Chernobyl Disaster Case Study Pdf Format


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Chernobyl Nuclear Power Plant  Accident – Case Study

 - Yu.A. Izrael


 Encyclopedia of Life Support Systems



Yu.A. Izrael

 Institute of Global Climate and Ecology, Russian Federal Service for  Hydrometeorology and Environment Monitoring, Russian Academy of Sciences, Russia


 close-in zone, dose, dose rate, gamma-radiation, nuclear power engineering,  plume, radioactive contamination, radioactive fallout, radioactivity release, radionuclide composition, radioactive aerosol products, reconstruction of dose, refractory and volatile radionuclides, remote zone, thermal explosion


1. Chernobyl Nuclear Power Plant Accident (Versions of Possible Causes of the Accident) 2. Formation and Description of Close-in and Remote Zone of the Environment contamination after the Accident: Radionuclide Composition of the Contamination 3. Analysis of Radioactive Contamination Transport with Consideration of Real Meteorological Situation after the Accident 4. Calculation and Reconstruction of Doses after Chernobyl 5. Penetration of Radioactive Substances into another Medium: Contamination of Agricultural and Natural Vegetation 6. Medical (for Human Health) Consequences 7. Biological and Ecological (for Nature) Consequences Glossary Bibliography Biographical Sketch


Under normal operating regime, nuclear power stations practically do not release aerosol products to the atmosphere leading to essential radioactive fallout. According to studies conducted by scientists from different countries, under the normal operating regime nuclear reactors can release to the atmosphere inert gases (









), in some cases with insignificant admixture of tritium (



) and iodine (



) isotopes. Some other isotopes are also mentioned in gaseous releases (for instance,









). Under normal operating regime, nuclear power stations can release (2-4)



 Ci/year in the form of gaseous products (mainly, due to relatively short-lived inert gas isotopes), up to 10 Ci/year of aerosol products, 0.5 Ci/year of radioactive iodine; only very small quantity of aerosol products can fall on the surface, including the Iodine isotopes. When this takes place, only a small quantity of the aerosol products can be felt onto the ground surface. However, accidents at nuclear reactors are an important exception, as well accidents of different type at atomic enterprises. Among notable accidents occurred at nuclear reactors, it should be noted here (in a chronological order) the following: the Windscale accident (the Great Britain, 1957),

This article is about the 1986 nuclear plant accident in Ukraine. For other uses, see Chernobyl (disambiguation).

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

Date26 April 1986 (1986-04-26)
Time01:23 (Moscow Time UTC+3)
LocationPripyat, Ukrainian SSR, Soviet Union
CauseInadvertent explosion of core during emergency shutdown of reactor whilst undergoing power failure experiment
Deaths31 (direct)
15 (estimated indirect deaths up to 2011)[1]

The Chernobyl disaster, also referred to as the Chernobyl accident, was a catastrophicnuclear accident. It occurred on 25–26 April 1986 in the No.4 light water graphite moderated reactor at the Chernobyl Nuclear Power Plant near Pripyat, a town in northern Ukrainian Soviet Socialist Republic which was part of the Soviet Union (USSR), approximately 104 km north of Kiev.[2]

The event occurred during a late-night safety test which simulated a station blackout power-failure, in the course of which safety systems were intentionally turned off. A combination of inherent reactor design flaws and the reactor operators arranging the core in a manner contrary to the checklist for the test, eventually resulted in uncontrolled reaction conditions. Water flashed into steam generating a destructive steam explosion and a subsequent open-air graphite fire.[note 1] This fire produced considerable updrafts for about nine days. These lofted plumes of fission products into the atmosphere. The estimated radioactive inventory that was released during this very hot fire phase approximately equaled in magnitude the airborne fission products released in the initial destructive explosion.[3] Practically all of this radioactive material would then go on to fall-out/precipitate onto much of the surface of the western USSR and Europe.

