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To provide radiologocal protection by prevention of airborne contamination, and prevent weathering of the reactor remains, a containment structure was planned. This was the largest civil engineering task in history, involving a quarter of a million construction workers who all reached their official lifetime limits of radiation.[59] Ukrainian filmmaker Vladimir Shevchenko captured film footage of an Mi-8 helicopter as its main rotor collided with a nearby construction crane cable, causing the helicopter to fall near the damage

To provide radiologocal protection by prevention of airborne contamination, and prevent weathering of the reactor remains, a containment structure was planned. This was the largest civil engineering task in history, involving a quarter of a million construction workers who all reached their official lifetime limits of radiation.[59] Ukrainian filmmaker Vladimir Shevchenko captured film footage of an Mi-8 helicopter as its main rotor collided with a nearby construction crane cable, causing the helicopter to fall near the damaged reactor building and killing its four-man crew on 2 October 1986.[92] By December 1986, a large concrete sarcophagus had been erected to seal off the reactor and its contents.[65] The greater urban decontamination liquidators similarly first washed buildings and roads with "Bourda", a sticky polymerizing fluid DeconGel, designed to entrain radioactive dust and, when dry, could then be peeled off and compacted into configurations, akin to carpet rolls, in preparation for burial.[93] A unique "clean up" medal was given to the workers.[94]

Investigations of the reactor condition

During the construction of the sarcophagus, a scientific team re-entered the reactor as part of an investigation dubbed "Complex Expedition", to locate and contain nuclear fuel in a way that could not lead to anoth

During the construction of the sarcophagus, a scientific team re-entered the reactor as part of an investigation dubbed "Complex Expedition", to locate and contain nuclear fuel in a way that could not lead to another explosion. These scientists manually collected cold fuel rods, but great heat was still emanating from the core. Rates of radiation in different parts of the building were monitored by drilling holes into the reactor and inserting long metal detector tubes. The scientists were exposed to high levels of radiation and radioactive dust.[59] After six months of investigation, in December 1986, with the help of a remote camera they discovered an intensely radioactive mass more than two metres wide in the basement of Unit Four, which they called "the elephant's foot" for its wrinkled appearance.[95] The mass was composed of melted sand, concrete and a large amount of nuclear fuel that had escaped from the reactor. The concrete beneath the reactor was steaming hot, and was breached by now-solidified lava and spectacular unknown crystalline forms termed chernobylite. It was concluded that there was no further risk of explosion.[59]

Area cleanup

The official contaminated zones saw a massive clean

The official contaminated zones saw a massive clean-up effort lasting seven months.[65]:177–183 The official reason for such early (and dangerous) decontamination efforts, rather than allowing time for natural decay, was that the land must be repopulated and brought back into cultivation. Indeed, within fifteen months 75% of the land was under cultivation, even though only a third of the evacuated villages were resettled. Defence forces must have done much of the work. Yet this land was of marginal agricultural value. According to historian David Marples, the administration had a psychological purpose for the clean-up: they wished to forestall panic regarding nuclear energy, and even to restart the Chernobyl power station.[65]:78–79, 87, 192–193 Although a number of radioactive emergency vehicles were buried in trenches, many of the vehicles used by the liquidators, including the helicopters, still remained, as of 2018, parked in a field in the Chernobyl area. Scavengers have since removed many functioning, but highly radioactive, parts.[96] Liquidators worked under deplorable conditions, poorly informed and with poor protection. Many, if not most of them, exceeded radiation safety limits.[65]:177–183[97]

IAEA used the International Nuclear Safety Advisory Group (INSAG), which had been created by the IAEA in 1985.[98] It produced two significant reports on Chernobyl; INSAG-1 in 1986, and a revised report, INSAG-7 in 1992. In summary, according to INSAG-1, the main cause of the accident was the operators' actions, but according to INSAG-7, the main cause was the reactor's design.[4]:24[99] Both IAEA reports identified an inadequate "safety culture" (INSAG-1 coined the term) at all managerial and operational levels as a major underlying factor of different aspects of the accident. This was stated to be inherent not only in operations but also during design, engineering, construction, manufacture and regulation.[4]:21,24

Views of the main causes were heavily lobbied by different groups, including the reactor's designers, power plant personnel, and the Soviet and Ukrainian

