Chernobyl disaster

Chernobyl disaster

File:Chernobyl Disaster.jpg The nuclear reactor after the disaster. Reactor 4 (center). Turbine building (lower left). Reactor 3 (center right).
Date	26 April 1986 (1986-04-26)
Time	01:23 a.m. (UTC+3)
Location	Pripyat, Soviet Union, now Ukraine
Casualties
56 direct deaths
as 4,000 cancer deaths over the lifetime of those exposed.

The abandoned city of Pripyat with Chernobyl plant in the distance.

File:Radio-operated bulldozer outside Chernobyl.jpg

Radio-operated bulldozers being tested prior to use.

Abandoned housing blocks in Pripyat.

The Chernobyl disaster (Template:Lang-uk, Čornobyľśka katastrofa) was a nuclear accident that occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in Ukraine (then in the Ukrainian Soviet Socialist Republic, part of the former Soviet Union). It is considered the worst nuclear power plant accident in history and is the only level 7 event on the International Nuclear Event Scale.

On April 26, 1986, reactor number four at the Chernobyl plant, near Pripyať in the Ukrainian Soviet Socialist Republic, had a meltdown. The resulting fire sent a plume of radioactive fallout into the atmosphere and over an extensive geographical area, including the nearby town of Pripyat. The plume drifted over large parts of the western Soviet Union, Eastern Europe, Western Europe, and Northern Europe. Large areas in Ukraine, Belarus, and Russia had to be evacuated, with over 336,000 people resettled. According to official post-Soviet data, about 60% of the fallout landed in Belarus.

The accident raised concerns about the safety of the Soviet nuclear power industry as well as nuclear power in general, slowing its expansion for a number of years while forcing the Soviet government to become less secretive.

The countries of Russia, Ukraine, and Belarus have been burdened with the continuing and substantial decontamination and health care costs of the Chernobyl accident. A 2005 report prepared by the Chernobyl Forum, led by the International Atomic Energy Agency (IAEA) and World Health Organization (WHO), attributed "fewer than 50" direct deaths (including nine children with thyroid cancer) and estimated that there may be up to 4,000 additional cancer deaths over time among the approximately 600,000 most highly exposed people. Although the Chernobyl Exclusion Zone and certain limited areas remain off limits, the majority of affected areas are now considered safe for settlement and economic activity.

Accident

On 26 April 1986 at 01:23 a.m. (UTC+3), reactor 4 suffered a massive, catastrophic power excursion. This caused a steam explosion, followed by a second (chemical, not nuclear) explosion from the ignition of generated hydrogen mixed with air, which tore the top from the reactor and its building and exposed the reactor core. This dispersed large amounts of radioactive particulate and gaseous debris containing fission products including caesium-137, strontium-90, and other highly radioactive reactor waste products. The open core also allowed atmospheric oxygen to contact the super-hot core containing 1,700 metric tons of combustible graphite moderator. The burning graphite moderator increased the emission of radioactive particles, carried by the smoke. The reactor was not contained by any kind of hard containment vessel (unlike all Western plants, Soviet reactors often did not have them). Radioactive particles were carried by wind across international borders.

Planning the test of a potential safety feature

A rotor of a modern day steam turbine.

During the daytime of 25 April 1986, reactor 4 was scheduled to be shut down for maintenance as it was near the end of its first fuel cycle. An experiment was scheduled to test a potential safety emergency core cooling feature during the shutdown procedure.

Following an emergency shutdown (scram), cooling is required to keep the temperature in the reactor core low enough to avoid fuel damage. The reactor consisted of about 1,600 individual fuel channels and each operational channel required a flow of 28 metric tons (28,000 liters (7,400 U.S. gal)) of water per hour. There was concern that in case of an external power failure the Chernobyl power station would overload, leading to an automated safety shut down, in which case there would be no external power to run the plant's cooling water pumps. Chernobyl's reactors had three backup diesel generators. The generators required 15 seconds to start up but took 60–75 seconds to attain full speed and reach the capacity of 5.5 MW required to run one main cooling water pump.

Nuclear power reactors require cooling flow to remove decay heat, even when not actively generating power. In case of an external power failure, the reactor would automatically scram: control rods would be inserted and stop the nuclear fission process (and hence steam generation). However, in the spent fuel, the fission products themselves are radioactive and continue to produce heat as they decay. This could amount to 6–7 percent of the normal output of the plant. If not immediately removed by coolant systems, the heat could lead to core damage.

This one-minute power gap was considered unacceptable and it was suggested that the mechanical energy (rotational momentum) of the steam turbine could be used to generate electricity to run the main cooling water pumps while it was spinning down. In theory, analyses indicated that the residual momentum had the potential to provide power for 45 seconds, which would bridge the power gap between the onset of the external power failure and the full availability of electric power from the emergency diesel generators, but this capability needed to be confirmed experimentally. Previous tests had ended unsuccessfully. An initial test in 1982 showed that the excitation voltage of the turbine-generator was insufficient; it did not maintain the desired magnetic field during the generator spin-down. The system was modified, and in 1984 the test was repeated, but the retest proved unsuccessful. In 1985 the test was attempted a third time, but it also yielded negative results. This test procedure was to be repeated again in 1986.

The test was focused on the switching sequences of the electrical supplies for the reactor. Since the test procedure was to begin when the reactor was scrammed automatically at the very beginning of the experiment, the experiment was not anticipated to have any detrimental effect on the safety of the reactor. Therefore, the test program was not formally coordinated with either the chief designer of the reactor (NIKIET) nor 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, at the start of the experiment, the thermal output of the reactor should have been no lower than 700 MW. If conditions had been as planned, the test almost certainly would have proceeded safely; the eventual disaster resulted from attempts to boost the reactor output once the experiment had started, inconsistent with the 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—an important safety feature. The station managers presumably wished to correct this at the first opportunity. This may explain why they continued the test even when serious problems arose, and why the requisite approval for the test was not sought from the Soviet nuclear oversight regulator (even though there was a representative at the complex of 4 reactors).

The experimental procedure was intended to run as follows:

• the reactor was to be running at a low power, but >700 MW
• the steam turbine was to be run up to full speed
• when these conditions were achieved, the steam supply was to be closed off
• the turbines would be allowed to freewheel down
• generator performance was to be recorded to determine whether it could provide the bridging power for coolant pumps

Turbogenerator vibrations measurement

Block 4 was equipped with the newer generation of the turbogenerators. These turned out to be prone to vibrations, and required periodic readjustment and balancing. For that purpose, the Kharkiv plant dispatched specialist technicians. In late 1985, the plant bought a mobile vibration analysis laboratory, a highly sophisticated, expensive, and unique electronics system mounted in a two-seat Mercedes-Benz van, from the Swiss company Vibro-meter. The laboratory was almost the only one in the entire Soviet Union. Its availability was an important reason against cancelling the test; it was not certain when it would be available again. So the testing had to proceed despite the known unfavorable conditions of the reactor, similar to the 1975 partial meltdown incident in the Leningrad plant.

Parallel to the safety test, analysis of the turbogenerator vibrations was planned. The vibrolaboratory with a driver and three vibration specialists was dispatched from the Kharkiv plant, with Savenkov and Strelkov (the driver) going by car and Popov and Kabanov by train. On arrival at the plant, the laboratory was parked directly inside the machine hall, on Level 0; the normal procedure was to park outside and connect sensors by remote cables; however, that would have required making a hole in the wall.

Particular attention had to be paid to bearing number 12 of turbogenerator 8. The Leningrad-based supplier of the generators, Elektrosila, attempted to integrate an emergency lubricant reservoir into the bearing housing, but such bearings were found to produce high vibrations. Numerous attempts by the power plant and L'vovenergoremont technicians to suppress the vibrations failed to get them below the limits allowed by GOST. Despite repeated urging, Elektrosila did not dispatch technicians to attend to the problem. Meanwhile, the vibrations resulted in a fatigue crack of the bearing oil line and leak of combustible oil. The problem was temporarily fixed by the turbine shop servicemen. Kharkiv Turbine Works specialists were willing not only to measure the vibration characteristics of the turbines, but also perform detailed investigation of the bearing. During the night of April 26, the vibrolaboratory was parked on Level 0 at the end of the machine hall, in the compartment of turbogenerator 8.

At the moment of the explosion, Popov was in the control room, Savenkov and Kabanov were in the vibrolaboratory, and Strelkov, the driver, was in the Pripyat hotel. The specialists attempted to save the laboratory buried in debris, but did not succeed. The radiation at Level 0 was too high and they received a high dose that for Popov and Savenkov proved to be lethal.

At Popov's burial, the Kharkiv plant general designer promised to name the vibrolaboratory after the two fallen specialists. However that did not happen, as after the vehicle was recovered in 1987, its decontamination proved impossible.

Conditions prior to the accident

The conditions to run the test were established prior to the day shift of 25 April 1986. The day shift workers had been instructed in advance about the test and were familiar with procedures. A special team of electrical engineers was present to test the new voltage regulating system. As planned, on 25 April a gradual reduction in the output of the power unit was begun at 01:06 a.m., and by the beginning of the day shift the power level had reached 50%. Another regional power station unexpectedly went off-line, 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.

At 11:04 p.m., the Kiev grid controller allowed the reactor shut-down to resume. This delay had some serious consequences: the day shift had long since departed, the evening shift was also preparing to leave, and the night shift would not take over until midnight, well into the job. Further rapid reduction in the power level from 50% was actually executed during the shift change-over. According to plan, the test should have been finalized during the daytime 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. Alexander Akimov was chief of the night shift, and Leonid Toptunov was the operator responsible for the reactor's operational regime, including the movement of the control rods. Toptunov was a young engineer who had worked independently as a senior engineer for approximately three months.

The test plan called for the power output of reactor 4 to be reduced from its nominal 3200 MW thermal to 700–1000 MW thermal. The power level established in the test program (700 MW) was achieved at 00:05 a.m. on 26 April; however, because of the natural production in the core of a neutron absorber, xenon-135, reactor power continued to decrease, even without operator action. As the power reached approximately 500 MW, Toptunov committed an error and inserted the control rods too far, bringing the reactor to a near-shutdown state. The exact circumstances will probably never be known, as both Akimov and Toptunov died from radiation sickness.

