Chernobyl disaster



The Chernobyl disaster, reactor accident at the Chernobyl nuclear power plant, or simply Chernobyl, was the worst nuclear power plant accident in history and the only instance so far of level 7 on the International Nuclear Event Scale, resulting in a severe release of radioactivity into the environment following a massive power excursion which destroyed the reactor. Thirty people died in the explosion, but most deaths from the accident were attributed to fallout.

On 26 April, 1986 at 01:23:44 a.m. (UTC+3) reactor number four at the Chernobyl Nuclear Power Plant located in the Soviet Union near Pripyat in Ukraine exploded. Further explosions and the resulting fire sent a plume of highly radioactive fallout into the atmosphere and over an extensive geographical area. Nearly thirty to forty times more fallout was released than had been by the atomic bombings of Hiroshima and Nagasaki.

The plume drifted all over parts of the western Soviet Union, Eastern Europe, Western Europe, Northern Europe, and eastern North America. 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 tell 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 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 deaths due to cancer among the approximately 600,000 most highly exposed and 5,000 among the 6 million living nearby. Although the Chernobyl Exclusion Zone and certain limited areas will 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 (51.38722°N, 30.11139°W) is located near the town of Pripyat, Ukraine, 18 km northwest of the city of Chernobyl, 16 km from the border of Ukraine and Belarus, and about 110 km north of Kiev. The station consisted of four reactors of type RBMK-1000, 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 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, capable of producing 1 GW each, were under construction at the time of the accident.

The accident
On April 26 1986 at 1:23:44 a.m., reactor 4 suffered a massive, catastrophic overflow of power, resulting in a steam explosion, which tore the top from the reactor and dispersed large amounts of radioactive particulate and gaseous debris, allowing oxygen to contact the super-hot graphite moderator and oxidize, increasing the dispersion of radioactive particles and eventually melting much of the fuel. The radioactivity was not contained by any kind of containment vessel and radioactive particles were carried by wind across international borders. Although much of the nuclear fuel in the reactor core did ultimately melt, it should be noted that the disaster was not a nuclear "meltdown" in the usual sense; the fuel melting was not a significant contribution to the radiological consequences of the accident, and the accident was not caused by a loss of coolant.

Test planning
During the daytime of April 25 1986, reactor 4 at 7 °N, 5.94883°W was scheduled to be shut down for maintenance. An experiment was proposed to test the ability of the steam turbines to supply power to the reactor's water pumps if there was a loss of steam pressure, as the turbines mechanically drove the water pumps. Chernobyl's reactors had a pair of backup diesel generators, but there was a 40-second delay before the generators could attain full speed and the operators wanted to find out if the turbine would be able to produce enough power under its own rotational momentum to drive the water pumps as it freewheeled to a stop. The experiment was important because, even if a reactor is quickly shut down, decay heat from nuclear fuel produces considerable amounts of heat which needs to be removed to prevent possible core damage.

For the experiement, 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 and the results recorded. Previously, the test had been successfully carried out on another unit (with all safety provisions active) with negative results — the turbines did not generate sufficient power, but because additional improvements were made to the reactor's four turbines, there was a need for another test.

Conditions prior to the accident
As conditions to run this test were prepared during the daytime of April 25, and the reactor electricity output had been gradually reduced to 50%, a regional power station unexpectedly went offline. The Kiev grid controller requested that the further reduction of output be postponed, as electricity was needed to satisfy the evening peak demand. The plant director agreed and postponed the test to comply. The ill-fated safety test was then left to be run by the night shift of the plant, a skeleton crew who would be working Reactor 4 that night and the early part of the next morning. This reactor crew had little or no experience in nuclear power plants, as many had been drafted in from coal powered plants, and Anatoly Dyatlov, deputy chief engineer of the plant and the effective crew chief during the experiment, had some experience installing nuclear reactors in submarines.

At 11:04 p.m., April 25, the grid controller allowed the reactor shut-down to continue. It was intended for the power output of reactor 4 to be reduced from its nominal 3.2 GW thermal to 0.7–1.0 GW thermal to conduct the test at the prescribed lower level of power. However, the skeleton crew was unaware of the prior postponement of the reactor slowdown, therefore assumed that the power output was already reduced, and followed the original test protocol, decreasing power too rapidly which caused the massive amounts of smoke.