Views of the main causes were heavily lobbied by different groups, including the reactor's designers, power plant personnel, and the Soviet and Ukrainian governments. This was due to the uncertainty about the actual sequence of events and plant parameters. After INSAG-1 more information became available, and more powerful computing has allowed better forensic simulations.[4]:10

The INSAG-7 conclusion of major factors contributory to the accident was:

"The Accident is now seen to have been the result of concurrance of the following major factors: specific physical characteristics of the reactor; specific design features of the reactor control elements; and the fact that the reactor was brought to a state not specified by procedures or investigated by an independent safety body. Most importantly, the physical characteristics of the reactor made possible its unstable behaviour."[4]:23

The first Soviet official explanation of the accident was by means of presentations from leading Soviet scientists and engineers to a large number of representatives from IAEA member states and other international organisations at the first Post-Accident Review Meeting, held at the IAEA in Vienna between 25 and 29 August 1986. This explanation effectively placed the blame on the power plant operators. The UKAEA INSAG-1 report followed shortly afterwards in September 1986, and on the whole also supported this view, based also on the information provided in discussions with the Soviet experts at the Vienna review meeting.[100] In this view, the catastrophic accident was caused by gross violations of operating rules and regulations. For instance; "During preparation and testing of the turbine generator under run-down conditions using the auxiliary load, personnel disconnected a series of technical protection systems and breached the most important operational safety provisions for conducting a technical exercise."[34]:311

It was stated that at the time of the accident the reactor was being operated with many key safety systems turned off, most notably the It was stated that at the time of the accident the reactor was being operated with many key safety systems turned off, most notably the Emergency Core Cooling System (ECCS), LAR (Local Automatic control system), and AZ (emergency power reduction system). Personnel had an insufficient understanding of technical procedures involved with the nuclear reactor, and knowingly ignored regulations to expedite the electrical test completion.[34] Several procedural irregularities also helped to make the accident possible, one of which was insufficient communication between the safety officers and the operators in charge of the test. The main process computer, SKALA, was running in such a way that the main control computer could not shut down the reactor or even reduce power. Normally the computer would have started to insert all of the control rods. The computer would have also started the "Emergency Core Protection System" that introduces 24 control rods into the active zone within 2.5 seconds, which is still slow by 1986 standards. All control was transferred from the process computer to the human operators.

It was held that the designers of the reactor considered this combination of events to be impossible and therefore did not allow for the creation of emergency protection systems capable of preventing the combination of events that led to the crisis, namely the intentional disabling of emergency protection equipment plus the violation of operating procedures. Thus the primary cause of the accident was the extremely improbable combination of rule infringement plus the operational routine allowed by the power station staff.[34]:312

On the disconnection of safety systems, Valery Legasov said in 1987, "It was like airplane pilots experimenting with the engines in flight."[101] In this analysis the operators were blamed, but deficiencies in the reactor design and in the operating regulations that made the accident possible were set aside and mentioned only casually. This view was reflected in numerous publications and artistic works on the theme of the Chernobyl accident that appeared immediately after the accident,[26] and for a long time remained dominant in the public consciousness and in popular publications.

The trial took place from 7 to 30 July 1987 in a temporary courtroom set up in the House of Culture in the city of Chernobyl, Ukraine. Five plant employees (the former deputy chief engineer Anatoly S. Dyatlov; the former plant director Viktor P. Bryukhanov; the former chief engineer Nikolai M. Fomin; the shift director of Reactor 4, Boris V. Rogozhin; and the chief of Reactor 4, Aleksandr P. Kovalenko) and Gosatomenergonadzor (USSR State Committee on Supervision of Safe Conduct of Work in Atomic Energy) inspector Yuri A. Laushkin were sentenced to 10, 10, 10, five, three and two years respectively in labor camps.[102] The families of Aleksandr Akimov, Leonid Toptunov and Valery Perevozchenko had received official letters but prosecution against the employees had been terminated at their deaths.

Anatoly Dyatlov was found guilty "of criminal mismanagement of potentially explosive enterprises" and sentenced to 10 years imprisonment—of which he would serve three[103]—for the role that his oversight of the experiment played in the ensuing accident.

Anatoly Dyatlov was found guilty "of criminal mismanagement of potentially explosive enterprises" and sentenced to 10 years imprisonment—of which he would serve three[103]—for the role that his oversight of the experiment played in the ensuing accident.

