Chernobyl disaster

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The Chernobyl disaster (Ukrainian: Чорнобильська катастрофа, Chornobylska Katastrofa – Chornobyl Catastrophe) was acatastrophic nuclear accident that occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in Ukraine (then officially Ukrainian SSR), which was under the direct jurisdiction of the central authorities of the Soviet Union. Sony VAIO PCG-3B1M Battery

An explosion and fire released large quantities of radioactive contamination into the atmosphere, which spread over much of 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). Sony VAIO PCG-3C1T Battery

The battle to contain the contamination and avert a greater catastrophe ultimately involved over 500,000 workers and cost an estimated 18 billion rubles.^[2] 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. Sony VAIO PCG-3D1M Battery

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 Prypiat and in proximity to the administrative border with Belarus and Dnieper river. There was a sudden power output surge, and when an emergency shutdown was attempted, a more extreme spike in power output occurred, which led to a reactor vessel rupture and a series of explosions. Sony VAIO PCG-3G2M Battery

These events exposed the graphite moderator of the reactor to air, causing it to ignite.^[3] The resulting fire sent a plume of highly radioactive smoke 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.Sony VAIO PCG-5R1M Battery

According to official post-Soviet data,^[6][7] 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. Sony VAIO PCG-7141M Battery

The government coverup of the Chernobyl disaster was a "catalyst" for glasnost, which "paved the way for reforms leading to the Soviet collapse."^[9]Russia, Ukraine, and Belarus have been burdened with the continuing and substantial decontamination and health care costs of the Chernobyl accident. A report of the International Atomic Energy Agency,^[7] examines the environmental consequences of the accident. Sony VAIO PCG-7143M Battery

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.Sony VAIO PCG-7151M Battery

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. Sony VAIO PCG-7154M Battery

Thirty one deaths are directly attributed to the accident, all among the reactor staff and emergency workers.^[13] An UNSCEAR report places the total confirmed deaths from radiation at 64 as of 2008. The Chernobyl Forum estimates 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); Sony VAIO PCG-7162M Battery

this figure includes some 50 emergency workers who died of acute radiation syndrome, nine children who died of thyroid cancer and an estimated total of 3940 deaths from radiation-induced cancer and leukemia.^[14]

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.^[15] Sony VAIO PCG-7181M Battery

For this broader group, the 2006 TORCH report predicts 30,000 to 60,000 excess cancer deaths,^[16] and a Greenpeace report puts the figure at 200,000 or more.^[17] The Russian publication Chernobyl, which has received criticism for its methodology and sourcing, 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. Sony VAIO PCG-41112M Battery

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 combustiblegraphite 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. Sony VAIO PCG-7153M Battery

The accident occurred during an experiment scheduled to test a potential safety emergency core cooling feature, which took place during a normal shutdown procedure.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. Sony VAIO PCG-71312M Battery

RBMK reactors, like those at Chernobyl, use water as coolant.^[20][21] Reactor 4 at Chernobyl consisted of about 1,600 individual fuel channels; each required a coolant flow of 28 metric tons (28,000 liters / 7,400 US gallons) per hour.^[19]:7

Since cooling pumps require electricity to cool a reactor after a SCRAM, in the event of a power grid failure, Sony VAIO PCG-7144M Battery

Chernobyl's reactors had three backup diesel generators; these could start up in 15 seconds, but took 60–75 seconds^[19]:15 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. Sony VAIO PCG-7191L Battery

Analysis indicated that this residual momentum and steam pressure might be sufficient to run the coolant pumps for 45 seconds,^[19]:16 bridging the gap between an external power failure and the full availability of the emergency generators.^[22]

This capability still needed to be confirmed experimentally, and previous tests had ended unsuccessfully. Sony VAIO PCG-3C1M Battery

An initial test carried out in 1982 showed that theexcitation 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. Sony VAIO PCG-3F1M Battery

The test procedure was to be repeated again in 1986, and it was scheduled to take place during the maintenance shutdown of Reactor Four.^[22]

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. Sony VAIO PCG-3H1M Battery

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).^[23]

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. Sony VAIO PCG-3J1M Battery

