   #copyright

Chernobyl disaster

2007 Schools Wikipedia Selection. Related subjects: Recent History

   SOS Children is still caring for Chernobyl victims in Belarus. For more
   information see SOS Children in Belarus

   Coordinates: 51°23′23″N, 30°5′58″E

   The Chernobyl disaster was an accident at the Chernobyl Nuclear Power
   Plant on April 26, 1986 at 01:23 a.m., consisting of an explosion at
   the plant and subsequent radioactive contamination of the surrounding
   geographic area. The power plant is located at 51°23′23″N, 30°5′58″E,
   near Pripyat, Ukraine, at the time part of the Soviet Union. It is
   regarded as the worst accident ever in the history of nuclear power. A
   plume of radioactive fallout drifted over parts of the western Soviet
   Union, Eastern and Western Europe, Scandinavia, the UK, Ireland and
   eastern North America. Large areas of Ukraine, Belarus, and Russia were
   badly contaminated, resulting in the evacuation and resettlement of
   over 336,000 people. About 60% of the radioactive fallout landed in
   Belarus, according to official post-Soviet data.

   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 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. Most of the expected long-term
   fatalities, especially those from cancer, have not yet actually
   occurred, and will be difficult or even impossible to attribute
   specifically to the accident. Dr Peter Boyle, director of the
   International Agency for Research on Cancer, put the discussion of the
   figures into perspective: "Tobacco smoking will cause several thousand
   times more cancers in the same (European) population." Estimates and
   figures vary widely. 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 as many as
   9,000 people among the approximately 6.6 million most highly exposed,
   may die from some form of cancer (one of the induced diseases). Nearly
   20 years after the disaster, according to the Chernobyl Forum, no
   evidence of increases in the solid cancers and, possibly more
   significantly, none of the widely expected increases in leukemia have
   been found in the population.

The plant

   The Chernobyl station (Чернобыльская АЭС им. В.И.Ленина – V.I. Lenin
   Memorial Chernobyl Nuclear Power Station) ( 51°23′14″N, 30°06′41″E) is
   located near the town of Prypiat, 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 GW of electric power (3.2 GW of
   thermal 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 Saturday April 26, 1986 at 1:23:58 a.m. reactor 4 suffered a
   catastrophic steam explosion that resulted in a fire, a series of
   additional explosions, and a nuclear meltdown. The accident can be
   thought of as an extreme version of the SL-1 accident where the core of
   a reactor was destroyed (killing three men) spreading radioactivity
   through the inside of the building that SL-1 was in.

Causes

   There are two conflicting official theories about the cause of the
   accident. The first was published in August 1986 and effectively placed
   the blame solely on the power plant operators. 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. Both commissions were heavily lobbied by different groups,
   including the reactor's designers, power plant personnel, and by the
   Soviet and Ukrainian governments.

   Another important factor contributing to the accident was 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. Contributing to this was that 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:
     * 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 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 produce a lot more energy 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 much fewer neutrons than the
       boiling light water. Thus for the first few seconds of control rod
       activation, reactor's power output increases, rather than reducing
       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 moderated 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.

     * The operators were careless and violated plant procedures, partly
       due to their lack of knowledge of the reactor's design, and lack of
       experience and training. Several procedural irregularities also
       contributed to cause the accident. One was insufficient
       communication between the safety officers and the operators in
       charge of an experiment being run that night. The operators
       switched off many of the safety systems, which was generally
       prohibited by the plant's published technical guidelines.

     * 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 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.

Test plan

   During the daytime of April 25, 1986, reactor 4 was scheduled to be
   shut down for maintenance. It had been decided to use this occasion as
   an opportunity 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. This type of reactor requires water being
   continuously circulated through the core, as long as the nuclear fuel
   is present. Chernobyl's reactors have a pair of diesel generators
   available as standby, but these do not activate instantaneously—the
   reactor was, therefore, to be used to spin up the turbine, at which
   point the turbine would be disconnected from the reactor and allowed to
   spin under its own rotational momentum, and 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 successfully
   carried out previously on another unit (with all safety provisions
   active) with negative results - the turbines did not generate
   sufficient power, but additional improvements were made to the
   turbines, which prompted the need for another test.

