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The Chernobyl disaster, also referred to as the Chernobyl accident, was a catastrophic nuclear accident that occurred on 26 April 1986 in the No.4 light water graphite moderated reactor at the Chernobyl Nuclear Power Plant near Pripyat , in what was then part of the Ukrainian Soviet Socialist Republic of the Soviet Union (USSR).
During a hurried late night power-failure stress testing , in which control rod were deliberately turned off, a combination of inherent RBMK , together with the reactor operators Iodine pit in a manner contrary to the checklist for the stress test, eventually resulted in prompt critical that flashed water into steam generating a destructive steam explosion and a subsequent open-air nuclear graphite "fire".
Although most reports on the Chernobyl accident refer to a number of graphite fires, it is highly unlikely that the graphite itself burned. According to the General Atomics website
: "It is often incorrectly assumed that the combustion behavior of graphite is similar to that of charcoal and coal. Numerous tests and calculations have shown that it is virtually impossible to burn high-purity, nuclear-grade graphites." On Chernobyl, the same source states: "Graphite played little or no role in the progression or consequences of the accident. The red glow observed during the Chernobyl accident was the expected color of luminescence for graphite at 700°C and not a large-scale graphite fire, as some have incorrectly assumed." Similarly, nuclear physicist Yevgeny Velikhov,
noted some decay heat after the accident that "Until now the possibility of a catastrophe really did exist: A great quantity of fuel and graphite of the reactor was in an incandescent state" That is, all the nuclear- decay heat that was generated inside the uranium fuel(heat which would normally be extracted by back-up coolant pumps, in an undamaged reactor) was instead responsible for making the fuel itself and any graphite in contact with it, to glow incandescent . This is contrary to the oft cited interpretation, which is that the graphite was red-hot chiefly because it was chemically oxidizing with the air.
This "fire" produced considerable for about 9 days, that lofted plumes of fission products into the atmosphere, with the estimated radioactive inventory that was released during this very hot "fire" phase, approximately equal in magnitude to the airborne fission products released in the initial destructive explosion.
Practically all of this radioactive material would then go on to nuclear fallout /precipitate onto much of the surface of the western USSR and Europe.
The Chernobyl accident dominates the Energy accidents sub-category, of most disasterous nuclear power plant accident in history, both in terms of cost and casualties. It is one of only two nuclear energy accidents classified as a level 7 event (the maximum classification) on the International Nuclear Event Scale, the other being the Fukushima Daiichi nuclear disaster in Japan in 2011.
The struggle to safeguard against scenarios which were, at many times falsely,
perceived as having the potential for greater catastrophe and the later decontamination efforts of the surroundings, ultimately involved over 500,000 workers and cost an estimated 18 billion .
Gorbachev, Mikhail (1996), interview in Johnson, Thomas, , film, Discovery Channel, retrieved 19 February 2014.
During the accident, blast effects caused 2 deaths within the facility and later 29 firemen and employees died in the days-to-months afterward from acute radiation syndrome, with the potential for long-term cancers still being investigated.
The remains of the No.4 reactor building were enclosed in a large sarcophagus ( radiation shield ) by December 1986, at a time when what was left of the reactor was entering the cold shut-down phase; the enclosure was built quickly as occupational safety for the crews of the other undamaged reactors at the power station, with No.3 continuing to produce electricity into 2000.
The accident motivated safety upgrades on all remaining Soviet-designed reactors in the RBMK (Chernobyl No.4) family, of which eleven continued to power electric grids as of 2013.
http://www.world-nuclear.org/information-library/nuclear-fuel-cycle/nuclear-power-reactors/appendices/rbmk-reactors.aspx RBMK Reactors Appendix to Nuclear Power Reactors. WNA.2016
Overview
The disaster began during a systems test on 26 April 1986 at reactor number four of the Chernobyl plant, which is near Pripyat and in proximity to the administrative border with Belarusian SSR and the Dnieper River. There was a sudden and unexpected power surge, and when an emergency shutdown was attempted, a much larger spike in power output occurred, which led to a reactor vessel rupture and a series of steam explosions. These events exposed the graphite neutron moderator of the reactor to air, causing it to ignite.
The resulting fire sent week long plumes of highly radioactive fallout into the atmosphere over an extensive geographical area, including Pripyat. The plumes drifted over large parts of the western Soviet Union and Europe.  According to official post-Soviet data,
(2017). 9789201147059, International Atomic Energy Agency. . ISBN 9789201147059
about 60% of the fallout landed in Belarus.