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.^[23]

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. Sony VAIO PCG-8141M Battery

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: Sony VAIO PCG-8161M Battery

The reactor was to be running at a low power level, between 700 MW and 800 MW.The steam-turbine generator was to be run up to full speed.When these conditions were achieved, the steam supply for the turbine generator was to be closed off.Turbine generator performance was to be recorded to determine whether it could provide the bridging power for coolant pumps until the emergency diesel Sony VAIO PCG-3C2M Battery

generators were sequenced to start and provide power to the cooling pumps automatically.After the emergency generators reached normal operating speed and voltage, the turbine generator would be allowed to freewheel down.

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. Sony VAIO PCG-5N2M Battery

A special team of electrical engineers was present to test the new voltage regulating system.^[24] 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 gridcontroller requested that the further reduction of Chernobyl's output be postponed, Sony VAIO PCG-5P1M Battery

as power was needed to satisfy the peak evening demand. The Chernobyl plant director agreed, and postponed 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. Sony VAIO PCG-5S1M Battery

According to plan, the test should have been finished during the day shift, and the night shift would only have had to maintain decay heatcooling systems in an otherwise shut down plant.^[19]:36–8

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. Sony VAIO PCG-9Z1M Battery

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.^[25] An output of 700 MW was reached at 00:05 on 26 April. Sony VAIO PCG-7171M Battery

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 both Akimov and Toptunov were killed during the later explosion. Sony VAIO PCG-7186M Battery

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. Sony VAIO PCG-81112M Battery

Control-room personnel consequently made the decision to restore power by extracting the majority of the reactor control rods to their upper limits.^[26] 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. Sony VAIO PCG-31311M Battery

The rapid reduction in the power during the initial shutdown, and the subsequent operation at a level of less than 200 MW led to increasedpoisoning of the reactor core by the accumulation of xenon-135.^[27][28] 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. Sony VAIO PCG-8152M Battery

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 (see reactor poisoning). Various alarms started going off at this point. The control room received repeated emergency signals regarding the levels in the steam/water separator drums, Sony VAIO PCG-31111M Battery

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. Sony VAIO PCG-61111M Battery

Emergency signals from the reactor emergency protection system (EPS-5) triggered a trip which turned off both turbine-generators.^[29]

After a while, a more or less stable state at a power level of 200 MW was achieved, and preparation for the experiment continued. Sony VAIO PCG-51112M Battery

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, which now more closely approached the nucleate boiling temperature of water, reducing the safety margin. Sony VAIO PCG-51211M Battery

The flow exceeded the allowed limit at 01:19. At the same time, the extra water flow lowered the overall core temperature and reduced the existing steam voids in the core.^[30] 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. Sony VAIO PCG-51212M Battery

This prompted the operators to remove the manual control rods further to maintain power.^[31]

All these actions led to an extremely unstable reactor configuration. Nearly all of the control rods were removed, which would limit the value of the safety rods when initially inserted in a SCRAM condition. Further, the reactor coolant had reduced boiling, but had limited margin to boiling, Sony VAIO PCG-41111M Battery

so any power excursion would produce boiling, reducing neutron absorptionby the water. The reactor was in an unstable configuration that was clearly outside the safe operating envelope established by the designers.

At 1:23:04 a.m. the experiment began. Four (of eight total) Main Circulating Pumps (MCP) were active. The steam to the turbines was shut off, and a run down of the turbine generator began. Sony VAIO PCG-41111V Battery

The diesel generator started and sequentially picked up loads, which was complete by 01:23:43. During this period, the power for the four MCPs was supplied by the turbine generator as it coasted down. As the momentum of the turbine generator decreased, the water flow rate decreased, leading to increased formation of steam voids (bubbles) in the core. Sony VAIO PCG-61412V Battery

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. Sony VAIO PCG-71112M Battery

However, 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.

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. Sony VAIO PCG-81111V Battery

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 fully inserted all control rods, 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 or simply as a routine method of shutting down the reactor upon completion of the experiment. Sony VAIO PCG-81111V Battery

There is a view^[who?] 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^[who?] 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. Sony VAIO PCG-81212M Battery

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^[who?] 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. Sony VAIO PCG-81212V Battery

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 coolant before inserting neutron-absorbing material to slow the reaction. Sony VAIO PCG-51111M Battery

As a result, the SCRAM actually increased the reaction rate in the lower half of the core.

