Chernobyl disaster

2008/9 Schools Wikipedia Selection. Related subjects: Engineering; Recent History

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 nuclear meltdown. On 26 April 1986 at 01:23:40 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.

The plume drifted 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 tally accurately 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 ( ) is located near the town of Pripyat, Ukraine, 18 km northwest of the city of Chernobyl, 16 km (10 mi) from the border of Ukraine and Belarus, and about 110 km (68 mi) 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 1:23:58 a.m. reactor 4 suffered a catastrophic steam explosion resulting in a nuclear meltdown, a series of additional explosions and a fire. The radiation was not contained and radioactive particles were carried by wind across international borders.

Test planning

During the daytime of April 25 1986, reactor 4 at was scheduled to be shut down for maintenance. A decision was made to test the ability of the reactor's turbine generator to generate sufficient electricity to power the reactor's safety systems (in particular, the water pumps), in the event of a loss of external electric power. A RBMK-1000 reactor requires water to be continuously circulated through the core for as long as the nuclear fuel is present.

Chernobyl's reactors had a pair of backup diesel generators, but because there was a 40-second delay before they could attain full speed, the reactor was going to be used to spin up the reactor's turbine generator. Once at full speed, the turbine would be disconnected from the reactor and allowed to spin under its own rotational momentum. The aim of the test was to determine whether the turbines in the rundown phase could power the pumps while the generators were starting up. The test was previously 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 reactor four's 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-advised 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 another had a little experience with nuclear submarine power plants.

At 11:00 p.m., April 25, the grid controller allowed the reactor shut-down to continue. The power output of reactor 4 was to be reduced from its nominal 3.2 GW thermal to 0.7–1.0 GW thermal in order to conduct the test at the prescribed lower level of power. However, the new crew were unaware of the prior postponement of the reactor slowdown, and followed the original test protocol, which meant that the power level was decreased too rapidly. In this situation, the reactor produced more of the nuclear poison product xenon-135 (the xenon production rate:xenon loss rate ratio initially goes higher during a reactor power down), which dropped the power output to 30 MW thermal (approximately 5% of what was expected). The operators believed that the rapid fall in output was due to a malfunction in one of the automatic power regulators, not because of reactor poisoning. In order to increase the reactivity of the underpowered reactor (caused unknowingly by neutron absorption of excess xenon-135), automatic control rods were pulled out of the reactor beyond what is allowed under safety regulations.

Despite this breach, the reactor's power only increased to 200MW, still less than a third of the minimum required for the experiment. Despite this, the crew's management chose to continue 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. – since water also absorbs neutrons, this further increase in the water flow necessitated the removal of the manual control rods, producing a very precarious operating situation where coolant and xenon-135 was substituting some of the role of the control rods of the reactor.

Fatal experiment

At 1:23:04 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. The turbine was disconnected from the reactor, increasing the level of steam in the reactor core. As the coolant heated, 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 rapidly, and the reactor operation became progressively less stable and more dangerous. As the reaction continued, the excess xenon-135 was burnt up, increasing the number of neutrons available for fission. The prior removal of manual and automatic control rods had no backup, leading to a runaway reaction.

At 1:23:40 the operators pressed the AZ-5 ("Rapid Emergency Defense 5") button 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, Anatoly Dyatlov, deputy chief engineer at the nuclear station at the time of the accident, 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 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, 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 traveled vertically along the rod channels in the reactor, displacing and destroying the reactor lid, rupturing the coolant tubes and then blowing the 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.

Possible causes of the disaster

There are two official theories about the main cause of the accident. The first was published in August 1986 and effectively placed the blame solely on the power plant operators. This is known as the flawed operators theory.

The second theory, proposed by Valeri Legasov and published in 1991, attributed the accident to flaws in the RBMK reactor design, specifically the control rods. This theory is called the flawed design theory.

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 one of them, 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.

In particular:

Flawed design theory

• 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-metre (3 ft) 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 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 behaviour 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.

Flawed operators theory

The operators violated plant procedures and were ignorant of the safety requirements needed by the RBMK design. This is 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.

The effects of the disaster

International spread of radioactivity

The nuclear meltdown provoked a radioactive cloud which floated over Russia, Belarus, Ukraine and Moldova, but also the European part of the Republic of Macedonia, Croatia, Turkey, 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) and the United Kingdom. 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 1,100 km (684 mi) from the Chernobyl site) were found to have radioactive particles on their clothes. It was Sweden's search for the source of radioactivity, after they had determined there was no leak at the Swedish plant, 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 not evenly spread across the surrounding countryside, but 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.

