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Chernobyl disaster
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The Chernobyl disaster was a nuclear reactor accident in the Chernobyl Nuclear Power Plant in Ukraine, then part of the Soviet Union. It is considered to be the worst nuclear power plant disaster in history and the only level 7 instance on the International Nuclear Event Scale. It resulted in a severe release of radioactivity into the environment following a massive power excursion which destroyed the reactor.
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The Chernobyl disaster was a nuclear reactor accident in the Chernobyl Nuclear Power Plant in Ukraine, then part of the Soviet Union. It is considered to be the worst nuclear power plant disaster in history and the only level 7 instance on the International Nuclear Event Scale. It resulted in a severe release of radioactivity into the environment following a massive power excursion which destroyed the reactor. Two people died in the initial steam explosion, but most deaths from the accident were attributed to radiation.
On 26 April 1986 01:23:45 a.m. (UTC+3) reactor number four at the Chernobyl plant, near Pripyat in the Ukrainian Soviet Socialist Republic, exploded. Further explosions and the resulting fire sent a plume of highly radioactive fallout into the atmosphere and over an extensive geographical area. Four hundred times more fallout was released than had been by the atomic bombing of Hiroshima.
The plume drifted over extensive parts of the western Soviet Union, Eastern Europe, Western Europe, Northern Europe, and eastern North America, with light nuclear rain falling as far as Ireland. Large areas in Ukraine, Belarus, and Russia were badly contaminated, resulting in the evacuation and resettlement of over 336,000 people. According to official post-Soviet data, about 60% of the radioactive fallout landed in Belarus.
The accident raised concerns about the safety of the Soviet nuclear power industry, slowing its expansion for a number of years, while forcing the Soviet government to become less secretive. The now-independent countries of Russia, Ukraine, and Belarus have been burdened with the continuing and substantial decontamination and health care costs of the Chernobyl accident. It is difficult to accurately quantify the number of deaths caused by the events at Chernobyl, as the Soviet-era cover-up made it difficult to track down victims. Lists were incomplete, and Soviet authorities later forbade doctors to cite "radiation" on death certificates.
The overall cost of the disaster is estimated at $200 billion USD, taking inflation into account. This places the Chernobyl disaster as the most costly disaster in modern history.
The 2005 report prepared by the Chernobyl Forum, led by the International Atomic Energy Agency (IAEA) and World Health Organization (WHO), attributed 56 direct deaths (47 accident workers, and nine children with thyroid cancer), and estimated that there may be 4,000 extra cancer deaths 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.
The Chernobyl nuclear power plant
The Chernobyl station is near the town of Pripyat, Ukraine, northwest of the city of Chernobyl, from the border of Ukraine and Belarus, and about north of Kiev. The station consisted of four RBMK-1000 nuclear reactors, each capable of producing 1 gigawatt (GW) of electric power, and the four together produced about 10% of Ukraine's electricity at the time of the accident. Construction of the plant began in the late 1970s, with reactor no. 1 commissioned in 1977, followed by no. 2 (1978), no. 3 (1981), and no. 4 (1983). Two more reactors, no. 5 and 6, also capable of producing 1 GW each, were under construction at the time of the disaster. Chernobyl wanted to build two more reactors for the Chernobyl nuclear power plant.
The accident
On 26 April 1986 at 1:23:45 a.m., reactor 4 suffered a massive, catastrophic power excursion, resulting in a steam explosion, which tore the top from the reactor, exposed the core, and dispersed large amounts of radioactive particulate and gaseous debris (mostly Cesium-137 and Strontium-90), allowing air (oxygen) to contact the super-hot core containing 1,700 tonnes of combustible graphite moderator; the burning graphite moderator increased the emission of radioactive particles. The radioactivity was not contained by any kind of containment vessel (unlike most Western plants, Soviet reactors often did not have them). Radioactive particles were carried by wind across international borders.
Planning the test of the safety device
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 proposed to test a safety emergency core cooling feature during the shut down procedure.
