of medical staff and people, who were calm and hopeful that they could soon return to their homes. Shevchenko returned home near midnight, stopping at a radiological checkpoint in Vilcha, one of the first that were set up soon after the accident.

There was a notification from Moscow that there was no reason to postpone the 1 May celebrations (including the annual parade), but on 30 April a meeting of the Political bureau of the Central Committee of CP(b)U took place to discuss the plan for the upcoming celebration. Scientists were reporting that the radiological background in Kiev city was normal. At the meeting, which was finished at 18:00, it was decided to shorten celebrations from the regular 3.5–4 to under 2 hours.

Steam explosion risk

Chernobyl corium lava flows formed by fuel-containing mass in the basement of the plant.

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, 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. 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.

The bubbler pool could be drained by opening its sluice gates. Volunteers in diving suits entered the radioactive water and managed to open the gates. These were the engineers Alexei Ananenko (who knew where the valves were) and Valeri Bezpalov, accompanied by a third man, Boris Baranov, who provided them with light from a lamp, though this lamp failed, leaving them to find the valves by feeling their way along a pipe. All of them returned to the surface and according to Ananenko, their colleagues jumped in joy when they heard they had managed to open the valves. Upon emerging from the water, the three were already suffering from radiation sickness and later died. Some sources claim incorrectly that they died in the plant.

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

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. This idea was soon scrapped and the bottom room where the cooling system would have been installed was filled with concrete.

Debris removal

Chernobyl power plant in 2003 with the sarcophagus containment structure.

The worst of the radioactive debris was collected inside what was left of the reactor, much of it shoveled in by liquidators wearing heavy protective gear (dubbed "bio-robots" by the military); these workers could only spend a maximum of 40 seconds at a time 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. The reactor itself was covered with bags of sand, lead and boric acid dropped from helicopters: some 5,000 metric tons of material were dropped during the week that followed the accident.

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. By December 1986, a large concrete sarcophagus had been erected to seal off the reactor and its contents.  A unique "clean up" medal was given to the workers.

Many of the vehicles used by the "liquidators" remain parked in a field in the Chernobyl area.

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.

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 and weighing hundreds of tons, which they called "the elephant's foot" for its wrinkled appearance. The mass was composed of sand, glass 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 solidified lava and spectacular unknown crystalline forms termed chernobylite. It was concluded that there was no further risk of explosion.

Causes

Operator error

There were two official explanations of the accident: the first, later acknowledged to be erroneous, was published in August 1986 and 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. 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."

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. 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.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. Several procedural irregularities also helped to make the accident possible. One 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 reactor 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, "It was like airplane pilots experimenting with the engines in flight."

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

Operating instructions and design deficiencies found

Reactor hall No. 1, Chernobyl nuclear power plant, Ukraine.

In 1991 a Commission of the USSR State Committee for the Supervision of Safety in Industry and Nuclear Power has 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, which reviewed "that part of the INSAG-1 report in which primary attention is given to the reasons for the accident". and included the USSR State Commission report as Appendix I.

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 another 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. Turning off the emergency system designed to prevent the two turbine generators from stopping was not a violation of regulations.

A simplified diagram of the major differences between the Chernobyl RBMK and the most common nuclear reactor design, the Light water reactor. 1. The use of a graphite moderator in a water cooled reactor. 2. A positive steam void coefficient that made the power excursion possible, which blew the reactor vessel. 3. The control rods were very slow, taking 18-20 seconds to be deployed. With the control rods having graphite tips that moderated, and therefore increased the fission rate in the beginning of the rod insertion 4. No reinforced containment building.

Human factors 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 program, 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.

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."

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:

• 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 vapor 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 feed-back). 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 behavior 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 meters 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 less 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 behavior 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.

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. 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.

Lumps of graphite moderator ejected from the core. The largest lump shows an intact control rod channel.

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 program 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."

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."