In 1991 a Commission of the USSR State Committee for the Supervision of Safety in Industry and Nuclear Power reassessed the causes and circumstances of the Chernobyl accident and came to new insights and conclusions. Based on that, INSAG published an additional report, INSAG-7,[4] which reviewed "that part of the INSAG-1 report in which primary attention is given to the reasons for the accident," and this included the text of the 1991 USSR State Commission report translated into English by the IAEA as Annex I.[4]

By the time of this report, Ukraine had declassified a number of KGB documents from the period between 1971 and 1988 related to the Chernobyl plant. It mentioned, for example, previous reports of structural damage caused by negligence during construction of the plant (such as splitting of concrete layers) that were never acted upon. They documented more than 29 emergency situations in the plant during this period, eight of which were caused by negligence or poor competence on the part of personnel.[105]

In the INSAG-7 report, most of the earlier accusations against staff for breach of regulations were acknowledged to be either erroneous, being based on incorrect information obtained in August 1986, or less relevant. The INSAG-7 report also reflected the view of the 1991 USSR State Commission account which held that the operators' actions in turning off the Emergency Core Cooling System, interfering with the settings on the protection equipment, and blocking the level and pressure in the separator drum did not contribute to the original cause of the accident and its magnitude, although they may have been a breach of regulations. In fact, turning off the emergency system designed to prevent the two turbine generators from stopping was not a violation of regulations.[4] Soviet authorities had identified a multitude of operator actions as regulation violations in the original 1986 report while no such regulations were in fact in place.[4]:18

The primary design cause of the accident, as determined by INSAG-7, was a major deficiency in safety features,[4]:22 in particular the "positive scram" effect due to the control rods' graphite tips that actually initially increased reactivity when control rods entered the core to reduce reactivity.[4]:16 There was also an overly positive void coefficient of the reactor, whereby steam generated voids in the fuel cooling channels would increase reactivity because neutron absorption was reduced, resulting in more steam generation, and thereby more voids; a regenerative process.KGB documents from the period between 1971 and 1988 related to the Chernobyl plant. It mentioned, for example, previous reports of structural damage caused by negligence during construction of the plant (such as splitting of concrete layers) that were never acted upon. They documented more than 29 emergency situations in the plant during this period, eight of which were caused by negligence or poor competence on the part of personnel.[105]

In the INSAG-7 report, most of the earlier accusations against staff for breach of regulations were acknowledged to be either erroneous, being based on incorrect information obtained in August 1986, or less relevant. The INSAG-7 report also reflected the view of the 1991 USSR State Commission account which held that the operators' actions in turning off the Emergency Core Cooling System, interfering with the settings on the protection equipment, and blocking the level and pressure in the separator drum did not contribute to the original cause of the accident and its magnitude, although they may have been a breach of regulations. In fact, turning off the emergency system designed to prevent the two turbine generators from stopping was not a violation of regulations.[4] Soviet authorities had identified a multitude of operator actions as regulation violations in the original 1986 report while no such regulations were in fact in place.[4]:18

The primary design cause of the accident, as determined by INSAG-7, was a major deficiency in safety features,[4]:22 in particular the "positive scram" effect due to the control rods' graphite tips that actually initially increased reactivity when control rods entered the core to reduce reactivity.[4]:16 There was also an overly positive void coefficient of the reactor, whereby steam generated voids in the fuel cooling channels would increase reactivity because neutron absorption was reduced, resulting in more steam generation, and thereby more voids; a regenerative process.[4]:13 To avoid such conditions, it was necessary for the operators to track the value of the reactor operational reactivity margin (ORM) but this value was not readily available to the operators[4]:17 and they were not aware of the safety significance of ORM on void and power coefficients.[4]:14 However, regulations did forbid operating the reactor with a small margin of reactivity. Yet "post-accident studies have shown that the way in which the real role of the ORM is reflected in the Operating Procedures and design documentation for the RBMK-1000 is extremely contradictory", and furthermore, "ORM was not treated as an operational safety limit, violation of which could lead to an accident".[4]:34–25

Even in this revised analysis, the human factor remained identified as a major factor in causing the accident; particularly the operating crew's deviation from the test programme. "Most reprehensibly, unapproved changes in the test procedure were deliberately made on the spot, although the plant was known to be in a very different condition from that intended for the test."[4]:24 This included operating the reactor at a lower power level than the prescribed 700 MW before starting the electrical test. The 1986 assertions of Soviet experts notwithstanding, regulations did not prohibit operating the reactor at this low power level.[4]:18