A very large amount of cooling water is needed to maintain a safe temperature in the reactor core. The reactor consisted of about 1,600 individual fuel channels and each operational channel required a flow of 28 tonnes 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 generator required 15 seconds to start up but took 60-75 seconds to attain full speed and reach its capacity of 5.5 MW required to run one main cooling water pump.
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. Because generator voltage varies with its rotational speed, a special device is required to provide stable voltage to the main cooling water pumps as the turbine spins down. This safety device—a voltage regulating system—was to be tested during a simulated power "blackout". In theory, it should have been able to provide power for 45 seconds and thus bridge the power gap between the onset of the external power failure and the full availability of electric power from the emergency diesel generators.
The reactor was designed such that it needed coolant even when not actively operating. 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 were highly radioactive, and continued to produce heat as they decayed. This could amount to 1-2 percent of the normal output of the plant. If not immediately removed by coolant systems, the heat could lead to core damage.
The amount of spent fuel, and thus the amount of decay heat that the cooling system must handle, increased throughout operation and attained its maximum value at the end of the fuel cycle. In the event of core damage, the end of the fuel cycle would present the worst possible point in time, with the maximum accumulated inventory of nuclides to be released into the environment. The experiment would have been far safer to carry out with fresh fuel. This means that the simulated power blackout experiment was performed at the most dangerous point in the reactor cycle.
It was a design requirement that the rotational momentum of the steam turbine, as it spun down, could be used to generate electricity to run the cooling water pumps to bridge the power gap. A previous test had been unsuccessful. Apparently, the test had not been completed successfully by March 1984 when the unit was brought into commercial operation ahead of schedule and celebrated as a "labour victory". Under pressure, the director of the Chernobyl station Viktor Bryukhanov signed an acceptance document on the last day of 1983, in order to declare that works planned for that year had been fulfilled. Had he not done so, thousands of workers, engineers and his own superiors would have lost bonuses, awards and other extras. Records were falsified to hide this fact.
The Chernobyl power plant had been in operation for two years without this important safety feature. The station managers must have wished to correct this at the first opportunity. This could explain why they were so determined to carry out the test, even when serious problems arose, and why the requisite approval for the test was not sought from the Soviet nuclear oversight regulatory body.
For the experiment, the reactor would be set at a low power setting and the steam turbine run up to full speed, at which point the steam supply would be closed off and the turbines allowed to freewheel, as the results were recorded.
Conditions prior to the accident
Conditions to run the test were prepared during the daytime of 25 April 1986. The day shift had been instructed in advance about the test and was familiar with procedures. A special team of electrical engineers was present to test the new voltage regulating system. As planned, the reactor's power output had been gradually reduced to 50%. Then a regional power station unexpectedly went offline. The Kiev grid controller requested that the further reduction of output be postponed, as power was consequently needed to satisfy the evening peak demand. The Chernobyl plant director agreed and postponed the test to comply.
At 11:04 p.m., the Kiev grid controller allowed the reactor shut-down to resume. This delay had serious consequences: the day shift had long since departed, the evening shift was also preparing to leave, and the night shift wouldn't take over until 12:00 midnight, well into the experiment. The special team of electrical engineers must have been exhausted from the long wait; according to plan, the test should have been finalized during the daytime and the night shift would only have to maintain basic cooling systems in a plant otherwise shut down, though the night shift was not prepared to 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 only worked independently as a senior engineer for about three months.
In Valeri Legasov's posthumous article, he maintains that the operators did not know what the test was about:
I have in my safe a transcript of the operators' telephone conversations on the eve of the accident. Reading the transcript makes one's flesh creep. One operator rings another and asks: What shall I do? In the programme there are instructions of what to do, and then a lot of things are crossed out. His interlocutor thought for a while and then replied: Follow the crossed out instructions.
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. For unknown reasons, Toptunov committed an error and inserted the control rods too far, causing the reactor to a near shut down. 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 (10 MW electrical) — almost complete shut down level and approximately 5 percent of what was expected. At this low power output a phenomenon called xenon poisoning, by which high levels of xenon-135 absorb neutrons and thus inhibit nuclear reaction, became predominant..