A major product of nuclear fission is the isotope iodine-135. I-135 decays with a half life of 6.7 hours into xenon-135. Xe-135 is a potent reactor poison, i.e., it is extremely effective at absorbing neutrons and slowing the chain reaction. Once an atom of Xe-135 absorbs a neutron, however, it becomes the stable Xe-136 that doesn't absorb neutrons. In normal high power operation, an equilibrium is reached where the Xe-135 is "burned" by the reactor's high neutron flux as fast as it is produced by I-135 decay. But because reactor 4's power and neutron flux were rapidly decreased, the decay of large amounts of I-135 from previous high power operation produced Xe-135 faster than it could be eliminated, so it built up and dampened the nuclear reaction further.

When the operators commanded a small power reduction, the reactor power dropped to 30 MW thermal, approximately 5% of what was expected. The operators, unaware of the poisoning phenomenon, believed that the rapid fall in output was due to a malfunction in one of the automatic power regulators. To increase power, automatic control rods were pulled out of the reactor beyond the correct position for the desired power output in normal operating conditions, and also beyond what is allowed under safety regulations.

The reactor's power still only increased to 200MW, less than a third of the minimum required for the experiment. Yet the crew's management continued the experiment. As part of the experiment, at 1:05 a.m. on April 26 the water pumps that were to be driven by the turbine generator were turned on, increasing the water flow beyond what is specified by safety regulations. The water flow increased at 1:19 a.m., and since water also absorbs neutrons, this decreased reactor power further and prompted the removal of the manual control rods. This produced an extremely hazardous condition; with nearly all of the control rods removed, the only thing keeping the reactor at such a low power level was the build-up of Xe-135.

Fatal experiment
At 1:23:04 AM the experiment began. The unstable state of the reactor was not reflected in any way on the control panel, and it did not appear that anyone in the reactor crew was aware of any danger. The steam to the turbines was shut off and, as the momentum of the turbine generator drove the water pumps, the water flow rate decreased, decreasing the absorption of neutrons by the coolant and causing the coolant to heat up. As the coolant boiled, pockets of steam formed voids in the coolant lines. Due to the RBMK reactor-type's large positive void coefficient, the steam bubbles increased the power of the reactor. As soon as the reactor power increased, the positive feedback that had acted to drive reactor power down now acted to increase it further. As power increased, the Xe-135 poison began to be burned faster than it was being produced by I-135 decay, which increased power, resulting in more steam generation, a faster Xe-135 burn, and so on. With the manual and automatic control rods removed, nothing prevented a runaway reaction.

With reactor output rapidy increasing, the operators pressed the AZ-5 ("Rapid Emergency Defense 5") button at 1:23:40, that 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). It is usually suggested that the SCRAM was 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 slow speed of the control rod insertion mechanism (18–20 seconds to complete), and the flawed graphite-tip rod design which initially reduces the amount of coolant present, meant that the SCRAM actually increased the reaction rate. At this point an energy spike occurred and some of the fuel rods began to fracture, placing fragments of the fuel rods in line with the control rod columns. The rods became stuck after being inserted only one-third of the way, and were therefore unable to stop the reaction. At this point nothing could be done to stop the disaster. By 1:23:47 the reactor jumped to around 30 GW thermal, ten times the normal operational output. The fuel rods began to melt and the steam pressure rapidly increased, causing a large steam explosion. Generated steam travelled vertically along the rod channels in the reactor, rupturing the coolant tubes and then blowing the 2,000 tonne lid off the reactor. After part of the roof blew off, the inrush of oxygen, combined with the extremely high temperature of the reactor fuel and graphite moderator, started a graphite fire. This fire greatly contributed to the spread of radioactive material and the contamination of outlying areas.

Radiation levels
At the time of the disaster, the plant's staff was not aware of the true radiation levels, which led to severe misassessments of the situation. The radiation levels in the worst-hit areas of the reactor building have been estimated to be 5.6 röntgen per second (R/s), which is equivalent to 20,000 röntgen per hour (R/h). A lethal dose is around 500 röntgen 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 was inaccessible due to the explosion, and another one failed when turned on. All remaining dosimeters had limits of 0.001 R/s and therefore read "off scale". Thus, the reactor crew could ascertain only that the radiation levels were somewhere above 0.001 R/s (3.6 R/h), while the true levels were 5,600 times higher in some areas.

Because of the fallacious 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 May 9, 1986. 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." The fires on the roof of the station and the area around the building containing Reactor No. 4 were extinguished by 5 a.m., but many firefighters received high doses of radiation. The fire inside Reactor No. 4 continued to burn until the fire was extinguished by helicopters dropping materials like sand, lead, clay and boron onto the burning reactor.