The reactor power dropped to 30 MW thermal (or less)—an almost completely shutdown power level that was approximately 5 percent of the minimum initial power level established as safe for the test. Control-room personnel made a decision to restore the power and extracted the reactor control rods, though several minutes elapsed after their extraction until the power output began to increase and subsequently stabilize at 160–200 MW (thermal). In this case the majority of control rods were withdrawn to their upper limits, but the low value of the operational reactivity margin restricted further rise of reactor power. Rapid reduction in the power and subsequent operation at the level of less than 200 MW led to increased poisoning of the reactor core by the accumulation of xenon-135, which made it necessary to extract additional control rods from the reactor core.

The operation of the reactor at the low power level with a small reactivity margin was accompanied by unstable temperature and flow and possibly by instability of neutron flux. The control room received repeated emergency signals of the levels in the steam/water separator drums, of relief valves opened to relieve excess steam into a turbine condenser, of large excursions in the flow rate of feed water, and from the neutron power controller. In the period between 00:35 a.m and 00:45 a.m., apparently to preserve the reactor power level, emergency alarm signals concerning thermal-hydraulic parameters were ignored. Emergency signals from the Reactor Emergency Protection System (EPS-5) triggered a trip, turning off both turbine-generators.

After a more or less stable state at a power level of 200 MW was achieved, preparation for the experiment continued. As part of the test plan, at 1:05 a.m. on 26 April extra water pumps were activated, increasing the water flow. The increased coolant flow rate through the reactor produced an increase in the inlet coolant temperature of the reactor core, which now more closely approached the nucleate boiling temperature of water, reducing the safety margin. The flow exceeded the allowed limit at 1:19 a.m. At the same time the extra water flow lowered the overall core temperature and reduced the existing steam voids in the core. Since water also absorbs neutrons (and the higher density of liquid water makes it a better absorber than steam), turning on additional pumps decreased the reactor power. This prompted the operators to remove the manual control rods further to maintain power.

All these actions led to an extremely unstable reactor configuration. Nearly all of the control rods were removed, which limited the value of the safety rods when initially inserted in a scram. Further, the reactor coolant had limited boiling, but had limited margin to boiling, so any power excursion would produce boiling, reducing neutron absorption by the water. This left the reactor in an unstable configuration that was clearly outside the safe operating envelope established by the designers.

Experiment and explosion

Aerial view of the damaged core. Roof of the turbine hall is damaged (image center). Roof of the adjacent reactor 3 (image lower left) shows minor fire damage.

Lumps of graphite moderator ejected from the core. The largest lump shows an intact control rod channel.

At 1:23:04 a.m. the experiment began. The steam to the turbines was shut off, and a run down of the turbine generator began, together with four (of the total eight) Main Circulating Pumps (MCP). The diesel generator started and sequentially picked up loads, which was complete by 01:23:43; during this period the power for these four MCPs was supplied by the coasting down turbine generator. As the momentum of the turbine generator that powered the water pumps decreased, the water flow rate decreased, leading to increased formation of steam voids 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 decreased the ability of the liquid water coolant to absorb neutrons, which increased the reactor's power output, causing yet more water to flash into steam, and yet a further power increase. However, during almost the entire period of the experiment the automatic control system successfully counteracted this positive feedback, continuously inserting control rods into the reactor core to limit the power rise.

At 1:23:40, as recorded by the SKALA centralized control system, the EPS-5 button (also known as the AZ-5 button) of the reactor emergency protection system was pressed, which initiated a scram — a shutdown of the reactor, fully inserting all control rods, including the manual control rods that had been incautiously withdrawn earlier. The reason the EPS-5 button was pressed is not known, whether it was done as an emergency measure or simply as a routine method of shutting down the reactor upon completion of the experiment. 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 convincingly testifying to this.

Some have suggested that the button was not pressed but rather that the signal was automatically produced by the emergency protection system; however, the SKALA clearly registered a manual scram signal. In spite of this, the question as to when or even whether the EPS-5 button was pressed was 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. Others assert that it happened earlier and in calm conditions. Dyatlov writes in his book:

Prior to 01:23:40, systems of centralized control … did not register any parameter changes that could justify the scram. The Commission … gathered and analyzed large amounts of material and, as stated in its report, failed to determine the reason why the scram was ordered. There was no need to look for the reason. The reactor was simply being shut down upon the completion of the experiment.

For whatever reason the EPS-5 button was pressed, insertion of control rods into the reactor core began. The control rod insertion mechanism operated at a relatively slow speed (0.4 m/s) taking 18–20 seconds to travel the full approximately 7-meter core length (height). A bigger problem was a flawed graphite-tip control rod design, which initially displaced coolant before neutron-absorbing material was inserted and the reaction slowed. As a result, the scram actually increased the reaction rate in the lower half of the core. At this point a massive power spike occurred, and the core overheated. Some of the fuel rods fractured, blocking the control rod columns and causing the control rods to become stuck after being inserted only one-third of the way. 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. According to some estimations, the reactor jumped to around 30 GW thermal, ten times the normal operational output, but not immediately. First a great rise in power caused an increase in fuel temperature and massive steam buildup with rapid increase in steam pressure. This destroyed fuel elements and ruptured the channels in which these elements were located.

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. There is a general understanding that it was steam from the wrecked channels entering the reactor inner structure that caused the destruction of the reactor casing, tearing off and lifting by force the 2,000 ton upper plate (to which the entire reactor assembly is fastened). Apparently this was the first explosion that many heard. This was a steam explosion like the explosion of a steam boiler from the excess pressure of vapor. This ruptured further fuel channels—as a result the remaining coolant flashed to steam and escaped the reactor core. The total water loss combined with a high positive void coefficient to increase the reactor power. A second, more powerful explosion occurred about two or three seconds after the first; evidence indicates that the second explosion resulted from a nuclear excursion.

There were initially several hypotheses about the nature of the second explosion. One view was that "the second explosion was caused by the hydrogen which had been produced either by the overheated steam-zirconium reaction or by the reaction of red-hot graphite with steam that produce hydrogen and carbon monoxide." According to another hypothesis, this explosion was of a nuclear nature, i.e., the 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. A third hypothesis was that the explosion was caused, exceptionally, by steam. According to this version, the flow of steam and the steam pressure caused all the destruction following the ejection from the shaft of a substantial part of the graphite and fuel.

According to observers outside Unit 4, burning lumps of material and sparks shot into the air above the reactor. Some of them fell onto the roof of the machine hall and started a fire. About 25 per cent 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.

However, the ratio of xenon radioisotopes released during the event provides compelling evidence that the second explosion was a nuclear power transient. This nuclear transient released ~0.01 kiloton of TNT equivalent (40 GJ) of energy; the analysis indicates that the nuclear excursion was limited to a small portion of the core.

The nuclear excursion dispersed the core and effectively terminated that phase of the event. However, the graphite fire continued, greatly contributing to the spread of radioactive material and the contamination of outlying areas.

Contrary to safety regulations, a combustible material (bitumen) 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 (still operating) adjacent reactor 3. It was imperative to put those fires out and protect the cooling systems of reactor 3. Inside reactor 3, the chief of the night shift, Yuri Bagdasarov, wanted to shut down the reactor immediately, but chief engineer Nikolai Fomin would not allow this. The operators were given respirators and potassium iodide tablets and told to continue working. At 05:00, however, Bagdasarov made his own decision to shut down the reactor, leaving only those operators there who had to work the emergency cooling systems.

Radiation levels

Approximate radiation levels at different locations shortly after the explosion:

location	radiation (roentgens per hour)
vicinity of the reactor core	30,000
fuel fragments	15,000–20,000
debris heap at the place of circulation pumps	10,000
debris near the electrolyzers	5,000–15,000
water in the Level +25 feedwater room	5,000
level 0 of the turbine hall	500–15,000
area of the affected unit	1,000–1,500
water in Room 712	1,000
control room, shortly after explosion	3–5
Gidroelektromontazh depot	30
nearby concrete mixing unit	10–15

Plant layout

based on the image of the plant

level	objects
49.6	roof of the reactor building, gallery of the refueling mechanism
39.9	roof of the deaerator gallery
35.5	floor of the main reactor hall
31.6	upper side of the upper biological shield, floor of the space for pipes to steam separators
28.3	lower side of the turbine hall roof
24.0	deaerator floor, measurement and control instruments room
16.4	floor of the pipe aisle in the deaerator gallery
12.0	main floor of the turbine hall, floor of the main circulation pump motor compartments
10.0	control room, floor under the reactor lower biological shield, main circulation pumps
6.0	steam distribution corridor
2.2	upper pressure suppression pool
0.0	ground level; house switchgear, turbine hall level
-0.5	lower pressure suppression pool
-5.2, -4.2	other turbine hall levels
-6.5	basement floor of the turbine hall

Individual involvement

Khodemchuk memorial, located outside the control room 4 where the observation gallery was

File:Kiev-UkrainianNationalChernobylMuseum 08.jpg

Model of the reactor and turbine hall

Most of the information here is based on Grigoriy Medvedev's work

In the night from 25 to 26 April, in the two power plant complexes there were 160 people on duty, including technicians and maintenance personnel of the various departments. Three hundred more workers were present at the building site of the third complex of the blocks 5 and 6.