At this low power output it was impossible to carry out the test. The operators seem to have been unaware of the xenon poisoning, perhaps believing that the rapid fall in output was due to a malfunction in one of the automatic power regulators. To increase power, control rods were pulled out of the reactor core, beyond the correct position for normal operations, and also beyond what is allowed under safety regulations. To do this, staff had to use manual controls to override the automatic system.
Slowly, the reactor's power only increased to 200 MW, less than a third of the minimum required for the experiment, yet the experiment was 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 flow exceeded the safe limit at 1:19 a.m. The extra water lowered the core temperature and reduced steam voids. However, since water also absorbs neutrons (and the higher density of liquid water makes it a better absorber than steam), this decreased reactor power further. This prompted the operators to remove the manual control rods.
This produced an extremely unstable condition with nearly all of the control rods removed; a setup for a run-away reaction. The only thing holding the reactor at such a low power level was the high levels of neutron-absorbing xenon. The increased water flow led to a fall in steam production and other changes in the operating parameters. At this point the automatic control system should have shut the reactor down. To avoid this, the operators had disabled the shut down system.
Fatal experiment
At 1:23:04 a.m. the experiment began. The extremely unstable condition of the reactor was not known to the reactor crew, and the steam to the turbines was shut off. As the momentum of the turbine generator drove the water pumps, the water flow rate decreased, leading to the formation of steam voids. The control rods that were removed earlier were never fully removed and were still partially in the reactor, preventing the heat from reaching the cooling water. The great rise in temperature resulted in a massive steam build up and, due to the fact that the RBMK type reactors are largely positive void coefficient, the power within the reactor only increased. As the reactor power increased, so did the neutron generation. Soon it exceeded what could be absorbed by the xenon poisoning, starting a dangerous cascade. With the manual and automatic neutron absorbing control rods removed, nothing prevented a runaway reaction.
With reactor output rapidly increasing, the operators pressed the AZ-5 ("Rapid Emergency Defense 5") button at 1:23:40, which ordered a "SCRAM" — a shutdown of the reactor, fully inserting all control rods, including the manual control rods that had been incautiously withdrawn earlier. It is unclear whether it was done as an emergency measure, or simply as a routine method of shutting down the reactor upon the completion of an experiment (the reactor was scheduled to be shut down for routine maintenance). The SCRAM may have been ordered as a response to the unexpected rapid power increase; on the other hand, Dyatlov writes in his book:
Prior to 01:23:40, systems of centralized control … didn't register any parameter changes that could justify the SCRAM. Commission … gathered and analyzed large amount of materials 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.
The control rod insertion mechanism operated at 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 the reaction was slowed. In this way, the SCRAM actually increased the reaction rate. At this point a massive energy 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. By 1:23:47 (seven seconds after the AZ-5 button was pressed) the reactor jumped to around 30 GW thermal, ten times the normal operational output. The rapid increase in steam pressure destroyed fuel channels and ruptured the large diameter cooling water pipes. Fuel rods began to melt and reached the cooling water in the flooded basement.
At 1:24, 20 seconds after the SCRAM was ordered, the first steam explosion took place. It blew the 2,000 ton lid off of the reactor, damaged the top of the reactor hall, and ejected fragments of material. This ruptured further fuel channels, lifted control rods and sheared off horizontal pipes. A second, more powerful explosion occurred about two or three seconds after the first:
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 oxygen. 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 blown 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.
The graphite fire greatly contributed 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 had 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 stop the reactor, leaving only those operators there who had to work the emergency cooling systems.
Immediate crisis management
Radiation levels
The radiation levels in the worst-hit areas of the reactor building have been estimated to be 5.6 röntgen per second (R/s) (0.056 Grays per second, or Gy/s), which is equivalent to 20,000 röntgen per hour (R/hr) (200 Gy per hour, or Gy/hr). A lethal dose is around 500 röntgen (5 Gy) 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 (10 Gy/s) was inaccessible due to the explosion, and another one failed when turned on. All remaining dosimeters had limits of 0.001 R/s (0.00001 Gy/s) and therefore read "off scale". Thus, the reactor crew could ascertain only that the radiation levels were somewhere above 0.001 R/s (3.6 R/hr, or 0.036 Gy/hr), 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
Shortly after the accident, firefighters arrived to try to extinguish the fires. The first one to the scene was a Chernobyl Power Station firefighter brigade under the command of Lieutenant Vladimir 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.