Prior to 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 demanded to postpone the further reduction of output 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.

   At 11:00pm, 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 a safer, lower level of power. However, due to a delay in
   starting the experiment the reactor operators reduced the power level
   too rapidly, and the actual power output fell to 30 MW thermal.
   According to operators, the rapid fall in output was due to
   malfunctioning of one of the automatic power regulators. As a result of
   output decline, the concentration of the nuclear poison product
   xenon-135 increased (the xenon production rate: xenon loss rate ratio
   goes initially higher during a reactor power down). Though the scale of
   the power drop was close to the maximum allowed by safety regulations,
   the crew's management chose not to shut down the reactor, and to
   continue the experiment. Further, it was decided to 'shortcut' the
   experiment and raise power output to only 200 MW. In order to overcome
   the neutron absorption of the excess xenon-135, the control rods were
   pulled out of the reactor somewhat further than normally allowed under
   safety regulations. 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; the water flow generated by this action exceeded that
   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 unstable and dangerous operating condition.

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 does not appear
   that anyone in the reactor crew was fully aware of any danger.
   Electricity to the water pumps was shut off and, as the momentum of the
   turbine generator drove them, the water flow rate decreased. 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 power of the reactor increased rapidly,
   and the reactor operation became progressively less stable and more
   dangerous. 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,
   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."

   Due to the slow speed of the control rod insertion mechanism (18–20
   seconds to complete), the hollow tips of the rods and the temporary
   displacement of coolant, the SCRAM caused the reaction rate to
   increase. Increased energy output caused the deformation of control rod
   channels. The rods became stuck after being inserted only one-third of
   the way, and were therefore unable to stop the reaction. 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 a hole in the roof. After part of the roof blew off, the inrush
   of oxygen, combined with the extremely high temperature of the reactor
   fuel and graphite moderator, sparked a graphite fire. This fire greatly
   contributed to the spread of radioactive material and the contamination
   of outlying areas.

The effects of the disaster

Long-term health effects

   Map showing caesium-137 contamination in Belarus, Russia, and Ukraine.
   In curies per square kilometer (1 curie is 37 gigabecquerels).
   Enlarge
   Map showing caesium-137 contamination in Belarus, Russia, and Ukraine.
   In curies per square kilometer (1 curie is 37 gigabecquerels).

   Right after the accident, the main health concern involved radioactive
   iodine, with a half-life of eight days. Today, there is concern about
   contamination of the soil with strontium-90 and caesium-137, which have
   half-lives of about 30 years. The highest levels of caesium-137 are
   found in the surface layers of the soil where they are absorbed by
   plants, insects and mushrooms, entering the local food supply. However,
   in 2006, hedgehogs from the area, an insectivorous species seem to have
   absorbed little if any radioactive material, whilst rodents are
   strongly radiating (20 millisieverts per day), although seem to suffer
   no ill effects.

   Some persons in the contaminated areas were exposed to large thyroid
   doses of up to 50 grays (Gy) because of an intake of radioactive
   iodine-131, a relatively short-lived isotope with a half-life of eight
   days, but which concentrates in the thyroid gland. This would have been
   absorbed from contaminated milk produced locally, particularly in
   children. Several studies have found that the incidence of thyroid
   cancer in Belarus, Ukraine and Russia has risen sharply, however there
   have barely been more than a handful of deaths. Some scientists think
   that most of the increase is caused by greatly increased monitoring.

   So far, no increase in leukemia in the general population is
   discernible.

   Some scientists fear that radioactivity will affect the local
   population for the next several generations. However, there is little
   evidence so far of this.