The Chernobyl accident dominates the energy accidents sub-category of most disastrous 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.[4] The struggle to safeguard against scenarios which were [3] perceived, in many cases falsely, as having the potential for greater catastrophe, together with later decontamination efforts of the surroundings, ultimately involved over 500,000 workers and cost an estimated 18 billion rubles.[5]

During the accident, blast effects caused two deaths within the facility; one immediately after the explosion, and the other later due to deadly doses of radiation. Later, 134 people were hospitalized with acute radiation symptoms, of which 28 firemen and employees died in the days-to-months afterward from the effects of acute radiation syndrome. In addition, approximately fourteen cancer deaths among this group of initially hospitalized survivors were to follow within the next ten years (1996).[6] While among the wider population, an excess of 15 childhood thyroid cancer deaths had been documented as of 2011.[1][7] It will take further time and investigative funding to definitively determine the elevated relative risk of cancer among both the surviving employees and the population at large.[8]

The remains of the No.4 reactor building were enclosed in a large cover which was named the "Object Shelter". It is often known as the sarcophagus, the purpose of which was to reduce the spread of the remaining radioactive dust and debris from the wreckage and the protection of the wreckage from further weathering. The sarcophagus was finished in December 1986 at a time when what was left of the reactor was entering the cold shut-down phase; the enclosure was not intended as a radiation shield, but 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.[9][10]

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.[11][12]


The disaster began during a systems test on 26 April 1986 at reactor 4 of the Chernobyl plant near Pripyat and in proximity to the administrative border with Belarus and the Dnieper River. There was a sudden and unexpected power surge. When operators attempted an emergency shutdown, a much larger spike in power output occurred. This second spike led to a reactor vessel rupture and a series of steam explosions. These events exposed the graphitemoderator of the reactor to air, causing it to ignite.[13][discuss] For the next week, the resulting fire sent 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,[14][15] about 60% of the fallout landed in Belarus.

Thirty-six hours after the accident, Soviet officials enacted a 10-kilometre exclusion zone, which resulted in the rapid evacuation of 49,000 people primarily from Pripyat, the nearest large population centre.[16] 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. Initially, the town itself was comparatively safe due to the favourable wind direction. Until the winds began to change direction, shelter in place was considered the best safety measure for the town.[16][17]

As plumes and subsequent fallout continued to be generated, the evacuation zone was increased from 10 to 30 km about one week after the accident. A further 68,000 persons were evacuated, including from the town of Chernobyl itself.[16] 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 agreeing to be moved.[16] The near tripling in the total number of permanently resettled persons between 1986 and 2000 from the most severely contaminated areas to approximately 350,000 [18][19] 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.[20] Many thousands of these evacuees would have been "better off staying home."[21]Risk analysis in 2007, supported by DNA biomarkers, 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."[22][17]

In 2017 Philip Thomas, Professor of Risk Management at the University of Bristol used the Years of potential life lost metric to conclude that, "Relocation was unjustified for 75% of the 335,000 people relocated after Chernobyl", finding that just 900 people within the 220,000 relocated during the second evacuation would have lost 3 months' of life expectancy by staying home and that, "none should have been asked to leave". For comparison, Thomas found that the average Londoner, a city of ~8 million, lose 4.5 months of life due to air pollution.[23][24]

Russia, Ukraine, and Belarus have been burdened with the continuing and substantial decontamination and monthly compensation costs [20][21][25] of the Chernobyl accident. Although certain initiatives are legitimate, as the director of the UN Development ProgramKalman Mizsei noted, “an industry has been built on this unfortunate event,” with a “vast interest in creating a false picture.”[21][26]

The accident raised the already heightened concerns about fission reactors 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.[27]

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.[28][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".[29]

Cancer projections

A report by the International Atomic Energy Agency examines the environmental consequences of the accident.[15] 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 Chernobyl liquidators) who averaged an effective dose equivalent to an extra 50 years of typical natural background radiation exposure each.[30][31][32]

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

In 1994, Thirty-one deaths were directly attributed to the accident, all among the reactor staff and emergency workers.[33] As of the 2008 UNSCEAR report, the total number of confirmed deaths from radiation was 64 and this is expected to continue to rise.