The immediate priority was to extinguish fires on the roof of the station and the area around the building containing Reactor No. 4 in order 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 tonnes of materials like sand, lead, clay and boron onto the burning reactor and injection of liquid nitrogen. Ukranian filmmaker Vladimir Shevchenko captured film footage of a Mi-8 helicopter as it lost its bearings while dropping its load and got its rotors tangled in the gibbets of a nearby construction crane, causing the wrecked copter to fall into the damaged reactor building and kill its two-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 particles of the nuclear fuel and also far more dangerous 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.
Evacuation of Pripyat
After radiation levels set off alarms at the Forsmark Nuclear Power Plant in Sweden, the Soviet Union did admit that an accident had occurred, but still tried to cover up the scale of the disaster. In order 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 impression that any damage and radiation was localized, although it was not.
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 hospitalized. During the night of 26 April / 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 the nearby city of Pripyat.
The evacuation began at 14:00, 27 April. In order to reduce baggage the residents were told that the evacuation would be temporary, lasting approximately three days. As a result, Pripyat still contains personal belongings.
Steam explosion risk
There was a bubbler pool beneath the reactor. It 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 pool and the basement were flooded due to ruptured cooling water pipes and accumulated fire water. It now constituted a serious steam explosion risk. The smouldering fuel and other material above were starting to burn their way through the reactor floor, mixing with molten concrete that had lined the reactor, and creating a radioactive semi-liquid material comparable to lava. If this mixture had melted through the floor into the pool of water, it would create a massive steam explosion which would eject 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. None of the three ever returned to the surface and it is thought one of them died before reaching the gates. Fire brigade pumps were then used to drain the basement. The operation was only completed by 8 May, after having pumped out 20,000 tonnes of highly radioactive water.
With the bubbler pool gone, a meltdown was less likely to produce a powerful steam explosion. 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; this would also stabilize the foundations. Using oil drilling equipment, injection of liquid nitrogen began on 4 May. It was estimated that 25 tonnes of liquid nitrogen per day would be required to keep the soil frozen at -100 °C.
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 due to 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 thrown off helicopters (some 5,000 metric tonnes 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/hr (0.1-0.3 Gy/hr) over 20 years after the disaster.
Causes of the disaster
There were two official explanations of the accident: the first, 'flawed operators explanation', was published in August 1986 and effectively placed the blame on the power plant operators. There is no question that the operators violated the reactor's design specifications, and were seemingly ignorant of the safety requirements needed by the RBMK design. This was probably due to their lack of knowledge of reactor physics and engineering, as well as lack of experience and training. 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).
In his book, "The Truth about Chernobyl," Grigori Medvedev (who was Deputy Director of the main industrial department in the Ministry of Energy that oversaw the construction of nuclear power plants at the time of the accident, and who also had been Deputy Chief Engineer for the Chernobyl No. 1 reactor in the 1970s) lists seven different serious violations of the reactor's operational safety specifications that were committed during the preparation and conduct of the fateful test. He blames the chief engineer of the Chernobyl No. 4 reactor, N.M. Fomin, saying "...only a man with no understanding of the processes of neutron physics inside a nuclear reactor could possibly have switched off the emergency core cooling system, which could in the critical seconds have prevented the blast by sharply reducing steam content in the core."
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 every safety system 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.
The second 'flawed design explanation' was discussed by Valeri Legasov and published in 1991, attributing the accident to flaws in the RBMK reactor design, specifically the control rods.