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 / April 27 — 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.

Thermal explosion risk
The water that had hurriedly been pumped into the reactor building in a futile attempt to extinguish the fire had run down to the space underneath the reactor floor. The smouldering fuel and other material still suspended above was starting to burn its 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 burned through the floor into the pool of water, the burst of radioactive steam would have killed everyone on site and increased the severity of the fallout.

In order to prevent this, soldiers and workers were sent in as clean-up staff by the government. Two of these were sent in wetsuits to open the sluice gates to vent the radioactive water, and thus prevent a thermal explosion. They are thought to be 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.

Debris removal
The worst of the radioactive debris was collected inside what was left of the reactor. 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.

Possible causes of the disaster
There are two official theories about the main cause of the accident: the first, 'flawed operators theory', was published in August 1986 and effectively placed the blame solely on the power plant operators. The operators violated plant procedures and were ignorant of the safety requirements needed by the RBMK design. This was partly due to their lack of knowledge of the reactor's design as well as lack of experience and training. Several procedural irregularities also contributed to causing the accident. One was insufficient communication between the safety officers and the operators in charge of the experiment being run that night. It is also important to note that the reactor operators disabled every safety system down to the generators, which the test was really about. The main process computer, "S.K.A.L.A", 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 who had very little or no experience with nuclear reactors.

The second 'flawed design theory' was proposed 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 1 m 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 with only partial containment. This allowed the radioactive contaminants to 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.

A variant theory holds 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.

International spread of radioactivity


The nuclear meltdown provoked a radioactive cloud that floated not over just the modern states of Russia, Belarus, Ukraine and Moldova, but also Turkish Thrace, Macedonia, Serbia, Croatia, 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, 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 April 27 workers at the Forsmark Nuclear Power Plant (approximately 1100 km 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.

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.

The IAEA and the former soviets maintain that less than 5% of the fuel was lost due to the explosion.

Health of plant workers


In the aftermath of the accident, two hundred and thirty-seven people suffered from acute radiation sickness, of whom thirty-one 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.

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 or 1 kBq/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 30 km exclusion zone around Chernobyl. Although there is a potential for off-site (i.e. out of the 30 km 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 6 km 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 in the area.

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 on 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 m 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 December 15, 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 shelter 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 shelter was carried out in 1998 and 1999. Some 200 tonnes 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 EUR 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 is in each unit's cooling pond, and some 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 the Ukraine government announced a 4-stage decommissioning plan which incorporates the above waste activities and progresses towards a cleared site. 

The lava (or fuel containing materials FCM)
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 1200 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 10 kg 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 that 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 Na4(UO2)(CO3)3 and uranyl carbonate. However the level of radioactivity is such that during one hundred years that the self irradiation of the lava (2 x 1016 α decays per gram and 2 to 5 x 105 Gy of β or γ) will fall short of the level of self irradiation which is 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 protective box which was placed over the wrecked reactor was named the object "Shelter" by the Soviets, 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 a reinforced concrete wall 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 63 m tall and has a series of cantilevers which extend through the western buttress wall and is intended to stablise the object "Shelter". . This was done because if the wall of the reactor building and subsequently 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), and rested atop 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, in that only debris is supporting it in a nearly upright position. The collapse of the bioshield would further exacerbate the dust conditions in the shelter, would probably spread 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 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


It is possible for grass or forest fires to occur on a regular basis within the contaminated zone. In 1986 a series of fires destroyed 23.36 km² (5,772 acres) of forest, and a series of other fires have since burned within the 30 km zone. During early May 1992 a serious fire occurred which affected 5 km² (1,240 acres) of land which included 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 Fund (SIP). The Shelter Implementation 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:
 * Span: 270 m
 * Height: 100 m
 * Length: 150 m

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.

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 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 assessments of the evidence for health effects of the Chernobyl accident.

"The Chernobyl Forum" has 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, many charities which help the "Children of Chernobyl" may be helping disadvantaged children, but the health problems of such children are not only to do with the Chernobyl accident, but also with the desperately poor state of post-Soviet health systems.

In response to the Chernobyl Forum, The Other Report on Chernobyl (TORCH) was produced. It predicted between 30,000 to 60,000 excess cancer deaths, and urged more research, stating that large uncertainties made it difficult to properly assess the full scale of the disaster. Another study critical of the Chernobyl Forum report was commissioned by Greenpeace. In its report, Greenpeace argued 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 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.