Nikolai Gorbachenko, a radiation monitoring technician, at the beginning of his shift checked Unit 3; he skipped the check of Unit 4 as it was being shut down, so at the moment of the accident he was located in the duty room. A flat and powerful thud shook the building; he and his assistant Pshenichnikov thought it was a water hammer occurring during a turbine shutdown. Another flat thud followed, accompanied by lights going out, the control panel of Unit 4 losing signal, latched double doors being blown apart by the blast, and black and red powder falling from the ventilation vent; emergency lights then switched on. Telephone connection with Unit 4 was cut. The corridor to the deaerator galleries was full of steam and white dust. The radiation counters went off-scale, and the high-range one burned out when switched on; the portable instruments were capable of showing at most 4 roentgens per hour (36 nA/kg), while the radiation on the roof ranged between 2,000 and 15,000 roentgens per hour (18 and 130 µA/kg). He went to the turbine hall to survey the damage, saw scattered pieces of concrete, and returned to the duty room. Meeting two men there, together with them he went to search for Shashenok, found him unconscious in a damaged instrument room, and carried him down. Gorbachenko returned to his post and changed clothes and shoes. He was then ordered to look for Khodemchuk, but the search was unsuccessful. He went to the control room and with Dyatlov went outside to survey the reactor building. At 5 am he began feeling weak and vomiting and was transported to hospital, from where he was released on October 27.

The night shift main circulating pump operator, Valery Khodemchuk, was likely killed immediately; he was located in the collapsed part of the building, in the far end of the southern main circulating pumps engine room at level +10. His body was never recovered and is entombed in the reactor debris.

Vladimir Sashenok, the automatic systems adjuster from Atomenergonaladka (the Chernobyl startup and adjustment enterprise), was supposed to be in Room 604, the location of the measurement and control instruments, on the upper landing across the turbine room, on level +24, under the reactor feedwater unit; he was reporting the states of the pressure gauges of the profile of the multiple forced circulation circuit to the computer room by telephone. The communication lines were cut during the explosion. Shashenok received deep thermal and radiation burns over his entire body when the overpressure spike destroyed the isolation membranes and the impulse pipes of the manometers in his instrument room just before the explosion, which then demolished the room itself. The landing was found damaged, covered with ankle-deep water, and there were leaks of boiling water and radioactive steam. Sashenok was found unconscious in Room 604, pinned under a fallen beam, with bloody foam coming out of his mouth. His body was severely contaminated by radioactive water. He was carried out by Gorbachenko and Palamarchuk and died at 6 am in the Pripyat hospital under care of the chief physician, Leonenko Vitaliy, without regaining consciousness. Gorbachenko suffered a radiation burn on his back where Sashenok's hand was located when he helped carry him out. Khodemchuk and Sashenok were the first two victims of the disaster.

Oleg Genrikh, an operator of the control room on Level +36, was taking a nap in a windowless room adjacent to the control room. The window in the control room was broken and the lights went out. His colleague, Anatoly Kurguz, was in the control room with three open doors between him and the reactor room; at the moment of the explosion, he suffered severe burns from steam entering the control room. Genrikh received less serious burns as he was protected by the windowless room. The stairs on the right side were damaged; he managed to escape by the stairs on the left. On the way back they were joined by Simeonov and Simonenko, the gas loop operators, all four heading to the control room. Kurguz was shortly afterwards evacuated by an ambulance; aware of dangers of radiation contamination, Genrikh took a shower and changed his clothes.

Alexander Yuvchenko was located in his office between reactors 3 and 4, on Level 12.5; he described the event as a shock wave that buckled walls, blew doors in, and brought a cloud of milky grey radioactive dust and steam. The lights went out. He met a badly burned, drenched and shocked pump operator, who asked him to rescue Khodemchuk; that quickly proved impossible as that part of the building did not exist anymore. Yuvchenko, together with the foreman Yuri Tregub, ran out of the building and saw half of the building gone and the reactor emitting a blue glow of ionized air. They returned to the building and met Valeri Perevozchenko and two junior technicians, Kudryavtsev and Proskuryakov, ordered by Dyatlov to manually lower the presumably seized control rods. Tregub went to report the extent of damage to the control room. Despite Yuvchenko's explanation that there were no control rods left, the four climbed a stairwell to level 35 to survey the damage; Yuvchenko held open the massive door into the reactor room and the other three proceeded in to locate the control rod mechanism; after no more than a minute of surveying the reactor debris, enough for all three to sustain fatal doses of radiation, they returned, their skin darkened with "nuclear tan" in reaction to the high radiation dose. The three were the first to die in the Moscow hospital. Yuvchenko meanwhile suffered serious beta burns and gamma burns to his left shoulder, hip and calf as he kept the radioactive-dust-covered door open. It was later estimated he received a dose of 4.1 Sv. At 3 am he began vomiting, at 6 am he could not walk anymore; he later spent a year in the Moscow hospital receiving blood and plasma transfusions and received numerous skin grafts.

Valeriy Ivanovich Perevozchenko, the reactor section foreman, was present on the open platform at level +50 shortly before the explosion. He witnessed the 350 kg blocks atop the fuel channels of the Upper Biological Shield jumping up and down and felt the shock waves through the building structure; the rupture of the pressure channels was in progress. He ran down the spiral staircase to Level +10, through the deaerator gallery and the corridor to the control room, to report his observations, arriving shortly after the explosions, then returned to search for his comrades. He witnessed the destruction of the reactor building from the broken windows of the deaerator gallery. With his face already tanned by the radiation, he went to the dosimetry room and asked Gorbachenko for radiation levels; Gorbachenko left with Palamarchuk to rescue Shashenok while Perevozchenko went through the graphite and fuel containing radioactive rubble on Level 10 to the remains of Room 306 in an unsuccessful attempt to locate Khodemchuk, close to debris emitting over 10,000 roentgens per hour (90 µA/kg). He then went to the control room of Genrikh and Kurguz and found it empty; vomiting and losing consciousness, he returned to the control room to report on the situation.

Vyacheslav Brazhnik, the senior turbine machinist operator, ran into the control room to report fire in the turbine hall. Pyotr Palamarchuk, the Chernobyl enterprise group supervisor, together with Razim Davletbayev, followed him back to the turbine room. They witnessed fires on levels 0 and +12, broken oil and water pipes, roof debris on top of turbine 7, and scattered pieces of reactor graphite and fuel, with the linoleum on the floor burning around them. Palamarchuk unsuccessfully attempted to contact Sashenok in Room 604, then ran around the turbogenerator 8, down to Level 0 and urged the two men from the Kharkov mobile laboratory, assigned to record the turbine 8 vibrations, to leave; they, however, had both already received a lethal radiation dose. Akimov asked Palamarchuk to look for Gorbachenko and then rescue Sashenok as the communication with the dosimetry room was cut. Palamarchuk met Gorbachenko by the staircase on Level +27, then they together found and recovered Shashenok's unconscious body.

Aleksandr Kudryavtsev and Viktor Proskuryakov, the SIUR trainees from other shifts, were present to watch Toptunov and learn. After the explosion they were sent by Akimov to the central hall to turn the handles of the system for manual lowering of the presumably seized control rods. They ran through the deaerator gallery to the right to the VRSO unit elevator, found it destroyed, so climbed up the staircase instead, towards Level 36; they missed Kurguz and Genrikh, who used another stairwell. Level 36 was destroyed, covered with rubble. They went through a narrow corridor towards the central hall, entered the reactor hall, and found it blocked with rubble and fragments; dangling fire hoses were pouring water into the remains of the reactor core, the firemen not there anymore. The Upper Biological Shield was slanted, jammed into the reactor shaft; a blue and red fire raged in the hole. The minute the two stood above the reactor was enough to darken their bodies with "nuclear tan" and give them a fatal radiation dose. They returned back to Level 10 and to the control room, reporting the situation; Dyatlov insisted they were wrong, the explosion had been caused by hydrogen-oxygen mixture in the 110 m³ emergency tank and the reactor itself was intact.

Anatoliy Dyatlov, the deputy chief engineer, supervised the test. At the moment reactor power slipped to 30 MW thermal, he insisted the operators continue the test. He overrode Akimov's and Toptunov's objections, threatening to hand the shift to Tregub (the previous shift operator who had remained on-site), intimidating them into attempting to increase the reactor power. The power stabilized at 200 MWt at around 1:00 a.m. and did not rise further, due to continued xenon poisoning of the core. Later the test was commenced, then the reactor showed power excursion and was scrammed; the control rods seized and the explosion occurred shortly afterwards. Dyatlov ordered reactor cooling with emergency speed, assuming the reactor was intact and the explosion had been caused by hydrogen accumulating in the emergency tank of the safety control system. Other employees went to the control room, reporting damage. Dyatlov went to the backup control room, pressing the AZ-5 button there and disconnecting power to the control rod servodrives; despite seeing the graphite blocks scattered on the ground outside the plant, he still believed the reactor was intact. Kudryavtsev and Proskuryakov returned to report the reactor damage they had seen, but Dyatlov insisted what they had seen was the results of an explosion of the emergency tank, claiming the explosion of the 110 m³ tank at Level +71 was sufficient to destroy the central hall roof. Dyatlov reported his assumptions as reality to Bryukhanov and Fomin, the higher-level managers. In the corridor, he met Genrikh and Kurguz and sent them to the medical station. He ran to the control room of Block 3, ordered Bagdasarov to shut reactor 3 down, then returned to control room 4 and ordered Akimov to call the daytime shift and get people to the affected unit; namely Lelechenko, whose crew had to remove hydrogen from the Generator 8 electrolyzer. At 2:00, Dyatlov ordered Akimov to feed water to the reactor, and together with Gorbachenko went to survey the plant from the outside. Despite seeing the fuel and graphite scattered around, he still believed the reactor was intact. They then returned to the control room. At 5 am, he got sick and together with Gorbachenko went to the medical unit. Fomin replaced him on his post with Sitnikov.