- The reactor had a dangerously large positive void coefficient. The void coefficient is a measurement of how the reactor responds to increased steam formation in the water coolant. Most other reactor designs produce less energy as they get hotter, 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. Chernobyl's RBMK reactor, however, used solid graphite as a neutron moderator to slow down the neutrons, and neutron-absorbing light water to cool the core. Thus neutrons are slowed down even if steam bubbles form in the water. Furthermore, because steam absorbs neutrons much less readily than water, increasing an RBMK reactor's temperature 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 dangerous level if the temperature rises. This was counter-intuitive and 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 control rod end tips were made of graphite and the extenders (the end areas of the control rods above the end tips, measuring in length) were hollow and filled with water, while the rest of the rod — the truly functional part which absorbs the neutrons and thereby halts the reaction — was made of boron carbide. With this design, when the rods are initially inserted into the reactor, the graphite ends displace some coolant. This greatly increases the rate of the fission reaction, since graphite is a more potent neutron moderator (a material that enables a nuclear reaction) and also 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.
- The water channels run through the core vertically, meaning that the water's temperature increases as it moves up and thus creates a temperature gradient in the core. This effect is exacerbated if the top portion turns completely to steam, since the topmost part of the core is no longer being properly cooled and reactivity greatly increases. (By contrast, the CANDU reactor's water channels run through the core horizontally, with water flowing in opposite directions among adjacent channels. Hence, the core has a much more even temperature distribution.)
- To reduce costs, and because of its large size, the reactor had been constructed without any secure containment. This allowed the radioactive contaminants to freely escape into the atmosphere after the steam explosion burst the primary pressure vessel.
- The reactor also had been running for over one year, and was storing fission byproducts; these byproducts pushed the reactor towards disaster.
- As the reactor heated up, design flaws caused the reactor vessel to warp and break up, making further insertion of control rods impossible as the heat deformed them.
Both commissions were heavily lobbied by different groups, including the reactor's designers, power plant personnel, and by the Soviet and Ukrainian governments. The IAEA's 1986 analysis attributed the main cause of the accident to the operators' actions. But in January 1993, the IAEA issued a revised analysis, attributing the main cause to the reactor's design.
Another contributing factor was that the operators were not informed about problems with the reactor. According to Anatoliy Dyatlov, the designers knew that the reactor was dangerous in some conditions but intentionally concealed this information. In addition, the plant's management was largely composed of non-RBMK-qualified personnel: the director, V.P. Bryukhanov, had experience and training in a coal-fired power plant. His chief engineer, Nikolai Fomin, also came from a conventional power plant. Dyatlov, deputy chief engineer of reactors 3 and 4, had only "some experience with small nuclear reactors", namely smaller versions of the VVER nuclear reactors that were designed for the Soviet Navy's nuclear submarines.
Many or all of these factors probably contributed to the disaster. A potentially unstable reactor design, shoddy and inadequate safety features, poorly-trained or incompetent operators, and a lack of containment building all worked together on that horrific April night in the Byelorussian-Ukrainian Woodlands. The underlying vulnerabilities and flaws in the Soviet nuclear industry which set the stage for the tragedy had been developing for as much as 35 years. Grigori Medvedev's book, "The Truth about Chernobyl" tells how the secretive, authoritarian Soviet bureaucracy (which valued party loyalty over competence) kept promoting incompetent personnel and choosing cheapness over safety until it engineered this terrible catastrophe.
The effects of the disaster
International spread of radioactivity
The nuclear meltdown produced a radioactive cloud that floated not over just the modern states of Russia, Belarus, Ukraine and Moldova, but also Turkish Thrace, the Southern coast of the Black Sea, Macedonia, Serbia, Croatia, Bosnia-Herzegovina, Bulgaria, Greece, Romania, Lithuania, Estonia, Latvia, Finland, Denmark, Norway, Sweden, Austria, Hungary, the Czech Republic and the Slovak Republic, The Netherlands, Belgium, Slovenia, Poland, Switzerland, Germany, Luxembourg, Italy, Ireland, France (including Corsica) the United Kingdom and the Isle of Man.
The initial evidence that a major exhaust of radioactive material was affecting other countries came not from Soviet sources, but from Sweden, where on 27 April workers at the Forsmark Nuclear Power Plant (approximately 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, which led to the first hint of a serious nuclear problem in the western Soviet Union. The rise of 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 countries around the area say that over one million people could have been affected by radiation.
Recently published data of a long-term monitoring programme (The Korma-Report) show a decrease of 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, measures were taken including seemingly arbitrary regulations pertaining to the legality of importation of certain foods but not others. In France some officials stated that the Chernobyl accident had no adverse effects.