36 hours after the accident Soviet officials enacted a 10-kilometre exclusion zone which resulted in the rapid evacuation of 49,000 people and their animals, primarily from the largest population centre near the reactor, the town of Pripyat.
Nuclear
Although not communicated at the time, an immediate evacuation of the town following the accident was not advisable as the road leading out of the town had heavy nuclear fallout "hotspots" deposited on it, while up to around that point, the town itself was comparatively safe due to the more favourable wind direction, permitting shelter in place to be the best safety measure to take for the town, before the winds began to change direction.
As the plumes and subsequent fallout continued to be generated, the exclusion zone was increased from 10 to 30 km about one week after the accident, resulting in a further 68,000 evacuated, including from the Chernobyl itself.
The surveying and detection of isolated fallout hotspots outside this zone over the following year eventually resulted in 135,000 "long-term evacuees" in total, accepting to be moved.
The near tripling in the total number of permanently resettled to some 350,000 over the decades, 1986 to 2000, from the most severely "contaminated" areas,
is regarded as largely political in nature, with the majority of the rest evacuated in an effort to redeem loss in trust in the government, which was most common around 1990.
Chernobyl in Perspective, James Peron, 2006
Epidemiology has determined that the "people still living unofficially in the abandoned lands around Chernobyl" have a lower risk of dying as a result of the elevated doses of radiation in the rural areas than "if they were exposed to the air pollution health risk in a large city such as nearby Kiev".
Chernobyl’s Myths and Misconceptions 2006, Mizsei
The accident raised the already heightened concerns about worldwide, and while most concern was focused on those of the same unusual design, hundreds of disparate electric-power reactor proposals, including those under construction at Chernobyl, reactor No.5 and 6, were eventually cancelled. With the worldwide issue generally being due to the ballooning in costs for new nuclear reactor safety system standards and the legal costs in dealing with the increasingly hostile/anxious public opinion. There was a precipitous drop in the prior rate of new "startups", after 1986.
The accident also raised concerns about the cavalier safety culture in the Soviet nuclear power industry, slowing industry growth and forcing the Soviet government to become less secretive about its procedures.
Boris Kagarlitsky
(1989). 9780860919629, United Nations University Press. ISBN 9780860919629
"No one believed the first newspaper reports, which patently understated the scale of the catastrophe and often contradicted one another. The confidence of readers was re-established only after the press was allowed to examine the events in detail without the original censorship restrictions. The policy of openness ( glasnost ) and 'uncompromising criticism' of outmoded arrangements had been proclaimed at the 27th Congress (of KPSS), but it was only in the tragic days following the Chernobyl disaster that glasnost began to change from an official slogan into an everyday practice. The truth about Chernobyl that eventually hit the newspapers opened the way to a more truthful examination of other social problems. More and more articles were written about drug abuse, crime, corruption and the mistakes of leaders of various ranks. A wave of 'bad news' swept over the readers in 1986–87, shaking the consciousness of society. Many were horrified to find out about the numerous calamities of which they had previously had no idea. It often seemed to people that there were many more outrages in the epoch of perestroika than before although, in fact, they had simply not been informed about them previously." -Kagarlitsky pp. 333–334
The government coverup of the Chernobyl disaster was a "catalyst" for glasnost , which "paved the way for reforms leading to the Soviet collapse".
Cancer projections
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 primarily male recovery workers (the " Liquidators ") 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.
William H Hallenbeck
(1994). 9780873719964, CRC Press. ISBN 9780873719964
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 leukaemia.
In a peer-reviewed paper in the International Journal of Cancer in 2006, the authors expanded the discussion on those exposed to all of Europe (but following a different conclusion methodology to the Chernobyl Forum study, which arrived at the total predicted death toll of 4,000 after cancer survival rates were factored in) they stated, without entering into a discussion on deaths, that in terms of total excess cancers attributed to the accident:
In terms of non peer-reviewed estimates, based on a simple linear no-threshold model multiplication, that the International Commission on Radiological Protection(ICRP) states "should not be done",
Nuclear Law In Progress. INLA congress 2014. pg 4-5
two anti-nuclear advocacy groups have publicised mortality estimates for those who were exposed to even smaller amounts of radiation. The Union of Concerned Scientists estimates that, among the hundreds of millions of people exposed worldwide, excluding thyroid cancer there will be an eventual 50,000 excess cancer cases resulting in 25,000 excess cancer deaths.
Chernobyl Cancer Death Toll Estimate More Than Six Times Higher Than the 4000 Frequently Cited, According to a New UCS Analysis Note: "The UCS analysis is based on radiological data provided by UNSCEAR, and is consistent with the findings of the Chernobyl Forum and other researchers."
