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Wikipedia Accidant Kyiv
الأربعاء 6 يناير - 9:56
Kiev (Київ,
Kyiv, in Ukrainian; Киев,
Kiev, in Russian) is the capital and largest city of Ukraine, located in the north central part of
the country on the Dnieper river.
As of 2003, Kiev officially had 2,642,486 inhabitants,
although the large number of unregistered migrants would probably raise this figure to
about 4 million.
Administratively, Kiev is a national-level
subordinated municipality,
independent from surrounding Kyivs'ka oblast'.
However, the governing bodies of that oblast' are situated in the city.
Kiev is an
important industrial, scientific, educational and cultural center of Eastern Europe. It is home to many high-tech
industries, higher education
institutions, world-famous museums and art
institutions.
The city has an
extensive infrastructure and highly-developed system of public transport, including metro
system.
Historically, Kiev is one of the most ancient (probably
1550 years old) and important cities of the region, the center of Rus
civilization, survivor of numerous wars, purges
and genocides. Many historical and architectural landmarks are preserved or
reconstructed in the city.
Chernobyl accident
From
Wikipedia, the free encyclopedia.
The Chernobyl Nuclear power plant
The Chernobyl
accident which occurred on 26 April 1986
at the Chernobyl nuclear power plant in Ukraine (then part of the Soviet Union) is widely regarded as the worst in
the history of nuclear power
generation. It produced a plume of radioactive debris that drifted over parts of
the western USSR, Eastern Europe, and Scandinavia. Large areas of the Ukrainian, Belarusian, and Russian republics of the USSR were contaminated, resulting
in the evacuation and resettlement of roughly 200,000 people. About 60% of
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 separate countries of Russia,
Ukraine and Belarus have been burdened with continuing and
substantial costs for decontamination and health care because of the Chernobyl accident.
The plant
The Chernobyl station
is situated at the settlement of Pripyat, Ukraine, 11 miles (18 km) northwest of the city
of Chernobyl, 10 miles (16 km) from the border of
Ukraine and Belarus, and 70 miles (110 km) north of Kiev.
The station consisted of four reactors, each capable of
producing 1 GW of electric power (3.2
gigawatts of thermal power), and the four together produced about 10 percent 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 No. 6, also capable of producing 1 gigawatt each)
were under construction at the time of the accident.
The four plants were
designed as a type of reactor called RBMK-1000.
The accident
On Saturday, April 26, 1986
at 1:23:58 am local time, the fourth reactor of the Chernobyl power plant - known as
Chernobyl-4 - suffered a catastrophic steam explosion that resulted in a fire, a
series of additional explosions, and a nuclear meltdown.
Causes
The catastrophe is
attributed to a flawed reactor design and mistakes made by the plant operators,
who violated procedures intended to ensure safe operation of the plant. As at Three Mile Island, a secondary factor
contributing to the accident was the fact that plant operators were
insufficiently trained and unfamiliar with many characteristics of the reactor.
Several procedural
irregularities contributed to the cause of the accident. One was insufficient
communication between the safety officers and the operators in charge of an
experiment being run that night. Moreover, because of insufficient training,
the operators had only a very imperfect understanding of how the reactor worked
under the low amount of reactivity caused by the experiment. Many of the
reactor's engineering features, such as the dangerously large positive void
coefficient (see below), were virtually treated as military secrets, and the
operators were unaware of them. In addition, several safety systems were
bypassed and ignored in order to conduct the experiment.
Events
The reactor was
undergoing an experiment to test the electrical backup supply which allows the
reactor to run safely during a power loss. The power output of the reactor was
reduced from its normal capacity of 3.2 to 1 GW in order to conduct the test at
a safer, low power. The actual power output fell to 30 MW, however, allowing
the concentration of the neutron absorbing fission product xenon-135
to rise; this product is typically consumed in a reactor under higher power
conditions. As the operators attempted to restore the power to the desired 1
GW, the concentration of Xenon-135 limited the power output to around 200 MW.
In order to overcome the neutron absorption of the Xenon-135, the control rods were pulled out of the reactor
farther than normally allowed under safety regulations.
As the coolant flow
was decreased, the coolant was heated rapidly so that much of it began to boil.
As the coolant heated, pockets of steam formed in the coolant lines. The
particular design of the RBMK graphite moderated reactor
at Chernobyl
has a large positive void coefficient,
which means that the power of the reactor increases rapidly in the absence of
the coolant. As the power output rapidly increased the operators attempted to shut
down the reactor by ordering an emergency manual "scram"--or rapid
full insertion of the control rods. However due to the slow speeds of the
control rod insertion mechanism, the graphite tips of the rods and the
temporary displacement of coolant the scram actually caused the reaction rate
to increase. Within seconds 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, displacing and destroying the
reactor lid, rupturing the coolant tubes and then blowing a hole in the roof.
When outside air contacted the graphite moderator of the core, the graphite began
to burn. The fire dispersed most of the radioactive
contamination.