Radioactive release (source term)
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 its core. While plutonium is often perceived as particularly dangerous nuclear fuel by the general population, 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 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 the 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 as 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
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 the radiation exposure (from the smoke) was (for a discussion of the more important isotopes in fallout, see fission product). 135,000 people were evacuated from the area, including 50,000 from Pripyat.
Residual radioactivity in the environment
Rivers, lakes and reservoirs
The Chernobyl nuclear power plant lies next to the Pripyat River which feeds into the Dnieper 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, activity 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, activity 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 concern in the short term (months) for parts of the UK and Germany and in the long term (years-decades) in the Chernobyl affected areas of Ukraine, Belarus and Russia as well as in parts of Scandinavia.
Groundwater
Groundwater was not badly affected by the Chernobyl accident since radionuclides with short half-lives decayed away a long time 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 groundwaters. Significant transfers of radionuclides to groundwaters have occurred from waste disposal sites in the exclusion zone around Chernobyl. Although there is a potential for off-site (i.e. out of the exclusion zone) transfer of radionuclides from these disposal sites, 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 kilometres of pine forest in the immediate vicinity of the reactor turned ginger 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 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.
In the years since the disaster, the exclusion zone abandoned by humans has become a haven for wildlife, with nature reserves declared (Belarus) or proposed (Ukraine) for the area. Many species of wild animals and birds, which were not seen in the area prior to the disaster, are now plentiful due to the absence of humans.
A robot sent into the reactor itself has returned with samples of black, melanin-rich fungi that are growing on the reactor's walls.
Chernobyl after the disaster
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 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. A fire broke out in the turbine building of reactor 2 in 1991; 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, effectively shutting down the entire plant transforming the Chernobyl plant from energy producer to energy consumer.
Chernobyl today
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. However, the structure is not strong or durable. Some major work on the sarcophagus was carried out in 1998 and 1999. Some 200 tons of highly radioactive material remains deep within it, and this poses an environmental hazard until it is better contained.
A New Safe Confinement structure will be built by the end of 2011, and then will be put into place on rails. It is to be a metal arch 105 meters high and spanning 257 metres, 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 at units 1 to 3 remained in the reactors, most of which is in each unit's cooling pond, as well as some material in a small interim spent fuel 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 will be filled with inert gas and welded shut. They will then be transported to the dry storage vaults in which 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 Ukrainian government announced a 4-stage decommissioning plan which incorporates the above waste activities and progresses towards a cleared site.
Lava-like Fuel Containing Materials (FCMs)
According to official estimates, about 95% of the fuel (about 180 tonnes) in the reactor at the time of the accident 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) 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 present within them. The porous lava is brown lava which had dropped into water thus being 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 which suggested that the self irradiation of the lava would convert all 1,200 tons into a submicrometre and mobile powder within a few weeks. But it has been reported that it is likely that the degradation of the lava is to be a slow and gradual process rather than a sudden rapid process. The same paper states that the loss of uranium from the wrecked reactor is only 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 and uranyl carbonate. However the level of radioactivity is such that during one hundred years the self irradiation of the lava a decays per gram and 2 to of ß or ?) will fall short of the level of self irradiation which is required to greatly change the properties of glass (1018 a decays per gram and 108 to 109 Gy of ß or ?). 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 protective box which 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 atop the ruins of the reactor building. The two "Mammoth Beams" that support the roof of the shelter are resting partly upon the structurally unsound west wall of the reactor building that was damaged by the accident. The western end of the shelter roof was supported by a wall (at a point designated axis 50). This wall is reinforced concrete, which was cracked by the accident. In December 2006 the Designed Stabilisation Steel Structure (DSSS) was extended until 50% of the roof load (circa 400 tons) was transferred from the axis-50 wall to the DSSS. The DSSS is a yellow steel object which has been placed next to the wrecked reactor; it is tall and has a series of cantilevers which extend through the western buttress wall, and intended to stabilise 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 sarcophagus was never designed to last for the 100 years needed to contain the worst of the radioactivity found within the remains of reactor 4. While present designs for a new shelter anticipate a lifetime of up to 100 years, that time is minuscule compared to the lifetime of the radioactive materials within the reactor. The construction and maintenance of a permanent sarcophagus that can completely contain the remains of reactor 4 will present a continuing task to engineers for many generations to come. If the Chernobyl plant were to collapse, a large release of radioactive dust would occur, but it would likely be a one-time event.