Aleksandr Akimov, the unit shift chief, was in charge of the test itself. He took over the shift at midnight from Tregub, who stayed on-site. The drop in reactor power from 1500 MWt to 30 MWt was disconcerting; he wanted to abort the test. He supported Toptunov's decision to shut down the poisoned reactor, but was overriden by Dyatlov and forced to continue. At 1:23:04 the test began, and the main circulation pumps started cavitating due to the too high temperature of inlet water. The coolant started boiling in the reactor, and the reactor power slowly increased. Toptunov reported a power excursion to Akimov. At 1:23:40 Akimov pressed the AZ-5 button, class-5 emergency. The control rods, according to the synchro indicators, seized at a depth of between 2 and 2.5 meters instead of the entire core depth of 7 meters. Akimov disconnected the clutches of the control rod servos to let the rods descend into the core by their own weight, but the rods did not move. The reactor was now making rumbling noises. Akimov was confused. The reactor control panel indicated no water flow and failure of pumps. The explosion occurred, the air filled with dust, power went out, and only battery-powered emergency lights stayed in operation. Perevozchenko ran into the control room, reporting the collapse of the reactor top. Brazhnik ran in from the turbine hall, reporting fire there. Brazhki, Akimov, Davletbayev, and Palamarchuk ran into the turbine hall, having seen scattered debris and multiple fires on Levels 0 and +12. Akimov called the fire station and the chiefs of electrical and other departments, asking for electrical power for coolant pumps, removal of hydrogen from hydrogen generators, and other emergency procedures to stabilize the plant and contain the damage. Internal telephone lines were disabled; Akimov sent Palamarchuk to contact Gorbachenko. Assuming the reactor was intact, he sent Kudryavtsev and Proskuryakov to the central hall to lower the reactor control rods manually. Kudryavtsev and Proskuryakov returned from the reactor and reported its state to Akimov and Dyatlov. Insisting the reactor was intact, Akimov ordered Stolyarchuk and Busygin to turn on the emergency feedwater pumps. Davletbayev reported loss of electrical power, torn cables, and electric arcs. Akimov sent Metlenko to help in the turbine hall with manual opening of the cooling system valves, which was expected to take at least 4 hours per valve. Perevozchenko returned and reported that the reactor was destroyed, but Akimov insisted it was intact. At 3:30, Telyatnikov contacted Akimov, asking what was happening to his firemen; Akimov sent him a dosimetrist. Akimov, already nauseous, was replaced at 6:00 by the unit chief Vladimir Alekseyevich Babychev, but together with Toptunov stayed in the plant. Believing the water flow to the reactor to be blocked by a closed valve somewhere, they went to the half-destroyed feedwater room on Level +24. Together with Nekhayev, Orlov, and Uskov, they opened the valves on the two feedwater lines, then climbed over to Level +27 and almost knee-deep in a mixture of fuel and water, opened two valves on the 300 line; due to advancing radiation poisoning, they did not have the strength to open the valves on the sides. Akimov and Toptunov spent several hours turning valves; the radioactive water in Room 712 was half submerging the pipeline. Smagin went in to open the third valves, spent 20 minutes in the room, and received 280 rads. Akimov was evacuated to hospital. Until his death, he insisted he had done everything correctly.

Bryukhanov, the plant manager, arrived at 2:30. Akimov reported a serious radiation accident but intact reactor, fires in the process of being extinguished, and a second emergency water pump being readied to cool the reactor. Due to limitations of available instruments, they seriously underestimated the radiation level. At 3 am, Bryukhanov called Maryin, the deputy secretary for the nuclear power industry, reporting Akimov's version of the situation. Maryin sent the message further up the chain of command, to Frolyshev, who then called Dolgikh, who called Gorbachev and other members of the Politburo. At 4 am, Moscow ordered feeding of water to the reactor.

Chief engineer Fomin arrived in the Block 4 control room at 4:30. Akimov reported an intact reactor and explosion of the emergency water feed tank. Fomin ordered continuous feeding of water into the reactor, which was already in progress by emergency pump 2 from the deaerators. Fomin kept pressing the staff to feed water to the reactor and transferred more people to Unit 4 to replace those being disabled by radiation. After Dyatlov left, Fomin ordered Sitnikov, his replacement, to climb to the roof of Unit C and survey the reactor; Sitnikov obeyed and received a fatal radiation dose there; at 10 am he returned and reported to Fomin and Bryukhanov that the reactor was destroyed. The managers refused to believe him and ordered continued feeding of water into the reactor; the water, however, flowed through the severed pipes into the lower levels of the plant, carrying radioactive debris and causing short circuits in the cableways common to all four blocks.

Deaths and survivors

Main article: Deaths due to 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) (1.45 amperes per kilogram), which is equivalent to more than 20,000 roentgens per hour. A lethal dose is around 500 roentgens (0.13 coulombs per kilogram) over 5 hours, so in some areas, unprotected workers received fatal doses within several minutes. However, a dosimeter capable of measuring up to 1,000 R/s (0.3 A/kg) was inaccessible because of the explosion, and another one failed when turned on. All remaining dosimeters had limits of 0.001 R/s (0.3 µA/kg) and therefore read "off scale." Thus, the reactor crew could ascertain only that the radiation levels were somewhere above 0.001 R/s (3.6 R/h, or 0.3 µA/kg), while the true levels were much higher in some areas.

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 4:30 a.m. were dismissed under the assumption that the new dosimeter must have been defective. Akimov stayed with his crew in the reactor building until morning, trying to pump water into the reactor. None of them wore any protective gear. Most of them, including Akimov, died from radiation exposure within three weeks.

Fire containment

Firefighter Leonid Telyatnikov, being decorated for bravery.

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."

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

We arrived there at 10 or 15 minutes to two in the morning ... We saw graphite scattered about. Misha asked: "What is graphite?" I kicked it away. But one of the fighters on the other truck picked it up. "It's hot," he said. The pieces of graphite were of different sizes, some big, some small enough to pick up ...
We didn't know much about radiation. Even those who worked there had no idea. There was no water left in the trucks. Misha filled the cistern and we aimed the water at the top. Then those boys who died went up to the roof—Vashchik Kolya and others, and Volodya Pravik ... They went up the ladder ... and I never saw them again.

However, Anatoli Zakharov, a fireman stationed in Chernobyl since 1980, offers a different description:

I remember joking to the others, "There must be an incredible amount of radiation here. We'll be lucky if we're all still alive in the morning."

20 years after the disaster, he claimed the firefighters from the Fire Station No. 2 were aware of the risks.

Of course we knew! If we'd followed regulations, we would never have gone near the reactor. But it was a moral obligation—our duty. We were like kamikaze.

The immediate priority was to extinguish fires on the roof of the station and the area around the building containing Reactor No. 4 to protect No. 3 and keep its core cooling systems intact. The fires were extinguished by 5 a.m., but many firefighters received high doses of radiation. The fire inside Reactor No. 4 continued to burn until 10 May 1986; it is possible that well over half of the graphite burned out. The fire was extinguished by a combined effort of helicopters dropping over 5,000 metric tons of materials like sand, lead, clay, and boron onto the burning reactor and injection of liquid nitrogen. Ukrainian filmmaker Vladimir Shevchenko captured film footage of a Mi-8 helicopter as it collided with a nearby construction crane, causing the helicopter to fall near the damaged reactor building and kill its four-man crew.

From eyewitness accounts of the firefighters involved before they died (as reported on the CBC television series Witness), one described his experience of the radiation as "tasting like metal," and feeling a sensation similar to that of pins and needles all over his face. (This is similar to the description given by Louis Slotin, a Manhattan Project physicist who died days after a fatal radiation overdose from a criticality accident.)

The explosion and fire threw hot particles of the nuclear fuel and also far more dangerous fission products, radioactive elements like caesium-137, iodine-131, strontium-90 and other radionuclides, into the air: the residents of the surrounding area observed the radioactive cloud on the night of the explosion.

Timeline

• 1:26:03 - fire alarm activated
• 1:28 - arrival of local firefighters, Pravik's guard
• 1:35 - arrival of firefighters from Pripyat, Kibenok's guard
• 1:40 - arrival of Telyatnikov
• 2:10 - turbine hall roof fire extinguished
• 2:30 - main reactor hall roof fires suppressed
• 3:30 - arrival of Kiev firefighters
• 4:50 - fires mostly localized
• 6:35 - all fires extinguished

Evacuation of Pripyat

The nearby city of Pripyat was not immediately evacuated.

File:Evacuation of Pripyat.jpg

Evacuation of Pripyat.

Only after radiation levels set off alarms at the Forsmark Nuclear Power Plant in Sweden did the Soviet Union admit that an accident had occurred, but authorities attempted to conceal the scale of the disaster. To evacuate the city of Pripyat, the following warning message was reported on local radio: "An accident has occurred at the Chernobyl Nuclear Power Plant. One of the atomic reactors has been damaged. Aid will be given to those affected and a committee of government inquiry has been set up." This message gave the false impression that any damage or radiation was localized.

The government committee formed to investigate the accident, led by Valeri Legasov, arrived at Chernobyl in the evening of 26 April. By that time two people were dead and 52 were in the hospital. During the night of 26–27 April—more than 24 hours after the explosion—the committee, faced with ample evidence of extremely high levels of radiation and a number of cases of radiation exposure, had to acknowledge the destruction of the reactor and order the evacuation of Pripyat.

The evacuation began at 2 p.m. on 27 April. To reduce baggage, the residents were told the evacuation would be temporary, lasting approximately three days. As a result, Pripyat still contains personal belongings. An exclusion zone of 30 km (19 mi) remains in place today.

Steam explosion risk

File:Pictureofchernobyllavaflow.jpg

Chernobyl Corium lava flows formed by fuel-containing mass in the basement of the plant. Lava flow (1). Concrete (2). Steam pipe (3). Electrical equipment (4).

Two floors of bubbler pools beneath the reactor served as a large water reservoir from the emergency cooling pumps and as a pressure suppression system capable of condensing steam from a (small) broken steam pipe; the third floor above them, below the reactor, served as a steam tunnel. The steam released from a broken pipe was supposed to enter the steam tunnel and be led into the pools to bubble through a layer of water. The pools and the basement were flooded because of ruptured cooling water pipes and accumulated fire water. They now constituted a serious steam explosion risk. The smoldering graphite, fuel and other material above, at more than 1200 °C, started to burn through the reactor floor and mixed with molten concrete that had lined the reactor, creating corium, a radioactive semi-liquid material comparable to lava. If this mixture had melted through the floor into the pool of water, it would have created a massive steam explosion that would have ejected more radioactive material from the reactor. It became an immediate priority to drain the pool.