   Soviet authorities started evacuating people from the area around the
   Chernobyl reactor 36 hours after the accident. By May 1986, about a
   month later, all those living within a 30-kilometre (18 mile) radius of
   the plant—about 116,000 people—had been relocated. This region is often
   referred to as the Zone of alienation. However, radiation affected the
   area in a much wider scale than this 30 km radius.
   A monument to victims of Chernobyl disaster in Luhansk, Ukraine
   Enlarge
   A monument to victims of Chernobyl disaster in Luhansk, Ukraine

   The issue of long-term effects of Chernobyl disaster on civilians is
   controversial. Over 300,000 people were resettled because of the
   accident; millions lived and continue to live in the contaminated area.
   On the other hand, most of those affected received relatively low doses
   of radiation, there is little evidence of increased mortality – cancers
   or birth defects among them – and, when such evidence is present,
   existence of a causal link to radioactive contamination is uncertain.

   Aside from obstacles posed by Soviet policies during and after the
   catastrophe, scientific studies may still be limited by a lack of
   democratic transparency. In Belarus, Yuri Bandazhevsky, a scientist who
   questioned the official estimates of Chernobyl's consequences and the
   relevance of the official maximum limit of 1000 Bq/kg, has allegedly
   been a victim of political repression. He was imprisoned from 2001 to
   2005 on a bribery conviction, after his 1999 publication of reports
   critical of the official research being conducted into the Chernobyl
   incident.

Farming

Cows

   Jiří Hála's text book (Radioactivity, Ionizing Radiation and Nuclear
   Energy, ISBN 807302053) explains how cattle only pass a minority of the
   strontium, cesium, plutonium and americium they ingest to the humans
   who consume milk and meat. For instance, for milk if the cow has a
   daily intake of 1000 Bq of the following isotopes then the milk will
   have the following activities. The meat and milk of cows was identified
   as a radioactive biproduct because of the extremely high amount of
   radiation that it contained.
     * ^90Sr, 2000 Bq m^-3
     * ^137Cs, 5000 Bq m^-3
     * ^239Pu, 1 Bq m^-3
     * ^241Am, 1 Bq m^-3

Soil

   Jiří Hála's text book states that soils vary greatly in their ability
   to bind radioisotopes, the clay particles and humic acids can alter the
   distribution of the isotopes between the soil water and the soil. The
   distribution coefficient K[d] is the ratio of the soil's radioactivity
   (Bq g^-1) to that of the soil water (Bq ml^-1). If the radioactivity is
   tightly bonded to by the minerals in the soil then less radioactivity
   can be absorbed by crops and grass growing on the soil.
     * Cs-137 K[d] = 1000
     * Pu-239 K[d] = 10000 to 100000
     * Sr-90 K[d] = 80 to 150
     * I-131 K[d] = 0.007 to 50

Food restrictions

   An abandoned village near Prypiat, close to Chernobyl
   Enlarge
   An abandoned village near Prypiat, close to Chernobyl

   In April 1986 several European countries, excluding France, had
   enforced food restrictions, most notably on mushrooms and milk. Twenty
   years after the catastrophe, restriction orders remain in place in the
   production, transportation and consumption of food contaminated by
   Chernobyl fallout, in particular caesium-137, in order to prevent them
   from entering the human food chain. In parts of Sweden and Finland,
   restrictions are in place on stock animals, including reindeer, in
   natural and near-natural environments. "In certain regions of Germany,
   Austria, Italy, Sweden, Finland, Lithuania and Poland, wild game,
   including boar and deer, wild mushrooms, berries and carnivore fish
   from lakes reach levels of several thousand Bq per kg of caesium-137",
   while "in Germany, caesium-137 levels in wild boar muscle reached
   40,000 Bq/kg. The average level is 6800 Bq/kg, more than ten times the
   EU limit of 600 Bq/kg", according to the TORCH 2006 report. The
   European Commission has stated that "The restrictions on certain
   foodstuffs from certain Member States must therefore continue to be
   maintained for many years to come".