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 – this was ridiculed as pretending that the radioactive cloud had stopped at the German and Italian borders.

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.

Residual radioactivity in the environment

Rivers, lakes and reservoirs

The Chernobyl nuclear power plant lies next to the river Pripyat 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 k 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 30 km (19 mi) exclusion zone around Chernobyl. Although there is a potential for off-site (i.e. out of the 30 km (19 mi) 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", according to the BBC. 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 metres (660 ft) of concrete was placed between the disaster site and the operational buildings. The Ukrainian government continued to let the three remaining reactors operate because of an energy shortage in the country. A fire broke out in 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. This transformed the Chernobyl plant from energy producer to energy consumer.

The need for future repairs

The sarcophagus is not an effective permanent enclosure for the destroyed reactor. Its hasty construction, in many cases conducted remotely with industrial robots, is aging poorly. If it collapses, another cloud of radioactive dust could be released. The sarcophagus is in such poor condition that a small earthquake or severe wind could cause the roof to collapse. A number of plans have been discussed for building a more permanent enclosure.

Water continues to leak into the shelter, spreading radioactive materials throughout the wrecked reactor building and potentially into the surrounding groundwater. The basement of the reactor building is slowly filling with water that is contaminated with nuclear fuel and is considered high-level radioactive waste. Though repairs were undertaken to fix some of the most gaping holes that had formed in the roof, it is by no means watertight, and will only continue to deteriorate.

The sarcophagus, while not airtight, heats up much more readily than it cools down. This is contributing to rising humidity levels inside the shelter. The high humidity inside the shelter continues to erode the concrete and steel of the sarcophagus.

Further, dust is becoming an increasing problem within the shelter. Radioactive particles of varying size are a portion of the debris inside the shelter. Convection currents compounded with increasing intrusion of outside airflow are increasingly stirring up and suspending the particles in the air inside the shelter. The installation of air filtration systems in 2001 has reduced the problem, but not eliminated it.

Some signs of a criticality were observed in June 24, 1990– July 1, 1990 inside room 304/3; to avoid any further nuclear fission reaction, a neutron poison was added to this room.

In September 2007, Ukraine approved the building of a steel casing over the reactor. The casing, to be built by the French firm Novarka, will be at a cost of $1.4bn. The arch shaped structure, which will measure 190 m (623 ft) wide and 200 m (660 ft) long, is scheduled to take 5 years to complete. Once the structure is complete, dismantling of Reactor 4 will begin.

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 (670000 TBq). 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, these are black, brown and a porous ceramic. These lavas are silicate glasses with inclusions of other materials present within them. The porous lava is brown lava which had dropped into water thus cooling it rapidly.

Degradation of the lava

It is unclear how long the ceramic form will retard the release of radioactivity. In 1997 to 2002 a series of papers were published which suggested that the α self irradiation of the lava would convert all 1200 tons into a submicron 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 (22 lb) per year. This low rate of uranium leaching suggests that the lava is resisting its environment. The paper also states that when the shelter is improved 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. These have been seen on the elephant foot already. However the level of radioactivity is such that during 100 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..The IAEA and the former soviets maintain that less than 5% of the fuel was lost due to the explosion.

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 metres (207 ft) 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.

Grass and forest fires

If the Chernobyl plant were to collapse, a large release of radioactive dust would occur, but it would likely be a one-time event. 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 (19 mi) 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. PDF (416  KiB) PDF (139  KiB)

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.

The Chernobyl Fund and the Shelter Implementation Plan

The Chernobyl Shelter Fund was established in 1997 at the Denver G7 summit to fund the Shelter Implementation Fund. The Shelter Implementation Plan (SIP) 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 (886 ft)
• Height: 100 m (330 ft)
• Length: 150 m (492 ft)

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 the year 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 can 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 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 behaviour 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."

( http://www.iaea.org/blog/Infolog/?page_id=25)

While it was commented by Fred Mettler ( http://www.iaea.org/Publications/Magazines/Bulletin/Bull472/htmls/chernobyls_legacy2.html) 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.

Reports by anti-nuclear power protest groups and irresponsible journalists , based on speculation rather than evidence, may have contributed to the anxiety and depression of people in the fallout zones. These include the TORCH report, the Greenpeace report, and the April 2006 International Physicians for Prevention of Nuclear Warfare (IPPNW) report.

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 tens of 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.

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 related illnesses since.
• 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.

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 (19 mi) 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 cesium-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.