The bubbler pool could be drained by opening its sluice gates. Volunteers in diving suits entered the radioactive water and managed to open the gates. These were engineers Alexei Ananenko (who knew where the valves were) and Valeri Bezpalov, accompanied by a third man, Boris Baranov, who provided them with light from a lamp, though this lamp failed, leaving them to find the valves by feeling their way along a pipe. All of them returned to the surface and according to Ananenko, their colleagues jumped for joy when they heard they had managed to open the valves. Despite their good condition after completion of the task, all of them suffered from radiation sickness, and at least two—Ananenko and Bezpalov—later died. Some sources claim incorrectly that they died in the plant. It is likely that intense alpha radiation hydrolyzed the water, generating a low-pH hydrogen peroxide (H₂O₂) solution akin to an oxidizing acid. Conversion of bubbler pool water to H₂O₂ is confirmed by the presence in the Chernobyl lavas of studtite and metastudtite, the only minerals that contain peroxide.

Fire brigade pumps were then used to drain the basement. The operation was not completed until 8 May, after 20,000 metric tons of highly radioactive water were pumped out.

With the bubbler pool gone, a meltdown was less likely to produce a powerful steam explosion. To do so, the molten core would now have to reach the water table below the reactor. To reduce the likelihood of this, it was decided to freeze the earth beneath the reactor, which would also stabilize the foundations. Using oil drilling equipment, injection of liquid nitrogen began on 4 May. It was estimated that 25 metric tons of liquid nitrogen per day would be required to keep the soil frozen at −100 °C. This idea was soon scrapped and the bottom room where the cooling system would have been installed was filled with cement.

Debris removal

The worst of the radioactive debris was collected inside what was left of the reactor, much of it shoveled in by liquidators wearing heavy protective gear (dubbed "bio-robots" by the military); these workers could only spend a maximum of 40 seconds at a time working on the rooftops of the surrounding buildings because of the extremely high doses of radiation given off by the blocks of graphite and other debris. The reactor itself was covered with bags containing sand, lead, and boric acid dropped from helicopters (some 5,000 metric tons during the week following the accident). By December 1986 a large concrete sarcophagus had been erected, to seal off the reactor and its contents.

Many of the vehicles used by the "liquidators" remain parked in a field in the Chernobyl area to this day, most giving off doses of 10–30 R/h (0.7–2 A/kg) over 20 years after the disaster.

Causes

There were two official explanations of the accident: the first, subsequently acknowledged as erroneous, was published in August 1986 and effectively placed the blame on the power plant operators. To investigate the causes of the accident the IAEA created a group known as the International Nuclear Safety Advisory Group (INSAG), which in its report of 1986, INSAG-1, on the whole also supported this view, based on the data provided by the Soviets and the oral statements of specialists. In this view, the catastrophic accident was caused by gross violations of operating rules and regulations. "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." This was probably due to their lack of knowledge of nuclear reactor physics and engineering, as well as lack of experience and training. According to these allegations, at the time of the accident the reactor was being operated with many key safety systems shut off, most notably the Emergency Core Cooling System (ECCS). Personnel had an insufficiently detailed understanding of technical procedures involved with the nuclear reactor, and knowingly ignored regulations to speed test completion.

The developers of the reactor plant 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.

In this analysis of the causes of the accident, deficiencies in the reactor design and in the operating regulations that made the accident possible were set aside and mentioned only casually. Serious critical observations covered only general questions and did not address the specific reasons for the accident. The following general picture arose from these observations. Several procedural irregularities also helped to make the accident possible. One was insufficient communication between the safety officers and the operators in charge of the experiment being run that night. The reactor operators disabled safety systems down to the generators, which the test was really about. 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 reactor 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.

This view is reflected in numerous publications and also artistic works on the theme of the Chernobyl accident that appeared immediately after the accident, and for a long time remained dominant in the public consciousness and in popular publications.

However, in 1993 the IAEA Nuclear Safety Advisory Group (INSAG) published an additional report, INSAG-7, which reviewed "that part of the INSAG-1 report in which primary attention is given to the reasons for the accident." In this later report, most of the accusations against staff for breach of regulations were acknowledged to be erroneous, based on incorrect information obtained in August 1986. This report reflected another view of the reasons for the accident, presented in appendix I.
According to this account, turning off the ECCS, and also 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, though they were possibly a breach of regulations. Turning off the emergency system designed to protect against the stopping of the two turbine generators was not a breach of regulations.

Human factors contributed to the conditions that led to the disaster, namely working with a small operational reactivity margin (ORM) and working at a low level of power in the reactor, less than 700 MW—the level documented in the run-down test program. Nevertheless, working at this low level of power was not forbidden in the regulations, despite what Soviet experts asserted in 1986.
Regulations forbade work with a small margin of reactivity. However, "... 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.", (see also ).

According to this report, the chief reasons for the accident lie in the peculiarities of physics and in the construction of the reactor. There are two such reasons:

• The reactor had a dangerously large positive void coefficient. 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. they attempt to decrease heat output when the vapor phase in the reactor increases, because if the coolant contains steam bubbles, fewer neutrons are slowed down. Faster neutrons are less likely to split uranium atoms, so the reactor produces less power (a negative feed-back). Chernobyl's RBMK reactor, however, used solid graphite as a neutron moderator to slow down the neutrons, and the water in it, on the contrary, acts like a harmful neutron absorber. Thus neutrons are slowed down even if steam bubbles form in the water. Furthermore, because steam absorbs neutrons much less readily than water, increasing the intensity of vaporization means that more neutrons are able to split uranium atoms, increasing the reactor's power output. This makes the RBMK design very unstable at low power levels, and prone to suddenly increasing energy production to a dangerous level. This behavior is counter-intuitive, and this property of the reactor was unknown to the crew.
• A more significant flaw was in the design of the control rods that are inserted into the reactor to slow down the reaction. In the RBMK reactor design, the lower part of each control rod was made of graphite and was 1.3 meters shorter than necessary, and in the space beneath the rods were hollow channels filled with water. The upper part of the rod—the truly functional part that absorbs the neutrons and thereby halts the reaction—was made of boron carbide. With this design, when the rods are inserted into the reactor from the uppermost position, the graphite parts initially displace some coolant. This greatly increases the rate of the fission reaction, since graphite (in the RBMK) is a more potent neutron moderator (absorbs far fewer neutrons than the boiling light water). Thus for the first few seconds of control rod activation, reactor power output is increased, rather than reduced as desired. This behavior is counter-intuitive and was not known to the reactor operators.
• Other deficiencies besides these were noted in the RBMK-1000 reactor design, as were its non-compliance with accepted standards and with the requirements of nuclear reactor safety.

Both views were heavily lobbied by different groups, including the reactor's designers, power plant personnel, and the Soviet and Ukrainian governments. According to the IAEA's 1986 analysis, the main cause of the accident was the operators' actions. But according to the IAEA's 1993 revised analysis the main cause was the reactor's design. One reason there were such contradictory viewpoints and so much debate about the causes of the Chernobyl accident was that the primary data covering the disaster, as registered by the instruments and sensors, were not completely published in the official sources.

Once again, the human factor had to be considered as a major element in causing the accident. INSAG notes that both the operating regulations and staff handled the disabling of the reactor protection easily enough: witness the length of time for which the ECCS was out of service while the reactor was operated at half power. INSAG’s view is that it was the operating crew's deviation from the test program that was mostly to blame. “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.”

As in the previously released report INSAG-1, close attention is paid in report INSAG-7 to the inadequate (at the moment of the accident) “culture of safety” at all levels. Deficiency in the safety culture was inherent not only at the operational stage but also, and to no lesser extent, during activities at other stages in the lifetime of nuclear power plants (including design, engineering, construction, manufacture and regulation). 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.”

Effects

Main article: Chernobyl disaster effects

International spread of radioactivity

Four hundred times more radioactive material was released than had been by the atomic bombing of Hiroshima. The fallout was detected over all of Europe except for the Iberian Peninsula.

The initial evidence that a major release of radioactive material was affecting other countries came not from Soviet sources, but from Sweden, where on the morning of April 28 workers at the Forsmark Nuclear Power Plant (approximately 1,100 km (680 mi) from the Chernobyl site) were found to have radioactive particles on their clothes. It was Sweden's search for the source of radioactivity, after they had determined there was no leak at the Swedish plant, that at noon on April 28 led to the first hint of a serious nuclear problem in the western Soviet Union. Hence the evacuation of Pripyat on April 27, 36 hours after the initial explosions, was silently completed before the disaster became known outside the Soviet Union. The rise in radiation levels had at that time already been measured in Finland, but a civil service strike delayed the response and publication.

Contamination from the Chernobyl accident was scattered irregularly depending on weather conditions. Reports from Soviet and Western scientists indicate that Belarus received about 60% of the contamination that fell on the former Soviet Union. However, the 2006 TORCH report stated that half of the volatile particles had landed outside Ukraine, Belarus, and Russia. A large area in Russia south of Bryansk was also contaminated, as were parts of northwestern Ukraine. Studies in surrounding countries indicate that over one million people could have been affected by radiation.

Recently published data from a long-term monitoring program (The Korma-Report) show a decrease in internal radiation exposure of the inhabitants of a region in Belarus close to Gomel. Resettlement may even be possible in prohibited areas provided that people comply with appropriate dietary rules.

In Western Europe, precautionary measures taken in response to the radiation included seemingly arbitrary regulations banning the importation of certain foods but not others. In France some officials stated that the Chernobyl accident had no adverse effects.

Radioactive release (source term)

The external gamma dose for a person in the open near the Chernobyl site.

Contributions of the various isotopes to the (atmospheric) dose in the contaminated area soon after the accident.

Like many other releases of radioactivity into the environment, the Chernobyl release was controlled by the physical and chemical properties of the radioactive elements in the core. While the general population often perceives plutonium as a particularly dangerous nuclear fuel, its effects are almost eclipsed by those of its fission products. Particularly dangerous are highly radioactive compounds that accumulate in the food chain, such as some isotopes of iodine and strontium.

