Chernobyl Disaster

[☢Radioactive☢]

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).[1] 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.

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.[3] 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.[4][5] 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.[8][notes 1] 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 by the International Atomic Energy Agency examines the environmental consequences of the accident.[7] 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.[10][11][12] 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.[13] 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.[14]

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:[15]


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.[16]

For this broader group, the 2006 TORCH report, commissioned by the European Greens political party, predicts 30,000 to 60,000 excess cancer deaths.[17]

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.[18] 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.[19]

The book however has failed the peer review process,[20][21] 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.[22]

Accident

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[23]:73 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.[24][25] 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.[23]:7[clarification needed]

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[23]:15 to attain full speed and reach the 5.5‑megawatt (MW) output required to run one main pump.[23]:30

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,[23]:16 bridging the gap between an external power failure and the full availability of the emergency generators.[26]

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.[26]

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).[27]

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.[27]

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 freewheel down.

Conditions prior to 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.[28] 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.[29] 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.[23]: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. 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.[23]:36–8

The test plan called for a gradual reduction in power output from reactor 4 to a thermal level of 700–1000 MW.[30] 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.[27]:73 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.[31] 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.[32][33] 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. 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.[34]

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.[35] 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.[29][36]

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
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.[37]:578[38]

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.[23]: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 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.[39]

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.[40]: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. 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.[41]


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.[23]:32

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.[42] 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.[43] 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.[43]

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



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.[23]: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, and the readings of another dosimeter brought in by 04:30 were dismissed under the assumption that the new dosimeter must have been defective.[23]:42–50 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.[46]:247–48

Fire containment
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