INSAG-7 also said, "The poor quality of operating procedures and instructions, and their conflicting character, put a heavy burden on the operating crew, including the chief engineer. The accident can be said to have flowed from a deficient safety culture, not only at the Chernobyl plant, but throughout the Soviet design, operating and regulatory organizations for nuclear power that existed at that time."[4]:24

In summary, the major factors were:[4]:18–24

The reactor had a dangerously large positive void coefficient of reactivity. The void coefficient is a measurement of how a reactor responds to increased steam formation in the water coolant. Most other reactor designs have a negative coefficient, i.e. the nuclear reaction rate slows when steam bubbles form in the coolant, since as the steam voids increase, fewer neutrons are slowed down. Faster neutrons are less likely to split uranium atoms, so the reactor produces less power (negative feedback effect).

Chernobyl's RBMK reactor, however, used solid graphite as a neutron moderator to slow down the neutrons, however the cooling water acts like a neutron absorber. Thus neutrons are moderated by the graphite even if steam bubbles form in the water. Furthermore, because steam absorbs neutrons much less readily than water, increasing the voids means that more moderated neutrons are able to split uranium atoms, increasing the reactor's power output. This was a positive feedback regenerative process which makes the RBMK design very unstable at low power levels, and prone to sudden energy surges to a dangerous level. This behaviour is counter-intuitive, and this property of the reactor was unknown to the crew.

There was a significant flaw in the design of the control rods that were inserted into the reactor to slow down the reaction rate by neutron absorption. In the RBMK design, the bottom tip of each control rod was made of graphite and was 1.3 metres (4.3 ft) shorter than necessary. Only the upper part of the rod was made of boron carbide, which absorbs neutrons and thereby slows the reaction. With this design, when a rod was inserted from the fully retracted position, the graphite tip displaced neutron-absorbing water, initially causing fewer neutrons to be absorbed and increasing reactivity. For the first few seconds of rod deployment, reactor core power was therefore increased, rather than reduced. This feature of control rod operation was counter-intuitive and not known to the reactor operators.

Management

Other deficiencies were noted in the RBMK-1000 reactor design, as were its non-compliance with accepted standards and with the requirements of nuclear reactor safety. While INSAG-1 and INSAG-7 reports both identified operator error as an issue of concern, the INSAG-7 identified that there were numerous other issues that were contributing factors that led to the incident. These contributing factors include:

  1. The plant was not designed to safety standards in effect and incorporated unsafe features
  2. "Inadequate safety a

    The force of the second explosion and the ratio of xenon radioisotopes released after the accident led Yuri V. Dubasov in 2009 to theorise that the second explosion could have been an extremely fast nuclear power transient resulting from core material melting in the absence of its water coolant and moderator. Dubasov argued that there was no delayed supercritical increase in power but a runaway prompt criticality which would have developed much faster. He felt the physics of this would be more similar to the explosion of a fizzled nuclear weapon, and it produced the second explosion.[106] His evidence came from Cherepovets, Vologda Oblast, Russia, 1,000 kilometres (620 mi) northeast of Chernobyl, where physicists from the V.G. Khlopin Radium Institute measured anomalous high levels of xenon-135 — a short half-life isotope — four days after the explosion. This meant that a nuclear event in the reactor may have ejected xenon to higher altitudes in the atmosphere than the later fire did, allowing widespread movement of xenon to remote locations.[107] This was an alternative to the more accepted explanation of a positive-feedback power excursion where the reactor disassembled itself by steam explosion.[4] [106]

    The more energetic second explosion, which produced the majority of the damage, was estimated by Dubasov in 2009 as equivalent to 40 billion joules of energy, the equivalent of about 10 tons of TNT. Both his 2009 and 2017 analyses argue that the nuclear fizzle event, whether producing the second or first explosion, consisted of a prompt chain reaction that was limited to a small portion of the reactor core, since self-disassembly occurs rapidly in fizzle events.[106][108][109]

    Dubasov's nuclear fizzle hypothesis was examined in 2017 by physicist Lars-Erik De Geer who put the hypothesized fizzle event as the more probable cause of the first explosion.[108][110]<