The Chernobyl Forum predicts that the eventual death toll could reach 4,000 among those exposed to the highest levels of radiation (200,000 emergency workers, 116,000 evacuees and 270,000 residents of the most contaminated areas); this figure is a total causal death toll prediction, combining the deaths of approximately 50 emergency workers who died soon after the accident from acute radiation syndrome, 15 children who have died of thyroid cancer and a future predicted total of 3935 deaths from radiation-induced cancer and leukaemia.[34]

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:[35]

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

Two anti-nuclear advocacy groups have publicized non-peer-reviewed estimates that include mortality estimates for those who were exposed to even smaller amounts of radiation. The Union of Concerned Scientists (UCS) calculated that, among the hundreds of millions of people exposed worldwide, there will be an eventual 50,000 excess cancer cases, resulting in 25,000 excess cancer deaths, excluding thyroid cancer.[36] However, these calculations are based on a simple linear no-threshold model multiplication and the misapplication of the collective dose, which the International Commission on Radiological Protection (ICRP) states "should not be done" as using the collective dose is "inappropriate to use in risk projections".[37]

Along similar lines to the UCS approach, the 2006 TORCH report, commissioned by the European Greens political party, likewise simplistically calculates an eventual 30,000 to 60,000 excess cancer deaths in total, around the globe.[38]

The Russian founder of that region's chapter of Greenpeace authored a book titled Chernobyl: Consequences of the Catastrophe for People and the Environment, 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.[39]Greenpeace itself advocates a figure of at least 200,000 or more.[40] The book was not peer reviewed prior to its publication,[41][42] and it has been heavily criticized; 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.[43]


On 26 April 1986, at 01:23 (UTC+3), reactor four suffered a catastrophic power increase, leading to explosions in its core. As the reactor had not been encased by any kind of hard containment vessel, this dispersed large quantities of radioactive isotopes into the atmosphere[44]:73 and caused an open-air fire that increased the emission of radioactive particles carried by the smoke. Ironically, the accident occurred during an experiment scheduled to test the viability of a potential safety emergency core cooling feature, which required a normal reactor 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 core damage.[45]RBMK reactors like those at Chernobyl use water as a coolant.[46][47] Reactor 4 at Chernobyl consisted of about 1,600 individual fuel channels, each of which required coolant flow of 28 metric tons (28,000 litres or 7,400 US gallons) per hour.[44]

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[44]:15 to attain full speed and reach the 5.5‑megawatt (MW) output required to run one main pump.[44]:30

To solve this one-minute gap – considered an unacceptable safety risk – it had been theorized that rotational energy from the steam turbine (as it wound down under residual steam pressure) could be used to generate the required electrical power. Analysis indicated that this residual momentum and steam pressure might be sufficient to run the coolant pumps for 45 seconds,[44]:16 bridging the gap between an external power failure and the full availability of the emergency generators.[48]

This capability still needed to be confirmed experimentally, and previous tests had ended unsuccessfully. An initial test carried out in 1982 indicated that the excitation voltage of the turbine-generator was insufficient; it did not maintain the desired magnetic field after the turbine trip. The system was modified, and the test was repeated in 1984 but again proved unsuccessful. In 1985, the tests were attempted a third time but also yielded negative results. The test procedure would be repeated in 1986, and it was scheduled to take place during the maintenance shutdown of Reactor Four.[48]

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).[49]

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.[49]

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

The experimental procedure was intended to run as follows:

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

Conditions before the accident

The conditions to run the test were established before the day shift of 25 April 1986. The day-shift workers had been instructed in advance and were familiar with the established procedures. A special team of electrical engineers was present to test the new voltage regulating system.[50] 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 Kievelectrical 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.[51] 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.[44]:36–38

The night shift had very limited time to prepare for and carry out the experiment. A further rapid decrease in the power level from 50% was executed during the shift change-over. Alexander Akimov was chief of the night shift, and Leonid Toptunov was the operator responsible for the reactor's operational regimen, including the movement of the control rods. Toptunov was a young engineer who had worked independently as a senior engineer for approximately three months.[44]:36–38