   In the United Kingdom, under powers in the 1985 Food and Environment
   Protection Act (FEPA), Emergency Orders have been used since 1986 to
   impose restrictions on the movement and sale of sheep exceeding the
   limit of 1000 Bq/kg. This safety limit was introduced in the UK in 1986
   based on advice from the European Commission's Article 31 group of
   experts. However, the area covered by these restrictions has decreased
   by 95% since 1986: while it covered at first almost 9000 farms and over
   4 million sheep, as of 2006 it covers 374 farms covering 750 km^2 and
   200,000 sheep. Only limited areas of Cumbria, South Western Scotland
   and Northern Wales are still covered by restrictions.

   In Norway, the Sami people were affected by contaminated food. Their
   reindeer had been contaminated by eating lichens, which extract
   radioactive particles from the atmosphere along with their nutrients.

Fauna and vegetation

   After the disaster, four square kilometres of pine forest in the
   immediate vicinity of the reactor went 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. Mice embryos simply
   dissolved, while horses left on an island 6 km from the power plant
   died when their thyroid glands disintegrated. Cattle on the same island
   were stunted due to thyroid damage, but the next generation were found
   to be surprisingly 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 never seen in the area prior to the
   disaster, are now plentiful, due to the absence of humans in the area.

Chernobyl after the disaster

   The completed (but crumbling) sarcophagus surrounding Chernobyl reactor
   4, viewed from the northwest.
   Enlarge
   The completed (but crumbling) sarcophagus surrounding Chernobyl reactor
   4, viewed from the northwest.

   Following the accident, questions arose on the future of the plant and
   its eventual fate. All work on the unfinished reactors 5 and 6 were
   immediately halted. 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 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 15th,
   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 badly. If it collapses
   another cloud of radioactive dust could be released. The sarcophagus is
   so badly damaged 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.
   The radioactivity levels of different isotopes in the FCM, this has
   been back calculated by Russian workers to April 1986
   Enlarge
   The radioactivity levels of different isotopes in the FCM, this has
   been back calculated by Russian workers to April 1986

   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.

   It is unclear how long the ceramic form will retard the release of
   radioactivity. By conservative estimates, there is at least four tons
   of radioactive dust inside the shelter. However, more recent estimates
   have strongly questioned the previously held assumptions regarding the
   quantity of fuel remaining in the reactor. Some estimates now place the
   total quantity of fuel in the reactor at only about 70% of the original
   fuel load, however the IAEA maintains that less than 5% of the fuel was
   lost due to the explosion. Moreover, some liquidators estimate that
   only 5–10% of the original fuel load remains inside the sarcophagus.

   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, most of similar consistency to
   ash makes up a large 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.

Consequences of further collapse

   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 upon the structurally unsound west wall of the reactor
   building that was damaged by the accident. 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
   radiation 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

   It is known that fires can make the radioactivity mobile again.

   It has been reported by V.I. Yoschenko et. al., Journal of
   Environmental Radioactivity, 2006, 86, 143-163 that grass and forest
   fires can make the caesium, strontium, and plutonium become mobile in
   the air again. As an experiment, fires were set and the levels of the
   radioactivity in the air down wind of these fires was measured.
   The rate of delivery of radioactivity which has been made mobile by a
   grass fire. The distance unit is meters
   Enlarge
   The rate of delivery of radioactivity which has been made mobile by a
   grass fire. The distance unit is meters

The Chernobyl Fund and the Shelter Implementation Plan

   A conceptual rendering of the New Safe Confinement to replace the aging
   sarcophagus.
   Enlarge
   A conceptual rendering of the New Safe Confinement to replace the aging
   sarcophagus.

   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). The original
   cost estimate for the SIP was US$768 million. 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 will be the largest movable structure ever built,
   and is expected to be completed in early 2009.