Along similar lines, the 2006 TORCH report , commissioned by the European Greens political party, predicts an eventual 30,000 to 60,000 excess cancer deaths in total, around the globe.
The Russian founder of that region's chapter of Greenpeace , authored a book titled , which suggests 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.
Greenpeace itself advocates a figure of at least 200,000 or more.
The book has failed the peer review process.
Correspondence (see reference 17) to George Monbiot from Douglas Braaten, Director and Executive Editor, Annals of the New York Academy of Sciences, 2 April 2011: "In no sense did Annals of the New York Academy of Sciences or the New York Academy of Sciences commission this work; nor by its publication do we intend to independently validate the claims made in the translation or in the original publications cited in the work. The translated volume has not been peer-reviewed by the New York Academy of Sciences, or by anyone else."
Of the five reviews published in the academic press, four considered the book severely flawed and contradictory, and one praised it while noting some shortcomings. The review by M. I. Balonov published by the New York Academy of Sciences concludes that the report is of negative value because it has very little scientific merit while being highly misleading to the lay reader. It characterized the estimate of nearly a million deaths as more in the realm of science fiction than science.
Verifiable human deaths
The largest verifiable death toll that followed as a result of the Chernobyl radioisotope releases is not a direct result of the radiation itself but instead was a result of the anxieties created by the media and some doctors, that capitalised on the varying levels of individual parental ignorance of the health physics at play.
Roger E. Kasperson
(1991). 9780792306016, Springer Science and Media. ISBN 9780792306016
Worldwide, an estimated excess of about 150,000 may have been performed on otherwise healthy pregnancies out of radiophobia from Chernobyl, according to Dr Robert Baker and ultimately a 1987 article published by Linda E. Ketchum in the Journal of Nuclear Medicine which mentions but does not reference an IAEA source on the matter.
Chernobyl's Hot Zone Holds Some Surprises
[ RadSafe Chernobyl-related abortions Bjorn Cedervall, 2010]
The available statistical data excludes the Soviet/Ukraine/Belarus abortion rates, as they are presently unavailable. From the available data, an increase in the number of abortions in what were healthy developing human offspring in Denmark occurred in the months following the accident, at a rate of about 400 cases.
In Greece, there was an observed 2500 excess of otherwise wanted pregnancies being terminated.
In Italy, a "slightly" above the expected number of occurred, approximately 100.
No evidence of changes in the prevalence of birth congenital anomalies(mutations) which might be associated with the accident are apparent in Belarus or the Ukraine, the two republics which had the highest exposure to fallout .
In Sweden,
and Finland where no increase in abortion rates occurred, it was likewise determined that "no association between the temporal and spatial variations in radioactivity and variable incidence of congenital malformations was."
A similar null increase in the abortion rate and a, healthy baseline situation, of no increase in birth defects was determined from assessing the Hungarian Congenital Abnormality Registry,
Findings also mirrored in Austria.
Larger, "mainly western European" data sets approaching a million births in the EUROCAT database, divided into "exposed" and control groups were assessed in 1999. As no Chernobyl impacts were detected, the researchers conclude "in retrospect the widespread fear in the population about the possible effects of exposure on the unborn fetus was not justified".
Despite studies from Germany and Turkey, the only robust evidence of negative pregnancy outcomes that transpired after the accident were these elective abortion indirect effects, in Greece, Denmark, Italy etc., due to the anxieties created.
In very high doses, it was known at the time that radiation can cause a physiological increase in the rate of pregnancy anomalies, but unlike the dominant linear-no threshold model of radiation and cancer rate increases, it was known, by researchers familiar with both the prior human exposure data and animal testing, that the "Malformation of organs appears to be a deterministic effect with a threshold dose" below which, no rate increase is observed.
Teratogen Update: Radiation and Chernobyl, Frank P. Castronovo Jr.TERATOLOGY 60:100–106 (1999)
This teratology (birth defects) issue was discussed by Frank Castronovo of the Harvard Medical School in 1999, publishing a detailed review of dose reconstructions and the available pregnancy data following the Chernobyl accident, inclusive of data from Kiev 's two largest obstetrics hospitals.
Castronovo concludes that "the mass media with newspaper reporters playing up anecdotal stories of children with birth defects" is, together with dubious studies that show selection bias , the two primary factors causing the persistent belief that Chernobyl increased the background rate of birth defects. When the vast amount of pregnancy data does not support this preception as no women took part in the most radioactive liquidator operations, no pregnant individuals were exposed to the threshold dose.