The test plan called for a gradual decrease in power output from reactor 4 to a thermal level of 700–1000 MW.[52] An output of 700 MW was reached at 00:05 on 26 April. Due to the reactor's production of a fission byproduct, xenon-135, which is a reaction-inhibiting neutron absorber, core power continued to decrease without further operator action—a process known as reactor poisoning. This continuing decrease in power occurred because in steady state operation, xenon-135 is "burned off" as quickly as it is created from decaying iodine-135 by absorbing neutrons from the ongoing chain reaction to become highly stable xenon-136. When the reactor power was lowered, previously produced high quantities of iodine-135 decayed into the neutron-absorbing xenon-135 faster than the reduced neutron flux could burn it off. As the reactor power output dropped further, to approximately 500 MW, Toptunov mistakenly inserted the control rods too far—the exact circumstances leading to this are unknown because Akimov 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.[49]:73 Control-room personnel decided to restore power by disabling the automatic system governing the control rods and manually extracting the majority of the reactor control rods to their upper limits.[53] 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.[54][55] This restricted any further rise of reactor power, and made it necessary to extract additional control rods from the reactor core in order to counteract the poisoning.

The operation of the reactor at the low power level and high poisoning level was accompanied by unstable core temperature and coolant flow, and possibly by instability of neutron flux, which triggered alarms. The control room received repeated emergency signals regarding the levels in the steam/water separator drums, and large excursions or variations in the flow rate of feed water, as well as from relief valves opened to relieve excess steam into a turbine condenser, and from the neutron power controller. Between 00:35 and 00:45, emergency alarm signals concerning thermal-hydraulic parameters were ignored, apparently to preserve the reactor power level.[56]

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

The flow exceeded the allowed limit at 01:19, triggering an alarm of low steam pressure in the steam separators. At the same time, the extra water flow lowered the overall core temperature and reduced the existing steam voids in the core and the steam separators.[57] Since water weakly absorbs neutrons (and the higher density of liquid water makes it a better absorber than steam), turning on additional pumps decreased the reactor power further still. The crew responded by turning off two of the circulation pumps to reduce feedwater flow, in an effort to increase steam pressure, and by removing more manual control rods to maintain power.[51][58]

All these actions led to an extremely unstable reactor configuration. Nearly all of the control rods were removed manually, including all but 18 of the "fail-safe" manually operated rods of the minimal 28 which were intended to remain fully inserted to control the reactor even in the event of a loss of coolant, out of a total 211 control rods.[59] While the emergency SCRAM system that would insert all control rods to shut down the reactor could still be activated manually (through the "AZ-5" button), the automated system that could do the same had been disabled to maintain the power level, and many other automated and even passive safety features of the reactor had been bypassed. Further, the reactor coolant pumping had been reduced, which had limited margin so any power excursion would produce boiling, thereby reducing neutron absorption by the water. The reactor was in an unstable configuration that was outside the safe operating envelope established by the designers. If anything pushed it into supercriticality, it was unable to recover automatically.

Experiment and explosion

At 1:23:04 a.m., the experiment began. Four of the main circulating pumps (MCP) were active; of the eight total, six are normally active during regular operation. The steam to the turbines was shut off, beginning a run-down of the turbine generator. The diesel generators started and sequentially picked up loads; the generators were to have completely picked up the MCPs' power needs by 01:23:43. In the interim, the power for the MCPs was to be supplied by the turbine generator as it coasted down. As the momentum of the turbine generator decreased, so did the power it produced for the pumps. The water flow rate decreased, leading to increased formation of steam voids (bubbles) in the core.

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 control rods into the reactor core to limit the power rise. This system had control of only 12 rods, and nearly all others had been manually retracted.

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

There is a view that the SCRAM may have been ordered as a response to the unexpected rapid power increase, although there is no recorded data proving this. Some have suggested that the button was not manually pressed, that the SCRAM signal was automatically produced by the emergency protection system, but the SKALA registered a manual SCRAM signal. Despite this, the question as to when or even whether the EPS-5 button was pressed has been the subject of debate. There have been assertions that the manual SCRAM was initiated due to the initial rapid power acceleration. Others have suggested that the button was not pressed until the reactor began to self-destruct, while others believe that it happened earlier and in calm conditions.[61]:578[62]

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

A few seconds into the SCRAM, a power spike occurred, and the core overheated, causing some of the fuel rods to fracture, blocking the control rod columns and jamming the control rods at one-third insertion, with the graphite displacers still in the lower part of the core. Within three seconds the reactor output rose above 530 MW.[44]:31