Two reports on the release of radioisotopes from the site were made available, one by the OSTI and a more detailed report by the OECD, both in 1998. At different times after the accident, different isotopes were responsible for the majority of the external dose. The dose that was calculated is that received from external gamma irradiation for a person standing in the open. The dose to a person in a shelter or the internal dose is harder to estimate.

The release of radioisotopes from the nuclear fuel was largely controlled by their boiling points, and the majority of the radioactivity present in the core was retained in the reactor.

• All of the noble gases, including krypton and xenon, contained within the reactor were released immediately into the atmosphere by the first steam explosion.
• About 55% of the radioactive iodine in the reactor was released, as a mixture of vapor, solid particles, and organic iodine compounds.
• Caesium and tellurium were released in aerosol form.

Two sizes of particles were released: small particles of 0.3 to 1.5 micrometers (aerodynamic diameter) and large particles of 10 micrometers. The large particles contained about 80% to 90% of the released nonvolatile radioisotopes zirconium-95, niobium-95, lanthanum-140, cerium-144 and the transuranic elements, including neptunium, plutonium and the minor actinides, embedded in a uranium oxide matrix.

Health of plant workers and local people

In the aftermath of the accident, 237 people suffered from acute radiation sickness, of whom 31 died within the first three months. Most of these were fire and rescue workers trying to bring the accident under control, who were not fully aware of how dangerous exposure to the radiation in the smoke was. Some 135,000 people were evacuated from the area, including 50,000 from Pripyat.

Residual radioactivity in the environment

Rivers, lakes and reservoirs

Earth Observing-1 image of the reactor and surrounding area in April of 2009.

The Chernobyl nuclear power plant is located next to the Pripyat River, which feeds into the Dnipro River reservoir system, one of the largest surface water systems in Europe. The radioactive contamination of aquatic systems therefore became a major issue in the immediate aftermath of the accident. In the most affected areas of Ukraine, levels of radioactivity (particularly radioiodine: I-131, radiocaesium: Cs-137 and radiostrontium: Sr-90) in drinking water caused concern during the weeks and months after the accident. After this initial period, however, radioactivity in rivers and reservoirs was generally below guideline limits for safe drinking water.

Bio-accumulation of radioactivity in fish resulted in concentrations (both in western Europe and in the former Soviet Union) that in many cases were significantly above guideline maximum levels for consumption. Guideline maximum levels for radiocaesium in fish vary from country to country but are approximately 1,000 Bq/kg in the European Union. In the Kiev Reservoir in Ukraine, concentrations in fish were several thousand Bq/kg during the years after the accident. In small "closed" lakes in Belarus and the Bryansk region of Russia, concentrations in a number of fish species varied from 0.1 to 60 kBq/kg during the period 1990–92. The contamination of fish caused short-term concern in parts of the UK and Germany and in the long term (years rather than months) in the affected areas of Ukraine, Belarus, and Russia as well as in parts of Scandinavia.

Groundwater

Map of radiation levels in 1996 around Chernobyl.

Groundwater was not badly affected by the Chernobyl accident since radionuclides with short half-lives decayed away long before they could affect groundwater supplies, and longer-lived radionuclides such as radiocaesium and radiostrontium were adsorbed to surface soils before they could transfer to groundwater. However, significant transfers of radionuclides to groundwater have occurred from waste disposal sites in the 30 km (19 mi) exclusion zone around Chernobyl. Although there is a potential for transfer of radionuclides from these disposal sites off-site (i.e. out of the 30 km (19 mi) exclusion zone), the IAEA Chernobyl Report argues that this is not significant in comparison to current levels of washout of surface-deposited radioactivity.

Flora and fauna

After the disaster, four square kilometers of pine forest in the immediate vicinity of the reactor turned reddish-brown and died, earning the name of the "Red Forest". Some animals in the worst-hit areas also died or stopped reproducing. Most domestic animals were evacuated from the exclusion zone, but horses left on an island in the Pripyat River 6 km (4 mi) from the power plant died when their thyroid glands were destroyed by radiation doses of 150–200 Sv. Some cattle on the same island died and those that survived were stunted because of thyroid damage. The next generation appeared to be normal.

A robot sent into the reactor itself has returned with samples of black, melanin-rich radiotrophic fungi that are growing on the reactor's walls.

Chernobyl after the disaster

The Pripyat Ferris wheel as seen from inside the town's Palace of Culture.

Following the accident, questions arose about the future of the plant and its eventual fate. All work on the unfinished reactors 5 and 6 was halted three years later. However, the trouble at the Chernobyl plant did not end with the disaster in reactor 4. The damaged reactor was sealed off and 200 meters (660 ft) of concrete was placed between the disaster site and the operational buildings. The Ukrainian government continued to let the three remaining reactors operate because of an energy shortage in the country. In 1991, a fire broke out in the turbine building of reactor 2; the authorities subsequently declared the reactor damaged beyond repair and had it taken offline. Reactor 1 was decommissioned in November 1996 as part of a deal between the Ukrainian government and international organizations such as the IAEA to end operations at the plant. On 15 December 2000, then-President Leonid Kuchma personally turned off Reactor 3 in an official ceremony, shutting down the entire site.

Chernobyl's Exclusion Zone

In his book Disasters: Wasted Lives, Valuable Lessons, economist and crisis consultant Randall Bell writes after his research at Chernobyl:

There is a 17-mile (sic) Exclusion Zone around Chernobyl where officially nobody is allowed to live, but people do. These "resettlers" are elderly people who lived in the region prior to the disaster. Today there are approximately 10,000 people between the ages of 60 and 90 living within the Zone around Chernobyl. Younger families are allowed to visit, but only for brief periods of time.

Eventually the land could be utilized for some sort of industrial purpose that would involve concrete sites. But estimates range from 60 – 200 years before this would be allowed. Farming or any other type of agricultural industry would be dangerous and completely inappropriate for at least 200 years. It will be at least two centuries before there is any chance the situation can change within the 1.5-mile Exclusion Zone. As for the #4 reactor where the meltdown occurred, we estimate it will be 20,000 years before the real estate will be fully safe.

Controversy over "Wildlife Haven" claim

The Exclusion Zone around the Chernobyl nuclear power station is reportedly a haven for wildlife. As humans were evacuated from the area 24 years ago, existing animal populations multiplied and rare species not seen for centuries have returned or have been reintroduced, for example lynx, wild boar, wolf, Eurasian brown bear, European bison, Przewalski's horse, and eagle owl. Birds even nest inside the cracked concrete sarcophagus shielding in the shattered remains of reactor number 4. The Exclusion Zone is so lush with wildlife and greenery that in 2007 the Ukrainian government designated it a wildlife sanctuary, "Chernobyl Special"; and at 488.7 km² it is one of the largest wildlife sanctuaries in Europe.

According to a 2005 U.N. report, wildlife has returned despite radiation levels that are presently 10 to 100 times higher than normal background radiation. Although they were significantly higher soon after the accident, the levels have fallen because of radioactive decay.

Some researchers claim that by halting the destruction of habitat, the Chernobyl disaster helped wildlife flourish. Biologist Robert J. Baker of Texas Tech University was one of the first to report that Chernobyl had become a wildlife haven and that many rodents he has studied at Chernobyl since the early 1990s have shown remarkable tolerance for elevated radiation levels.

However Møller et al. (2005) suggested that reproductive success and annual survival rates of barn swallows are much lower in the Chernobyl exclusion zone; 28% of barn swallows inhabiting Chernobyl return each year, while at a control area at Kanev 250 km to the southeast, the return rate is around 40%. A later study by Møller et al. (2007) furthermore claimed an elevated frequency of 11 categories of subtle physical abnormalities in barn swallows, such as bent tail feathers, deformed air sacs, deformed beaks, and isolated albinistic feathers.

However, another researcher criticized these findings and instead proposed that a lack of human influence in the exclusion zone locally reduced the swallows' insect prey and that radiation levels across the vast majority of the exclusion zone are now too low to have an observable negative effect. But the criticisms raised were responded to in Møller et al. (2008). It is possible that barn swallows are particularly vulnerable to elevated levels of ionizing radiation because they are migratory; they arrive in the exclusion area exhausted and with depleted reserves of radio-protective antioxidants after an arduous journey.

Several research groups have suggested that plants in the area have adapted to cope with the high radiation levels, for example by increasing the activity of DNA cellular repair machinery and by hypermethylation. (see Radiation Hormesis). Given the uncertainties, further research is needed to assess the long-term health effects of elevated ionizing radiation from Chernobyl on flora and fauna.

Chernobyl today

File:Chernobyl Monument Rivne-commons.jpg

Monument in Rivne, Ukraine.

The Chernobyl reactor is now enclosed in a large concrete sarcophagus, which was built quickly to allow continuing operation of the other reactors at the plant.

A New Safe Confinement was to have been built by the end of 2005; however, it has suffered ongoing delays and is currently expected to be completed in 2012. The structure will be built adjacent to the existing shelter and then will be slid into place on rails. It is to be a metal arch 105 meters (344.4 feet) high and spanning 257 meters (842.9 feet), to cover both unit 4 and the hastily built 1986 structure. The Chernobyl Shelter Fund, set up in 1997, has received €810 million from international donors and projects to cover this project and previous work. It and the Nuclear Safety Account, also applied to Chernobyl decommissioning, are managed by the European Bank for Reconstruction and Development (EBRD).

As of 2006, some fuel remained in the reactors at units 1 through 3, most of it in each unit's cooling pond, as well as some material in a small spent fuel interim storage facility pond (ISF-1).