   Dimensions:
     * Span: 270 m
     * Height: 100 m
     * Length: 150 m

Controversy over fatality estimates

   The majority of premature deaths caused by Chernobyl are expected to be
   the result of cancers and other diseases induced by radiation in the
   decades after the event. This will be the result of a large population
   (some studies have considered the entire population of Europe) exposed
   to relatively low doses of radiation increasing the risk of cancer
   across that population. It will be impossible to attribute specific
   deaths to Chernobyl, and many estimates indicate that the rate of
   excess deaths will be so small as to be statistically undetectable,
   even if the ultimate number of extra premature deaths is large.
   Furthermore, interpretations of the current health state of exposed
   population is subject vary. Therefore, estimates of the ultimate human
   impact of the disaster have relied on numerical models of the effects
   of radiation on health. Furthermore, the effects of low-level radiation
   on human health are not well understood, and so the models used,
   notably the linear no threshold model, are open to question.

   Given these factors, several different studies of Chernobyl's health
   effects have come up with substantially different conclusions and are
   the subject of considerable scientific and political controversy. The
   following section presents some of the major studies on this topic.

The Chernobyl Forum report

   In September 2005, a draft summary report by the Chernobyl Forum,
   comprising a number of UN agencies including the International Atomic
   Energy Agency (IAEA), the World Health Organization (WHO), the United
   Nations Development Programme (UNDP), other UN bodies and the
   Governments of Belarus, the Russian Federation and Ukraine, put the
   total predicted number of deaths due to the accident at 4000. This
   death toll predicted by the WHO included the 47 workers who died of
   acute radiation syndrome as a direct result of radiation from the
   disaster and nine children who died from thyroid cancer, in the
   estimated 4000 excess cancer deaths expected among the 600,000 with the
   highest levels of exposure. The full version of the WHO health effects
   report adopted by the UN, published in April 2006, included the
   prediction of 5000 additional fatalities from significantly
   contaminated areas in Belarus, Russia and Ukraine and predicted that,
   in total, 9000 will die from cancer among the 6.8 million most-exposed
   Soviet citizens. This report is not free of controversy, and has been
   accused of trying to minimize the consequences of the accident.

The TORCH report

   In 2006 German Green MEP (member of the European Parliament) Rebecca
   Harms, commissioned two UK scientists for an alternate report (TORCH
   ,The Other Report on Chernobyl) in response to the UN report. The
   report included areas not covered by the Chernobyl forum report, and
   also lower radiation doses. It predicted about 30,000 to 60,000 excess
   cancer deaths and warned that predictions of excess cancer deaths
   strongly depend on the risk factor used, and urged more research
   stating that large uncertainties made it difficult to properly asses
   the full scale of the disaster.

Greenpeace

   Greenpeace claimed contradictions in the Chernobyl Forum reports,
   quoting a 1998 WHO study referenced in the 2005 report, which projected
   212 dead from 72,000 liquidators. In its report, Greenpeace suggested
   there will be 270,000 cases of cancer attributable to Chernobyl
   fallout, and that 93,000 of these will probably be fatal, but state in
   their report 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.” Blake Lee-Harwood, campaigns director at Greenpeace, believes
   that cancer was likely to be the cause of less than half of the final
   fatalities and that "intestinal problems, heart and circulation
   problems, respiratory problems, endocrine problems, and particularly
   effects on the immune system," will also cause fatalities. However,
   concern has been expressed about the methods used in compiling the
   Greenpeace report.

The April 2006 IPPNW report

   According to an April 2006 report by the German affiliate of the
   International Physicians for Prevention of Nuclear Warfare (IPPNW),
   entitled "Health Effects of Chernobyl", more than 10,000 people are
   today affected by thyroid cancer and 50,000 cases are expected. The
   report projected tens of thousands dead among the liquidators. In
   Europe, it alleges that 10,000 deformities have been observed in
   newborns because of Chernobyl's radioactive discharge, with 5000 deaths
   among newborn children. They also claimed that several hundreds of
   thousands of the people who worked on the site after the accident are
   now sick because of radiation, and tens of thousands are dead.