On 26 April 1986, at 01:23 ( Moscow Time ), reactor four suffered a catastrophic power increase, leading to explosions in its core. This dispersed large quantities of radioactive isotopes into the atmosphere
Zhores A. Medvedev
(1990). 9780393308143, W. W. Norton & Company. ISBN 9780393308143
and caused an open-air fire. The fire increased the emission of hot particle , 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
In steady state operation, a significant fraction (over 6%) of the power from a nuclear reactor is derived not from fission but from the decay heat of its accumulated fission products. This heat continues for some time after the chain reaction is stopped (e.g., following an emergency SCRAM ) and active cooling may be required to prevent nuclear meltdown .
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 () 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 ; 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 gapconsidered an unacceptable safety riskit had been theorized that rotational energy from the steam turbine (as it wound down under residual steam pressure) could be used to generate the required electrical power. Analysis indicated that this residual momentum and steam pressure might be sufficient to run the coolant pumps for 45 seconds,
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 indicated that the excitation voltage of the turbine-generator was insufficient; it did not maintain the desired magnetic field after the turbine trip. The system was modified, and the test was repeated in 1984 but again proved unsuccessful. In 1985, the tests were attempted a third time but also yielded negative results. The test procedure would be repeated 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 expected to begin with an automatic emergency shutdown. No detrimental effect on the safety of the reactor was anticipated, so the test programme was not formally coordinated with either the chief designer of the reactor (NIKIET) or the scientific manager. Instead, it was approved only by the director of the plant (and even this approval was not consistent with established procedures).
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 mere fact that the operators were carrying out an experiment that had not been approved by higher officials indicates that something was wrong with the chain of command. The State Committee on Safety in the Atomic Power Industry is permanently represented at the Chernobyl station. Yet the engineers and experts in that office were not informed about the program. In part, the tragedy was the product of administrative anarchy or the attempt to keep everything secret." Medvedev, Z., pp. 18–20
The experimental procedure was intended to run as follows:
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 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 continue to freewheel down.
Conditions before the accident
The conditions to run the test were established before the day shift of 25 April 1986. The day-shift workers had been instructed in advance and were familiar with the established procedures. A special team of electrical engineers was present to test the new voltage regulating system.
As planned, a gradual reduction in the output of the power unit was begun at 01:06 on 25 April, and the power level had reached 50% of its nominal 3200 MW thermal level by the beginning of the day shift.
At this point, another regional power station unexpectedly went offline, and the Kiev electrical grid controller requested that the further reduction of Chernobyl's output be postponed, as power was needed to satisfy the peak evening demand. The Chernobyl plant director agreed, and postponed the test. Despite this delay, preparations for the test not affecting the reactor's power were carried out, including the disabling of the emergency core cooling system or ECCS, a passive/active system of core cooling intended to provide water to the core in a loss-of-coolant accident. Given the other events that unfolded, the system would have been of limited use, but its disabling as a "routine" step of the test is an illustration of the inherent lack of attention to safety for this test.
In addition, had the reactor been shut down for the day as planned, it is possible that more preparation would have been taken in advance of the test.
At 23:04, the Kiev grid controller allowed the reactor shutdown to resume. This delay had some serious consequences: the day shift had long since departed, the evening shift was also preparing to leave, and the night shift would not take over until midnight, well into the job. According to plan, the test should have been finished during the day shift, and the night shift would only have had to maintain decay heat cooling systems in an otherwise shut-down plant.
The night shift had very limited time to prepare for and carry out the experiment. A further rapid decrease in the power level from 50% was executed during the shift change-over. Alexander Akimov was chief of the night shift, and Leonid Toptunov was the operator responsible for the reactor's operational regimen, including the movement of the . Toptunov was a young engineer who had worked independently as a senior engineer for approximately three months.
The test plan called for a gradual decrease 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. Due to the reactor's production of  a fission byproduct, xenon-135, which is a reaction-inhibiting neutron absorber , core power continued to decrease without further operator action—a process known as Iodine pit . This continuing decrease in power occurred because in steady state operation, xenon-135 is "burned off" as quickly as it is created from decaying iodine-135 by absorbing neutrons from the ongoing chain reaction to become highly stable xenon-136. When the reactor power was lowered, previously produced high quantities of iodine-135 decayed into the neutron-absorbing xenon-135 faster than the reduced neutron flux could burn it off. As the reactor power output dropped further, to approximately 500 MW, Toptunov mistakenly inserted the control rods too far—the exact circumstances leading to this are unknown because Akimov and Toptunov both died in the hospital on 10 and 14 May respectively. This combination of factors put the reactor into an unintended near-shutdown state, with a power output of 30 MW thermal or less.