In 1999 a contract was signed for construction of a radioactive waste management facility to store 25,000 used fuel assemblies from units 1–3 and other operational wastes, as well as material from decommissioning units 1–3 (which will be the first RBMK units decommissioned anywhere). The contract included a processing facility able to cut the RBMK fuel assemblies and to put the material in canisters, which were to be filled with inert gas and welded shut. The canisters were to be transported to dry storage vaults, where the fuel containers would be enclosed for up to 100 years. This facility, treating 2500 fuel assemblies per year, would be the first of its kind for RBMK fuel. However, after a significant part of the storage structures had been built, technical deficiencies in the concept emerged, and the contract was terminated in 2007. The interim spent fuel storage facility (ISF-2) will now be completed by others by mid-2013.

Another contract has been let for a liquid radioactive waste treatment plant, to handle some 35,000 cubic meters of low- and intermediate-level liquid wastes at the site. This will need to be solidified and eventually buried along with solid wastes on site.

In January 2008, the Ukrainian government announced a 4-stage decommissioning plan that incorporates the above waste activities and progresses towards a cleared site.

Lava-like fuel-containing materials (FCMs)

The radioactivity levels of different isotopes in the FCM, as back-calculated by Russian workers to April 1986.

Main article: corium (nuclear reactor)

According to official estimates, about 95% of the fuel in the reactor at the time of the accident (about 180 metric tons) remains inside the shelter, with a total radioactivity of nearly 18 million curies (670 PBq). The radioactive material consists of core fragments, dust, and lava-like "fuel-containing materials" (FCM, also called "corium") that flowed through the wrecked reactor building before hardening into a ceramic form.

Three different lavas are present in the basement of the reactor building: black, brown, and a porous ceramic. They are silicate glasses with inclusions of other materials within them. The porous lava is brown lava that dropped into water and thus cooled rapidly.

Degradation of the lava

It is unclear how long the ceramic form will retard the release of radioactivity. From 1997 to 2002 a series of papers were published that suggested that the self-irradiation of the lava would convert all 1,200 metric tons into a submicrometer and mobile powder within a few weeks. But it has been reported that the degradation of the lava is likely to be a slow and gradual process rather than sudden and rapid. The same paper states that the loss of uranium from the wrecked reactor is only 10 kg (22 lb) per year. This low rate of uranium leaching suggests that the lava is resisting its environment. The paper also states that when the shelter is improved, the leaching rate of the lava will decrease.

Some of the surfaces of the lava flows have started to show new uranium minerals such as Na
₄(UO
₂)(CO
₃)
₃ and uranyl carbonate. However, the level of radioactivity is such that during one hundred years the self irradiation of the lava (2 × 10¹⁶ α decays per gram and 2 to 5 × 10⁵ Gy of β or γ) will fall short of the level of self irradiation required to greatly change the properties of glass (10¹⁸ α decays per gram and 10⁸ to 10⁹ Gy of β or γ). Also the rate of dissolution of the lava in water is very low (10⁻⁷ g-cm⁻² day⁻¹), suggesting that the lava is unlikely to dissolve in water.

Possible consequences of further collapse of the Sarcophagus

The Sarcophagus, the concrete block surrounding reactor #4

The protective box that was placed over the wrecked reactor was named object "Shelter" by the Soviet government, but the media and the public know it as the "sarcophagus."

The present shelter is constructed over the ruins of the reactor building. The two "Mammoth Beams" that support the roof of the shelter rest partly on the structurally unsound west wall of the reactor building that was damaged by the accident. The western end of the shelter roof is supported by a wall at a point designated axis 50. This wall is reinforced concrete, and was cracked by the accident. In December 2006 the "Designed Stabilisation Steel Structure" (DSSS) was extended until 50% of the roof load (about 400 tons) was transferred from the axis 50 wall to the DSSS. The DSSS is a yellow steel object that has been placed next to the wrecked reactor; it is 63-meter (207 ft) tall and has a series of cantilevers that extend through the western buttress wall, and is intended to stabilize the sarcophagus. This was done because if the wall of the reactor building or the roof of the shelter were to collapse, then large amounts of radioactive dust and particles would be released directly into the atmosphere, resulting in a large new release of radioactivity into the environment.

A further threat to the shelter is the concrete slab that formed the "Upper Biological Shield" (UBS), situated above the reactor prior to the accident. This concrete slab was thrown upwards by the explosion in the reactor core and now rests at approximately 15° from vertical. The position of the upper bioshield is considered inherently unsafe, as only debris supports it in its nearly upright position. A collapse of the bioshield would further exacerbate the dust conditions in the shelter, possibly spreading some quantity of radioactive materials out of the shelter, and could damage the shelter itself. The UBS is a circle 15 meters in diameter, weighing 1000 tons and consisting of 2000 cubes, each located above a fuel channel. The shield, called Pyatachok ("five kopek coin") before the disaster, was afterwards named Component "E" and nicknamed "Elena"; the twisted fuel bundles still attached to it are called "Elena's hair."

Grass and forest fires

It is known that fires can make radioactivity mobile again. In particular V.I. Yoschenko et al. reported on the possibility of increased mobility of caesium, strontium, and plutonium due to grass and forest fires. As an experiment, fires were set and the levels of the radioactivity in the air downwind of these fires was measured.

Grass and forest fires have happened inside the contaminated zone, releasing radioactive fallout into the atmosphere. In 1986 a series of fires destroyed 23.36 km² (5,772 acres) of forest, and several other fires have since burned within the 30 km (19 mi) zone. A serious fire in early May 1992 affected 5 km² (1,240 acres) of land including 2.7 km² (670 acres) of forest. This resulted in a great increase in the levels of caesium-137 in airborne dust.

Recovery process

Recovery projects

The Chernobyl Shelter Fund

The Chernobyl Shelter Fund was established in 1997 at the Denver 23rd G8 summit to finance the Shelter Implementation Plan (SIP). The plan calls for transforming the site into an ecologically safe condition by means of stabilization of the Sarcophagus followed by construction of a New Safe Confinement (NSC). While the original cost estimate for the SIP was US$768 million, the 2006 estimate was $1.2 billion. The SIP is being managed by a consortium of Bechtel, Battelle, and Electricité de France, and conceptual design for the NSC consists of a movable arch, constructed away from the shelter to avoid high radiation, to be slid over the sarcophagus. The NSC is expected to be completed in 2012, and will be the largest movable structure ever built.

Dimensions:

• Span: 270 m (886 ft)
• Height: 100 m (330 ft)
• Length: 150 m (492 ft)

The United Nations Development Programme

The United Nations Development Programme has launched in 2003 a specific project called the Chernobyl Recovery and Development Programme (CRDP) for the recovery of the affected areas. The programme was initiated in February 2002 based on the recommendations in the report on Human Consequences of the Chernobyl Nuclear Accident. The main goal of the CRDP’s activities is supporting the Government of Ukraine in mitigating long-term social, economic, and ecological consequences of the Chernobyl catastrophe. CRDP works in the four most Chernobyl-affected areas in Ukraine: Kyivska, Zhytomyrska, Chernihivska and Rivnenska.

The International Project on the Health Effects of the Chernobyl Accident

The International Project on the Health Effects of the Chernobyl Accident (IPEHCA) was created and received US $20 million, mainly from Japan, in hopes of discovering the main cause of health problems due to ¹³¹I radiation. These funds were divided between Ukraine, Belarus, and Russia, the three main affected countries, for further investigation of health effects. As there was significant corruption in former Soviet countries, most of the foreign aid was given to Russia, and no positive outcome from this money has been demonstrated.

Assessing the disaster's effects on human health

Mental Health Effects

The anxiety caused by the accident, which appears to show no sign of diminishing, and its negative impact on the living conditions in the affected areas, may be the principal reason for the increase in poor reported health. Beyond this, it has been argued that a culture has developed in which people perceive themselves to be victims, where there is a lack of trust, where ill-health is expected, where there are general feelings of anxiety, instability, helplessness, and a generalized fear about the future; all of which have resulted in an increased inability to adjust to changed circumstances. The negative impact of the accident has been compounded by a breakdown in the infrastructure of community life, increased poverty, poor diet and generally poor living conditions, which have further deteriorated following the break-up of the USSR in 1991.

Physical Health Effects

• Down's syndrome (trisomy 21). In West Berlin, Germany, prevalence of Down's syndrome (trisomy 21) peaked 9 months following the main fallout.[ 11, 12] Between 1980 and 1986, the birth prevalence of Down's syndrome was quite stable (i.e., 1.35–1.59 per 1,000 live births [27–31 cases]). In 1987, 46 cases were diagnosed (prevalence = 2.11 per 1,000 live births). Most of the excess resulted from a cluster of 12 cases among children born in January 1987. The prevalence of Down's syndrome in 1988 was 1.77, and in 1989, it reached pre-Chernobyl values. The authors noted that the isolated geographical position of West Berlin prior to reunification, the free genetic counseling, and complete coverage of the population through one central cytogenetic laboratory support completeness of case ascertainment; in addition, constant culture preparation and analysis protocols ensure a high quality of data.
• Chromosomal aberrations. Reports of structural chromosome aberrations in people exposed to fallout in Belarus and other parts of the former Soviet Union, Austria, and Germany argue against a simple dose-response relationship between degree of exposure and incidence of aberrations. These findings are relevant because a close relationship exists between chromosome changes and congenital malformations. Inasmuch as some types of aberrations are almost specific for ionizing radiation, researchers use aberrations to assess exposure dose. On the basis of current coefficients, however, one cannot assume that calculation of individual exposure doses resulting from fallout would not induce measurable rates of chromosome aberrations.
• Neural tube defects (NTDs) in Turkey. During the embryonic phase of fetal development, the neural tube differentiates into the brain and spinal cord (i.e., collectively forming the central nervous system). Chemical or physical interactions with this process can cause NTDs. Common features of this class of malformations are more or less extended fissures, often accompanied by consecutive dislocation of central nervous system (CNS) tissue. NTDs include spina bifida occulta and aperta, encephalocele, and—in the extreme case—anencephaly. The first evidence in support of a possible association between CNS malformations and fallout from Chernobyl was published by Akar et al. in 1988. The Mustafakemalpasa State Hospital, Bursa region, covers a population of approximately 90,000. Investigators have documented the prevalence of malformations since 1983. The prevalence of NTDs was 1.7 to 9.2 per 1,000 births, but during the first 6 months of 1987 increased to 20 per 1,000 (12 cases). The excess was most pronounced for the subgroup of anencephalics, in which prevalence increased 5-fold (i.e., 10 per 1,000 [6 cases]). In the consecutive months that followed (i.e., July-December 1987), the prevalence decreased again (1.3 per 1,000 for all NTDs, 0.6 per 1,000 for anencephaly), and it reached pre-Chernobyl levels during the first half of 1988 (all NTDs: 0.6 per 1,000; anencephaly: 0.2 per 1,000). This initial report was supported by several similar findings in observational studies from different regions of Turkey.