The subsequent course of events was not registered by instruments; it is known only as a result of mathematical simulation. Apparently, the power spike caused an increase in fuel temperature and steam buildup, leading to a rapid increase in steam pressure. This caused the fuel cladding to fail, releasing the fuel elements into the coolant, and rupturing the channels in which these elements were located.[63]

Then, according to some estimations, the reactor jumped to around 30,000 MW thermal, ten times the normal operational output. The last reading on the control panel was 33,000 MW. It was not possible to reconstruct the precise sequence of the processes that led to the destruction of the reactor and the power unit building, but a steam explosion, like the explosion of a steam boiler from excess vapour pressure, appears to have been the next event. There is a general understanding that it was explosive steam pressure from the damaged fuel channels escaping into the reactor's exterior cooling structure that caused the explosion that destroyed the reactor casing, tearing off and blasting the upper plate, to which the entire reactor assembly is fastened, through the roof of the reactor building. This is believed to be the first explosion that many heard.[64]:366 This explosion ruptured further fuel channels, as well as severing most of the coolant lines feeding the reactor chamber, and as a result the remaining coolant flashed to steam and escaped the reactor core. The total water loss in combination with a high positive void coefficient further increased the reactor's thermal power.

A second, more powerful explosion occurred about two or three seconds after the first; this explosion dispersed the damaged core and effectively terminated the nuclear chain reaction. This explosion also compromised more of the reactor containment vessel and ejected hot lumps of graphite moderator. The ejected graphite and the demolished channels still in the remains of the reactor vessel caught fire on exposure to air, greatly contributing to the spread of radioactive fallout and the contamination of outlying areas.[51]

According to observers outside Unit 4, burning lumps of material and sparks shot into the air above the reactor. Some of them fell onto the roof of the machine hall and started a fire. About 25 percent of the red-hot graphite blocks and overheated material from the fuel channels was ejected. Parts of the graphite blocks and fuel channels were out of the reactor building. As a result of the damage to the building an airflow through the core was established by the high temperature of the core. The air ignited the hot graphite and started a graphite fire.[44]:32[better source needed]

After the larger explosion, a number of employees at the power station went outside to get a clearer view of the extent of the damage. One such survivor, Alexander Yuvchenko, recounts that once he stopped outside and looked up towards the reactor hall, he saw a "very beautiful" LASER-like beam of light bluish light caused by the ionization of air that appeared to "flood up into infinity".[65][66][67]

There were initially several hypotheses about the nature of the second explosion. One view was that the second explosion was caused by hydrogen, which had been produced either by the overheated steam-zirconium reaction or by the reaction of red-hot graphite with steam that produced hydrogen and carbon monoxide. Another hypothesis, by Checherov, published in 1998, was that the second explosion was a thermal explosion of the reactor as a result of the uncontrollable escape of fast neutrons caused by the complete water loss in the reactor core.[68] A third hypothesis was that the second explosion was another steam explosion. According to this version, the first explosion was a more minor steam explosion in the circulating loop, causing a loss of coolant flow and pressure that in turn caused the water still in the core to flash to steam. This second explosion then did the majority of the damage to the reactor and containment building.

The force of the second explosion and the ratio of xenon radioisotopes released after the accident (a vital tool in nuclear forensics) indicated to Yuri V. Dubasov in a 2009 publication (suggested before him by Checherov in 1998), that the second explosion could have been a nuclear power transient resulting from core material melting in the absence of its water coolant and moderator. Dubasov argues that the reactor did not simply undergo a runaway delayed-supercritical/exponential increase in power into the multi-gigawatt power range, which is somewhat similar to the conditions of a normal reactor coming up to its commercial power level (with the notable exception that Chernoyl's older RBMK reactor design had the largest positive void coefficient of reactivity of any reactor then operating commercially), permitting a dangerous "positive feedback"/runaway condition, given the lack of inherent safety stops when power levels began to increase above the commercial level. Although a positive-feedback power excursion that increased until the reactor disassembled itself by means of its internal energy and external steam explosions[69] is the more accepted explanation for the cause of the explosions, Dubasov argues instead that a runaway prompt supercriticality occurred, with the internal physics being more similar to the explosion of a fizzled nuclear weapon, and that this failed/fizzle event produced the second explosion.[70]