Grass and forest fires
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 zone. In early May 1992 a serious fire occurred which 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 in airborne dust.
It is known that fires can make the 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 down wind of these fires was measured.
Recovery process
The Chernobyl Shelter Fund was established in 1997 at the Denver G7 summit to finance the Shelter Implementation Plan (SIP). The plan calls for transforming the site into an ecologically safe condition through stabilization of the sarcophagus, followed by construction of a New Safe Confinement (NSC). While original cost estimate for the SIP was US$768 million, the 2006 estimate is $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:
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 launched its activities based on the Human Consequences of the Chernobyl Nuclear Accident report recommendations and has been initiated in February 2002. The main goal of the CRDP’s activities is supporting the Government of Ukraine to mitigate long-term social, economic and ecological consequences of the Chernobyl catastrophe, among others. 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 (IPEHCA) was created and received $20 million US, mainly from Japan, in hopes of discovering the main cause of health problems due to I131 radiation. These funds that were given to IPEHCA where divided between Ukraine, Belarus, and Russia, the three main affected countries, for further investigation of health effects. As corruption played an important role of the former Soviet countries, most of the foreign aid was given to Russia, and no positive outcome from this money was ever shown.
Assessing the disaster's effects on human health
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 insinuate 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 Belarus, 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. The five year survival rate of thyroid cancer is 96%, and 92% after 30 years, with proper treatment.
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, which 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, 15 patients died from thyroid cancer, and it roughly estimated that cancers deaths caused by Chernobyl may reach a total of about 4000 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."
While it was commented by Fred Mettler 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 which are not only to do with the Chernobyl accident, but also with the desperately 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 system and any separate effects of the Chernobyl accident.
Lastly, in its report Health Effects of Chernobyl, 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. This said, it is important to bear in mind that many of the conclusions from reports such as UNSCEAR remain disputed by other commentators and scientists in the field.
In the popular consciousness
The Chernobyl accident attracted a great deal of interest. Because of the distrust that many people had in the Soviet authorities (people both within and outside the USSR) a great deal of debate about the situation at the site occurred in the first world during the early days of the event. Due to defective intelligence based upon photographs taken from space, it was thought that unit number three had also suffered a dire accident.
A few authors claim that the official reports underestimate the scale of the Chernobyl tragedy, counting only 30 victims; some estimate the Chernobyl radioactive fallout as hundreds of times that of the atomic bomb dropped on Hiroshima, Japan, counting millions of exposed.
In general the public knew little about radioactivity and radiation and as a result their degree of fear was increased. It was the case that many professionals (such as the spokesman from the UK NRPB) were mistrusted by journalists, who in turn encouraged the public to mistrust them.
It was noted that different governments tried to set contamination level limits which were stricter than the next country. In the dash to be seen to be protecting the public from radioactive food, it was often the case that the risk caused by the modification of the nations' diet was greater and un-noticed.
In Italy, the fear of nuclear accidents was dramatically increased by the Chernobyl accident: this reflected in the outcome of the 1987 referendum about the construction of new nuclear plants in Italy. As effect of that referendum, Italy began phasing out its nuclear power plants in 1988.
Commemoration of the disaster
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 in German, English, Danish, Dutch, Russian and Ukrainian.
In Kyiv, Ukraine, the exhibition was launched at the "Chernobyl 20 Remembrance for the Future" conference on 23 April 2006. It was then exhibited during 2006 in Australia, Denmark, the
Netherlands, Switzerland, Ukraine, the United Kingdom, and the United States.
Representations in games
See also
Selected Chernobyl articles
General nuclear articles
Other
Further reading
External links
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- Annotated bibliography on Chernobyl from the Alsos.
- , by the IAEA
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