Other studies and claims

     * The Ukrainian Health Minister claimed in 2006 that more than 2.4
       million Ukrainians, including 428,000 children, suffer from health
       problems related to the catastrophe. Psychological after-effects,
       as the 2006 UN report pointed out, have also had adverse effects on
       internally displaced persons.
     * Another study alleged heightened mortality in Sweden.
     * The UNSCEAR 2000 report on worldwide sources and effects of
       ionizing radiation, Volume II, Annex J is dedicated to exposures
       and effects of Chernobyl
     * According to the Union Chernobyl, the main organization of
       liquidators, 10% of the 600,000 liquidators are now dead, and
       165,000 disabled.
     * The Abstract of the April 2006 International Agency for Research on
       Cancer report Estimates of the cancer burden in Europe from
       radioactive fallout from the Chernobyl accident stated "It is
       unlikely that the cancer burden from the largest radiological
       accident to date could be detected by monitoring national cancer
       statistics. Indeed, results of analyses of time trends in cancer
       incidence and mortality in Europe do not, at present, indicate any
       increase in cancer rates - other than of thyroid cancer in the most
       contaminated regions - that can be clearly attributed to radiation
       from the Chernobyl accident." However, while undetectable, they
       estimate, based on the linear no threshold model of cancer effects,
       that 16,000 excess cancer deaths could be expected from the effects
       of the Chernobyl accident up to 2065. Their estimates have very
       wide 95% confidence intervals from 6,700 deaths to 38,000.
     * A report from the European Committee on Radiation Risk (a body
       sponsored by the European Green Party) claims that the World Health
       Organization, together with most other international and national
       health bodies, has marginalized or ignored, perhaps purposely, the
       terrible consequences of the Chernobyl fallout to protect the
       vested interests of the nuclear industry.
     * The application of the linear no threshold model to predict deaths
       from low levels of exposure to radiation was disputed in a BBC
       (British Broadcasting Corporation) “Horizon” documentary, broadcast
       on 13 July 2006. It offered statistical evidence to suggest that
       there is an exposure threshold of about 200 millisieverts below
       which there is no increase in radiation-induced disease. Indeed it
       went further, suggesting that low exposures to radiation can have a
       protective effect. The program interviewed scientists who believe
       that the increase in thyroid cancer in the immediate area of the
       explosion had been over-recorded, and predicted that the estimates
       for widespread deaths in the long term would be proved wrong. It
       also noted that whilst most cancers can take decades to manifest,
       Leukemia manifests within a decade or so, and none of the
       previously expected peak of leukemia deaths has been found, and
       none is now expected.

French legal action

   Since March 2001 400 lawsuits have been filed in France against 'X' by
   the French Association of Thyroid-affected People, including 200 in
   April 2006. These persons are affected by thyroid cancer or goitres,
   and have filed lawsuits alleging that the French government, at the
   time led by Prime Minister Jacques Chirac, had not adequately informed
   the population of the risks linked to the Chernobyl radioactive
   fallout. The complaint contrasts the health protection measures put in
   place in nearby countries (warning against consumption of green
   vegetables or milk by children and pregnant women) with the relatively
   high contamination suffered by the east of France and Corsica. Although
   the 2006 study by the French Institute of Radioprotection and Nuclear
   Safety said that no clear link could be found between Chernobyl and the
   increase of thyroid cancers in France, it also stated that papillary
   thyroid cancer had tripled in the following years.

Comparison with other disasters

   The Chernobyl disaster caused a few tens of immediate deaths due to
   radiation sickness; thousands of 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) — 171,000
       killed.
     * The Bhopal disaster (India, 1984) — 15,000 killed.
     * The Great Smog (London, England, 1952) — 12,000 killed.
     * The Johnstown Flood ( Pennsylvania, United States, 1889) — 2,209
       killed.

Chernobyl 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.

   In general the public knew little about radioactivity and radiation
   (they still know very little) 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 in Chernobyl ten years on 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.

   As a result of all these events, "Chernobyl" has entered the public
   consciousness.

   After the disaster, the American nuclear-powered aircraft carrier USS
   Enterprise, the first nuclear-powered capital ship, gained the
   unofficial nickname "Mobile Chernobyl."
   Retrieved from " http://en.wikipedia.org/wiki/Chernobyl_disaster"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