A few seconds after the start of the SCRAM, a massive power spike occurred, the core overheated, and seconds later this overheating resulted in the initial explosion. Some of the fuel rods fractured, blocking the control rod columns and causing the control rods to become stuck at one-third insertion. Sony VAIO VPCS13X9E/B battery

Within three seconds the reactor output rose above 530 MW.^[19]:31

The subsequent course of events was not registered by instruments: it is known only as a result of mathematical simulation. Apparently, a great rise in power first caused an increase in fuel temperature and massive steam buildup, leading to a rapid increase in steam pressure. Sony VAIO VPCS12V9E/B battery

This destroyed fuel elements and ruptured the channels in which these elements were located.^[34]

Then, according to some estimations^[who?], the reactor jumped to around 30 GW thermal, ten times the normal operational output. The last reading on the control panel was 33 GW. 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, Sony VAIO VPCS12V9E/B battery

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 channels entering the reactor's inner 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. Apparently, this was the first explosion that manyheard.Sony VAIO VPCF13M8E/B battery

his explosion ruptured further fuel channels, 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 power.

A second, more powerful explosion occurred about two or three seconds after the first; Sony VAIO VPCF13Z0E/B battery

evidence indicates that the second explosion was from the core itself undergoing runaway criticality.^[36] The nuclear excursion dispersed the core and effectively terminated the nuclear chain reaction. However, a graphite fire was burning by now, greatly contributing to the spread of radioactive material and the contamination of outlying areas.^[37]

There were initially several hypotheses about the nature of the second explosion. Sony VAIO VPCM13M1E/L battery

One view was that "the second explosion was caused by the 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. Sony VAIO VPCM13M1E/W battery

A third hypothesis was that the explosion was caused by steam. According to this version, the flow of steam and the steam pressure caused all the destruction that followed the ejection from the shaft of a substantial part of the graphite and fuel.

According to observers outside Unit 4, burning lumps of material and sparks shot into the air above the reactor. Sony VAIO VPCF22M1E battery

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. Sony VAIO VPCF11M1E/H battery

However, the ratio of xenon radioisotopes released during the event indicates that the second explosion could be a nuclear power transient. This nuclear transient released 40 GJ 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.^[36]

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. Sony VAIO VPCF13M0E/B battery

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.^[19]: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. Sony VAIO VPCYB2M1E battery

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.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. Sony VAIO VPCYB3V1E battery

A lethal dose is around 500 roentgens (0.13 coulombs per kilogram) over 5 hours, so in some areas, unprotected workers received fatal doses within minutes. However, a dosimeter capable of measuring up to 1,000 R/s (0.3 A/kg) was inaccessible because of the explosion, 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." Sony VAIO VPCY11M1E battery

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.^[19]:42–50

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, Sony VAIO VPCS12L9E/B battery

and the readings of another dosimeter brought in by 04:30 were dismissed under the assumption that the new dosimeter must have been defective.^[19]:42–50 Akimov stayed with his crew in the reactor building until morning, trying to pump water into the reactor. None of them wore any protective gear. Most, including Akimov, died from radiation exposure within three weeks. Sony VAIO VPCF11S1E/B battery

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."^[42] Sony VAIO VPCYB3V1E/R Battery

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

e 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... Sony VAIO VPCF23P1E Battery

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. Sony VAIO VPCF23N1E Battery

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.^[19]:73Sony VAIO VPCY21S1E/L Battery

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, Sony VAIO VPCY21S1E/G battery

causing the helicopter to fall near the damaged reactor building and killing its four-man crew.^[45] 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. Sony VAIO VPCF24M1E battery

(This is similar to the description given byLouis Slotin, a Manhattan Project physicist who died days after a fatal radiation overdose from a criticality accident.)^[47]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: Sony VAIO VPCS115EC Battery

the residents of the surrounding area observed the radioactive cloud on the night of the explosion.