To reduce costs, and
because of its large size, the reactor only had partial containment. This
allowed the radioactive contaminants to escape into the atmosphere after the
steam explosion burst the primary pressure vessel. 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.
Immediate crisis management
In order to try to
limit the scale of the disaster, the Soviet government immediately sent in
workers to try to clean up. Firefighters were sent in to try to extinguish the
fires, although they were not told how dangerously radioactive the smoke was.
In the next months, many "liquidators" - members of the army and
other workers - were sent in as cleanup staff, and again, most were not told
anything about the danger. Effective protective gear was unavailable. The worst
of the radioactive debris was collected inside what was left of the reactor, and
a large steel sarcophagus was hastily erected to seal off the reactor and its
contents.
Immediate
results
203 people were
hospitalized immediately, of whom 31 died (28 of them died from acute radiation
exposure). 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. 135,000
people were evacuated from the area, including 50,000 from the nearby town of Pripyat, Ukraine. Health officials have
predicted that over the next 70 years there will be a 2% increase in cancer
rates in much of the population which was exposed to the 5-12 (depending on
source) EBq of radioactive contamination
released from the reactor. An additional 10 individuals have already died of
cancer as a result of the accident.
In January 1993,
the IAEA issued a revised analysis of the Chernobyl accident,
attributing the main root cause to the reactor's design and not to operator
error. The IAEA's 1986 analysis had cited the operators' actions
as the principal cause of the accident.
Soviet scientists have reported that the
Chernobyl Unit 4 reactor contained about 190 metric tons of uranium dioxide
fuel and fission products. Estimates of the amount of this material that
escaped range from 13 percent to 30 percent.
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
percent of the contamination that fell on the former Soviet
Union. But a large area in the Russian Federation
south of Bryansk was also contaminated, as were parts of
northwestern Ukraine.
Chernobyl was a secret disaster at first. The
initial evidence that a major nuclear accident had occurred came not from
Soviet sources, but from Sweden, where on April 27 workers at a nuclear power plant were
found to have radioactive particles on their clothes. It was Sweden's search for the source of radioactivity,
after they had determined there was no leak at the Swedish plant, that led to
the first hint of a nuclear problem in the Soviet Union.
Short-term
impact
Workers and liquidators
Workers involved in
the recovery and cleanup after the accident received high doses of radiation.
In most cases, these workers were not equipped with individual dosimeters to measure the amount of radiation
received, so experts can only estimate their doses. Even where dosimeters were
used, dosimetric procedures varied. Some workers are thought to have been given
more accurate estimated doses than others. According to Soviet estimates,
between 300,000 and 600,000 people were involved in the cleanup of the 30 km evacuation zone around
the reactor, but many of them entered the zone two years after the accident.
(Estimates of the number of "liquidators" - workers brought into the
area for accident management and recovery work - vary; the World Health
Organization, for example, puts the figure at about 800,000, also Russia lists as
liquidators some people who did not work in contaminated areas.) In the first
year after the accident, the number of cleanup workers in the zone was
estimated to be 211,000, and these workers received an estimated average dose
of 165 millisievert (16.5 rem).
Civilians
Some children in the
contaminated areas were exposed to high thyroid doses up to 50 gray (Gy) because of an
intake of radioactive iodine, a relatively
short-lived isotope, from contaminated local milk. Several studies have found
that the incidence of thyroid cancer
among children in Belarus, Ukraine and Russia has risen sharply. The IAEA
notes "1800 documented cases of thyroid cancer in children who were
between 0 and 14 years of age when the accident occurred, which is far higher
than normal" but fails to note the expected rate. The childhood thyroid
cancers that have appeared are of a large and aggressive type, and if detected
early, can be treated. Treatment entails surgery followed by iodine-131 therapy
for any metastases. To date, such treatment appears to
have been successful in all diagnosed cases.
Longer-term
impact
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 and mushrooms and enter the local food
supply. Recent tests have shown that caesium-137 levels in trees of the area
are continuing to rise. The main source of elimination is predicted to be
natural decay of caesium-137 to stable barium-137, since runoff by rain and groundwater has been
demonstrated to be negligible.
Global impact
The IAEA notes that,
while the Chernobyl accident released as much as
400 times the radioactive contamination of the Hiroshima bomb, it was 100 to 1000 times less
than the contamination caused by atmospheric nuclear weapons testing in the
mid-20th century. One can conclude that while the Chernobyl accident was a local disaster, it
was not a global one.
Impact on the natural world
According to reports
from Soviet scientists at the First International Conference on the Biological
and Radiological Aspects of the Chernobyl Accident (September 1990),
fallout levels in the 10 km
zone around the plant were as high as 4.81 GBq/m². The so-called "red forest" of
pine trees killed by heavy radioactive fallout lies immediately behind the
reactor complex within the 10
km zone. The "red forest" covered some 4 km²
and only pine trees died while birch and aspen survived. The "red
forest" is so named because it was reported by evacuees that in the days
following the accident the trees glowed red, apparently due to heavy
radioactive fallout.