The reactor was now producing 5 percent of the minimum initial power level established as safe for the test.
Control-room personnel decided to restore power by disabling the automatic system governing the control rods and manually extracting the majority of the reactor control rods to their upper limits.
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.
The accumulation of Xe-135 in the core is burned out by neutrons. Higher power settings bring higher neutron flux and burn the xenon out more quickly. Conversely, low power settings result in the accumulation of xenon.
This restricted any further rise of reactor power, and made it necessary to extract additional control rods from the reactor core in order to counteract the poisoning.
The operation of the reactor at the low power level and high poisoning level was accompanied by unstable core temperature and coolant flow, and possibly by instability of neutron flux, which triggered alarms. The control room received repeated emergency signals regarding the levels in the steam/water separator drums, and large excursions or variations in the flow rate of feed water, as well as from opened to relieve excess steam into a turbine condenser, and from the neutron power controller. Between 00:35 and 00:45, emergency alarm signals concerning thermal-hydraulic parameters were ignored, apparently to preserve the reactor power level.
Experiment and explosion
At 1:23:04 a.m., the experiment began. Four of the main circulating pumps (MCP) were active; of the eight total, six are normally active during regular operation. The steam to the turbines was shut off, beginning a run-down of the turbine generator. The diesel generators started and sequentially picked up loads; the generators were to have completely picked up the MCPs' power needs by 01:23:43. In the interim, the power for the MCPs was to be supplied by the turbine generator as it coasted down. As the momentum of the turbine generator decreased, so did the power it produced for the pumps. The water flow rate decreased, leading to increased formation of steam voids (bubbles) in the core.
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 a further power increase. During almost the entire period of the experiment the automatic control system successfully counteracted this positive feedback, inserting into the reactor core to limit the power rise. This system had control of only 12 rods, and nearly all others had been manually retracted.
At 1:23:40, as recorded by the SKALA centralized control system, an emergency shutdown (SCRAM) of the reactor was initiated. The SCRAM was started when the EPS-5 button (also known as the AZ-5 button) of the reactor emergency protection system was pressed: this engaged the drive mechanism on all control rods to fully insert them, including the manual control rods that had been withdrawn earlier. The reason why the EPS-5 button was pressed is not known, whether it was done as an emergency measure in response to rising temperatures, or simply as a routine method of shutting down the reactor upon completion of the experiment.
There is a view that the SCRAM may have been ordered as a response to the unexpected rapid power increase, although there is no recorded data proving this. Some have suggested that the button was not pressed, and instead the signal was automatically produced by the emergency protection system;  the SKALA 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.
(2017). 9785987060186, GUP NIKIET. . ISBN 9785987060186
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 metres. A bigger problem was the design of the RBMK control rods, which had graphite neutron moderators attached to boost reactor output when the rod was withdrawn. Those displacers had a 1.25 m column of water above and below them when the rods were at maximum extration, and lowering the rods displaced the neutron-absorbing water in the lower portion of the reactor with moderating graphite. As a result, the SCRAM increased the reaction rate in the lower part of the core as the graphite displaced the coolant. This behaviour was known after the initial insertion of control rods in another RBMK reactor at Ignalina Nuclear Power Plant in 1983 induced a power spike, but as the subsequent SCRAM of that reactor was successful, the information was disseminated but deemed of little importance.
A few seconds after the start of the SCRAM, a power spike occurred, and the core overheated, causing some of the to fracture, blocking the control rod columns and jamming the control rods at one-third insertion, with the graphite displacers still in the lower part of the core. Within three seconds the reactor output rose above 530 MW.
The subsequent course of events was not registered by instruments; it is known only as a result of mathematical simulation. Apparently, the power spike caused an increase in fuel temperature and steam buildup, leading to a rapid increase in steam pressure. This caused the fuel cladding to fail, releasing the fuel elements into the coolant, and rupturing the channels in which these elements were located.
Then, according to some estimations, the reactor jumped to around 30,000 MW thermal, ten times the normal operational output. The last reading on the control panel was 33,000 MW. It was not possible to reconstruct the precise sequence of the processes that led to the destruction of the reactor and the power unit building, but a steam explosion , like the explosion of a steam boiler from excess vapour pressure, appears to have been the next event. There is a general understanding that it was explosive steam pressure from the damaged fuel channels escaping into the reactor's exterior cooling structure that caused the detonation that destroyed the reactor casing, tearing off and blasting the 2000-ton upper plate, to which the entire reactor assembly is fastened, through the roof of the reactor building. This is believed to be the first explosion that many heard.
RI Davletbaev