Demonstration on Chernobyl day near WHO in Geneva

An international assessment of the health effects of the Chernobyl accident is contained in a series of reports by the United Nations Scientific Committee of the Effects of Atomic Radiation (UNSCEAR). UNSCEAR was set up as a collaboration between various UN bodies, including the World Health Organisation, after the atomic bomb attacks on Hiroshima and Nagasaki, to assess the long-term effects of radiation on human health.

UNSCEAR has conducted 20 years of detailed scientific and epidemiological research on the effects of the Chernobyl accident. Apart from the 57 direct deaths in the accident itself, UNSCEAR originally predicted up to 4,000 additional cancer cases due to the accident. However, the latest UNSCEAR reports suggest that these estimates were overstated. In addition, the IAEA states that there has been no increase in the rate of birth defects or abnormalities, or solid cancers (such as lung cancer) corroborating UNSCEAR's assessments.

Precisely, UNSCEAR states:

Among the residents of Belaruss 09, the Russian Federation and Ukraine there had been, up to 2002, about 4,000 cases of thyroid cancer reported in children and adolescents who were exposed at the time of the accident, and more cases are to be expected during the next decades. Notwithstanding problems associated with screening, many of those cancers were most likely caused by radiation exposures shortly after the accident. Apart from this increase, there is no evidence of a major public health impact attributable to radiation exposure 20 years after the accident. There is no scientific evidence of increases in overall cancer incidence or mortality rates or in rates of non-malignant disorders that could be related to radiation exposure. The risk of leukaemia in the general population, one of the main concerns owing to its short latency time, does not appear to be elevated. Although those most highly exposed individuals are at an increased risk of radiation-associated effects, the great majority of the population is not likely to experience serious health consequences as a result of radiation from the Chernobyl accident. Many other health problems have been noted in the populations that are not related to radiation exposure.

Thyroid cancer is generally treatable. With proper treatment, the five-year survival rate of thyroid cancer is 96%, and 92% after 30 years.

The Chernobyl Forum is a regular meeting of IAEA, other United Nations organizations (FAO, UN-OCHA, UNDP, UNEP, UNSCEAR, WHO, and the World Bank), and the governments of Belarus, Russia, and Ukraine that issues regular scientific assessments of the evidence for health effects of the Chernobyl accident. The Chernobyl Forum concluded that twenty-eight emergency workers died from acute radiation syndrome including beta burns and 15 patients died from thyroid cancer, and it roughly estimated that cancer deaths caused by Chernobyl may reach a total of about 4,000 among the 600,000 people having received the greatest exposures. It also concluded that a greater risk than the long-term effects of radiation exposure is the risk to mental health of exaggerated fears about the effects of radiation:

The designation of the affected population as “victims” rather than “survivors” has led them to perceive themselves as helpless, weak and lacking control over their future. This, in turn, has led either to over cautious behavior and exaggerated health concerns, or to reckless conduct, such as consumption of mushrooms, berries and game from areas still designated as highly contaminated, overuse of alcohol and tobacco, and unprotected promiscuous sexual activity.

Fred Mettler commented that 20 years later:

The population remains largely unsure of what the effects of radiation actually are and retain a sense of foreboding. A number of adolescents and young adults who have been exposed to modest or small amounts of radiation feel that they are somehow fatally flawed and there is no downside to using illicit drugs or having unprotected sex. To reverse such attitudes and behaviors will likely take years although some youth groups have begun programs that have promise.

In addition, disadvantaged children around Chernobyl suffer from health problems that are attributable not only to the Chernobyl accident, but also to the poor state of post-Soviet health systems.

Another study critical of the Chernobyl Forum report was commissioned by Greenpeace, which is well known for its anti-nuclear positions. In its report, Greenpeace alleges that: "the most recently published figures indicate that in Belarus, Russia and Ukraine alone the accident could have resulted in an estimated 200,000 additional deaths in the period between 1990 and 2004." However, the Greenpeace report failed to discriminate between the general increase in cancer rates that followed the dissolution of the USSR's health care system and any separate effects of the Chernobyl accident.

The German affiliate of the International Physicians for the Prevention of Nuclear War (IPPNW) argued that more than 10,000 people are today affected by thyroid cancer and 50,000 cases are expected in the future. According to some commentators, both the Greenpeace and IPPNW reports suffer from a lack of any genuine or original research and failure to understand epidemiologic data.

In popular culture

Main article: Cultural impact of the Chernobyl disaster

See also: Nuclear power debate

The Chernobyl accident attracted a great deal of interest. Because of the distrust that many people (both within and outside the USSR) had in the Soviet authorities, a great deal of debate about the situation at the site occurred in the first world during the early days of the event. Because of defective intelligence based on photographs taken from space, it was thought that unit number three had also suffered a dire accident.

In general the public knew little about radioactivity and radiation and as a result their degree of fear was increased. Journalists mistrusted many professionals (such as the spokesman from the UK NRPB), and in turn encouraged the public to mistrust them.

In Italy, the Chernobyl accident was reflected in the outcome of the 1987 referendum. As a result of that referendum, Italy began phasing out its nuclear power plants in 1988, a decision that was reversed in 2008.

Chernobyl is the backdrop for an Arkady Renko detective novel by Martin Cruz Smith called Wolves Eat Dogs. One section of the video game Call of Duty 4: Modern Warfare takes place in the abandoned city of Prypiat, Ukraine. The video games S.T.A.L.K.E.R.: Shadow of Chernobyl, S.T.A.L.K.E.R.: Call of Pripyat and S.T.A.L.K.E.R.: Clear Sky take place in a fictional alternate reality in which a secondary explosion occurred.

Commemoration of the disaster

The Front Veranda (1986), a lithograph by Susan Dorothea White in the National Gallery of Australia, exemplifies worldwide awareness of the event. Heavy Water: A film for Chernobyl was released by Seventh Art in 2006 to commemorate the disaster through poetry and first-hand accounts. The film secured the Cinequest Award as well as the Rhode Island "best score" award [2] along with a screening at Tate Modern.

Chernobyl 20

This exhibit presents the stories of 20 people who have each been affected by the disaster, and each person's account is written on a panel. The 20 individuals whose stories are related in the exhibition are from Belarus, France, Latvia, Russia, Sweden, Ukraine, and the United Kingdom.

Developed by Danish photo-journalist Mads Eskesen, the exhibition is prepared in multiple languages including German, English, Danish, Dutch, Russian, and Ukrainian.

In Kiev, Ukraine, the exhibition was launched at the "Chernobyl 20 Remembrance for the Future" conference on April 23, 2006. It was then exhibited during 2006 in Australia, Denmark, the Netherlands, Switzerland, Ukraine, the United Kingdom, and the United States.

See also

• Chernobyl compared to other radioactivity releases
• Chernobyl disaster effects
• Chernobyl Shelter Fund
• Liquidator (Chernobyl)
• List of Chernobyl-related articles
• Red Forest
• Threat of the Dnieper reservoirs
• Zone of alienation
• Ukrainian National Chernobyl Museum

Other

• National Geographic Seconds From Disaster episodes
• Zero Hour episode, showing the inside of the power plant with remarkable accuracy
• EnglishRussia photos of a RBMK-based power plant, showing details of the reactor hall, pumps, and the control room

Further reading

• Cheney, Glenn Alan (1995). Journey to Chernobyl: Encounters in a Radioactive Zone. Academy Chicago. ISBN 0-89733-418-3. OCLC 231661295.
• Medvedev, Grigori (1989). The Truth About Chernobyl. VAAP. First American edition published by Basic Books in 1991. ISBN 2-226-04031-5 (Hardcover). {{cite book}}: Check |isbn= value: invalid character (help)
• Medvedev, Zhores A. (1990). The Legacy of Chernobyl. W. W. Norton & Company (paperback 1992). First American edition in 1990. ISBN 978-0393308143 (paperback). {{cite book}}: Check |isbn= value: invalid character (help)
• Shcherbak, Yurii (1991). Chernobyl (in Russian). Sowiet Writers.
• Dyatlov, Anatoly (2003). Chernobyl. How did it happen (in Russian). Nauchtechlitizdat, Moscow. ISBN 5-93728-006-7 (paperback). {{cite book}}: Check |isbn= value: invalid character (help)
• Karpan, Nikolaj V. (2006). Chernobyl. Vengeance of peaceful atom. (in Russian). IKK “Balance Club”, Dnepropetrovsk. ISBN 966-8135-21-0 (paperback). {{cite book}}: Check |isbn= value: invalid character (help)
• Hoffmann, Wolfgang (2001). Fallout From the Chernobyl Nuclear Disaster and Congenital Malformations in Europe. Archives of Environmental Health.
• Abbott, Pamela (2006). Chernobyl: Living With Risk and Uncertaity. Health, Risk & Socitey 8.2. pp. 105–121.
• Cohen, Bernard Leonard (1990). "(7) The Chernobyl accident — can it happen here?". The Nuclear Energy Option: An Alternative for the 90's. Plenum Press. ISBN 978-0306435676. {{cite book}}: Check |authorlink= value (help); External link in |authorlink= (help)

Documents

The source documents, which relate to the emergency, published in the unofficial sources:

• Technological Regulations on operation of 3 and 4 power units of Chernobyl NPP (in force at the moment of emergency)
• Tables and graphs of some parameters variation of the unit before the emergency