This nuclear fizzle hypothesis, then mostly defended by Dubasov, was examined further in 2017 by retired physicist Lars-Erik De Geer in an analysis that puts the hypothesized fizzle event as the more probable cause of the first explosion.[71][72][73] The more energetic second explosion, which produced the majority of the damage, has been estimated by Dubasov in 2009 as equivalent to 40 billion joules of energy, the equivalent of about ten tons of TNT. Both the 2009 and 2017 analyses argue that the nuclear fizzle event, whether producing the second or first explosion, consisted of a prompt chain reaction (as opposed to the consensus delayed neutron mediated chain-reaction) that was limited to a small portion of the reactor core, since expected self-disassembly occurs rapidly in fizzle events.[70][74][75]

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.[44]:42 Inside reactor 3, the chief of the night shift, Yuri Bagdasarov, wanted to shut down the reactor immediately, but chief engineer Nikolai Fomin would not allow this. The operators were given respirators and potassium iodide tablets and told to continue working. At 05:00, Bagdasarov made his own decision to shut down the reactor, leaving only those operators there who had to work the emergency cooling systems.[44]:44

Radiation levels

Approximate radiation intensity levels at different locations at Chernobyl reactor site shortly after the explosion are shown in the below table.[76] A dose of 500 roentgens (~5 Sv) delivered over an hour is usually lethal for human beings.

LocationRadiation (roentgens per hour)Sieverts per hour (SI Unit)
Vicinity of the reactor core30,000300
Fuel fragments15,000–20,000150–200
Debris heap at the place of circulation pumps10,000100
Debris near the electrolyzers5,000–15,00050–150
Water in the Level +25 feedwater room5,00050
Level 0 of the turbine hall500–15,0005–150
Area of the affected unit1,000–1,50010–15
Water in Room 7121,00010
Control room3–50.03–0.05
Hydropower Installation300.3
Nearby concrete mixing unit10–150.10–0.15

Plant layout

Based on the image of the plant[77]
MetresLevels are distances above (or below for minus values) ground level at the site.
49.6Roof of the reactor building, gallery of the refuelling mechanism
39.9Roof of the deaerator gallery
35.5Floor of the main reactor hall
31.6Upper side of the upper biological shield, floor of the space for pipes to steam separators
28.3Lower side of the turbine hall roof
24.0Deaerator floor, measurement and control instruments room
16.4Floor of the pipe aisle in the deaerator gallery
12.0Main floor of the turbine hall, floor of the main circulation pump motor compartments
10.0Control room, floor under the reactor lower biological shield, main circulation pumps
6.0Steam distribution corridor
2.2Upper pressure suppression pool
0.0Ground level; house switchgear, turbine hall level
−0.5Lower pressure suppression pool
−5.2, −4.2Other turbine hall levels
−6.5Basement floor of the turbine hall

Individual involvement

Main article: Individual involvement in the Chernobyl disaster

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), equivalent to more than 20,000 roentgens per hour. A lethal dose is around 500 roentgens (~5 Gy) 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 1000 R/s 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 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), while the true levels were much higher in some areas.[44]: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.[44]: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.[59]:247–48

Fire containment

Shortly after the accident, firefighters arrived to try to extinguish the fires. First on the scene was a Chernobyl Power Station firefighter brigade under the command of Lieutenant Volodymyr Pravik, who died on 9 May 1986 of acute radiation sickness. They were not told how dangerously radioactive the smoke and the debris were, and may not even have known that the accident was anything more than a regular electrical fire: "We didn't know it was the reactor. No one had told us."[78]

Grigorii Khmel, the driver of one of the

Location of Chernobyl nuclear power plant
The abandoned city of Pripyat with the Chernobyl facility visible in the distance
The number of nuclear power plant constructions started each year worldwide, from 1954 to 2013.
A schematic diagram of the reactor
Radioactive steam plumes continued to be generated days after the initial explosion, as evidenced here on 3 May 1986 due to decay heat. The roof of the turbine hall is damaged (image centre). Roof of the adjacent reactor 3 (image lower left) shows minor fire damage. Igor Kostin would take some of the clearer pictures of the roof of the buildings when he was physically present on the roof of reactor 3, in June of that year.[60]
Extremely high levels of radioactivity in the lava under the Chernobyl number four reactor in 1986
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