Comparison with other disasters
The Chernobyl disaster caused a few dozen immediate deaths due to radiation poisoning; a few thousand premature deaths are predicted over the coming decades. Since it is often not possible to prove the origin of the cancer which causes a person's death, it is difficult to estimate Chernobyl's long-term death toll, which is still a hotly-debated issue.

Other man-made disasters with very high death tolls include:
 * The failure of the Banqiao Dam (Henan, China, 1975) – where an estimated 26,000 people died from flooding and another 145,000 died during subsequent epidemics and famine.
 * The Bhopal disaster (India, 1984) – the BBC gives the death toll as nearly 3,000 people dead initially, and at least another 15,000 have died from subsequent illnesses.
 * The Great Smog (London, United Kingdom, 1952) – where medical services compiled statistics and found that the fog had killed 4,000 people initially, and another 8,000 died in the weeks and months that followed.
 * The MV Doña Paz disaster, (Philippines, 1987) - this petroleum products fire at sea killed over 4000.
 * The Johnstown Flood (Pennsylvania, United States, 1889) – 2,209 killed.

Comparisons with other various incidents concerning radioactivity are at Chernobyl compared to other radioactivity releases.

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.

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 Kiev, 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.

Some alternative views on Chernobyl
Strong arguments do exist in support of (at the very least) some aspects of 'alternative' assessments such as the TORCH report. The ECRR's publication "Chernobyl: 20 Years On" has summarised thousands of Ukrainian, Belarusian and Russian papers, and scientists studying those regions who have claimed, among other effects, genetic defects in plants and animals transmitted over 22 generations. However, these findings have not been confirmed by all workers within the subject, it is known that many species of wild animals found within the 30 km exclusion zone are alive and well.

Furthermore, it has been reported by the UN that in the contaminated areas in the general public that "no evidence or likelihood of decreased fertility has been seen among males or females. Also, because the doses were so low, there was no evidence of any effect on the number of stillbirths, adverse pregnancy outcomes, delivery complications or overall health of children. A modest but steady increase in reported congenital malformations in both contaminated and uncontaminated areas of Belarus appears related to better reporting, not radiation."

The Low Level Radiation Campaign has claimed that a 40% increase in Belarusian cancer rates has occurred, with a similar increase in northern Sweden. However it is important to bear in mind that some of the LLRC's work on cancer in humans could not be repeated by other scientists.

It is also important to note that the (presumed) low effect of chronic doses of radiation may be different to acute exposure due to self repair processes, and scientific opinion is divided on the subject of how dangerous small doses (delivered at low dose rates) of radiation are. One common model is the LNT model which states that the increased likelihood of disease is directly proportional to the dose, while other models suggest that below a threshold that radiation is harmless or even good for human health. However, the effects of internal doses of radiation on the body (due to inhaled/ ingested isotopes) are often very different from their external effects, even at low doses.

Some caution is needed in adopting a purely local view of the Chernobyl disaster, as the major effects of the accident go far beyond the intense on-site radiation fields (these were sometimes in the range of circa 10 Gy min-1 on day one, but they have now decayed to far lower levels). While the majority of the emitted radioactivity was deposited close to the reactor some activity was deposited at remote locations such as Wales, Sweden and other parts of Western Europe. It is now the case that Chernobyl caesium-137 can be found in many topsoils and sedimentary deposits in Europe, which has been documented in numerous studies. While the fallout outside the former Soviet Union did not result in radiation fields with the intensity to cause deterministic effects such as radiation sickness, it has resulted in a situation where restrictions on the sale and movement of food were considered wise and necessary in some cases, and many governments imposed cautious new regulations.

Due to the long latent period between exposure and the clinical appearance of many radiation related diseases (mainly cancer), it is unwise to rule out at least the possibility of additional major long-term health effects (both within and outside the Chernobyl region). In any case, while there is no doubt that socio-economic stress - coupled with the psychological effects of anxiety and relocation - must play a part in the Chernobyl health debacle, the indirect health impacts of clean-up costs over many years (through diverted expenditure) cannot be dismissed as somehow 'outside' the event. These 'collateral impacts' are part and parcel of the socio-economic aftermath of disasters on this scale and (where they have occurred) need to be incorporated in the impact equation. These costs are real and include an erosion of national confidence, loss of trade and land area (Exclusion Zones contaminated above the WHO limits), an intensification of existing health issues and the 'socio-economic deaths' which arise through social underprovision.