Evacuation
Soviet authorities
started evacuating people from the area around Chernobyl within 36 hours of the accident. By
May
1986, about a month later, all those living within a 30 km (18 mile) radius of the
plant-- about 116,000 people-- had been relocated.
According to reports
from Soviet scientists, 28,000 km² (10,800 mile²) were contaminated by caesium-137 to levels greater than 185 kBq/m².
Roughly 830,000 people lived in this area. About 10,500 km ² (4,000 mile²)
were contaminated by caesium-137 to levels greater than 555 kBq/m². Of this
total, roughly 7,000 km² (2,700 square miles) lie in Belarus, 2,000 km² (800
square miles) in the Russian Federation and 1,500 km² (580 square miles) in
Ukraine. About 250,000 people lived in this area. These reported data were
corroborated by the International
Chernobyl Project.
Comparison with other disasters
The Chernobyl accident was a unique event, on a
scale by itself. It was the first time in the history of commercial nuclear
electricity generation that radiation-related fatalities occurred. (note: an
accident at the Japanese Tokaimura nuclear fuel reprocessing plant on September
30, 1999 resulted in the radiation related death of one worker on December 22
of that same year)
Long-term effects on civilians
Epidemiological
studies have been hampered in the former Soviet Union
by a lack of funds, an infrastructure with little or no experience in chronic
disease epidemiology, poor communication facilities and
an immediate public health problem with many dimensions. Emphasis has been
placed on screening rather than on well-designed epidemiological studies.
International efforts to organize epidemiological studies have been slowed by
some of the same factors, especially the lack of a suitable scientific
infrastructure.
An increased
incidence of thyroid cancer among children in areas of Belarus, Ukraine
and Russia affected by the Chernobyl accident has been firmly established as a result
of screening programs and, in the case of Belarus, an established cancer registry. The findings of most
epidemiological studies must be considered interim, say experts, as analysis of
the health effects of the accident is an ongoing process.
The activities
undertaken by Belarus and Ukraine in response to the accident--remediation of
the environment, evacuation and resettlement, development of noncontaminated
food sources and food distribution channels, and public health measures-- have
overburdened the governments of those countries. International agencies and
foreign governments have provided extensive logistic and humanitarian
assistance. In addition, the work of the European Commission
and World Health Organization in strengthening the epidemiological research
infrastructure in Russia, Ukraine and Belarus is laying the basis for
major advances in these countries' ability to carry out epidemiological studies
of all kinds.
Wildlife
In marked contrast to the human
cost, the evacuation of the area surrounding the plant has created a lush and
unique wildlife refuge. It is unknown whether fallout contamination will have
any long-term adverse affect on the flora and fauna of the region, as plants
and animals have significantly different and varying radiologic tolerance
compared with humans. However, it seems that the biodiversity around the
massive radiation spill has increased due to the removal of human influence
(see the first hand account of the wildlife preserve . There are
reports of mutations in some plants in the area, leading to unsubstantiated
tales of a "forest of wonders" containing many strangely mutated
plants. Also, the area is reported to be silent, suggesting that birds have not
yet recolonised it.
Chernobyl after the
accident
The trouble at the Chernobyl plant itself
did not end with the disaster in Reactor No. 4. 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 No. 2 in 1991; the authorities
subsequently declared the reactor damaged beyond repair and had it taken
offline. Reactor No. 1 was decomissioned 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. In November 2000, Ukrainian President Leonid Kuchma personally turned off the switch
to Reactor No. 3 in
an official ceremony, effectively shutting down the entire plant.
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,
means it is aging badly, and if it collapses, another cloud of radioactive dust
could be released. A number of plans have been discussed for building a more
permanent enclosure, but all of the plans so far have been too costly and
dangerous to be attempted.
Chernobyl in the popular consciousness
The Chernobyl accident riveted international
attention. Around the world, people read the story and were profoundly
affected. As a result, "Chernobyl"
has entered the public consciousness in a number of different ways.
Political
Outcome
The Chernobyl accident was clearly a major
disaster, and it received worldwide media attention. Public awareness of the
risks of nuclear power increased signficantly. Organizations, both pro- and
anti-nuclear, have made great efforts to sway public opinion. Casualty figures,
reactor safety estimates, and estimates of the risks associated to other
reactors differ greatly depending on which position is favored by the author of
any given document. For example, the UN scientific committee on the effects of
radiation has publicly criticized the UN office on humanitarian affairs with
respect to some of its publications. The true facts of the affair are therefore
rather difficult to uncover.
Chernobyl and the
Bible
Because of a controversial translation of "chernobyl"
as wormwood, an urban myth started among English-speaking people
(for evident reasons, these people have been assumed to be Christians) that the Chernobyl accident was
mentioned in the Bible:
And the third angel sounded, and there fell a great star from heaven,
burning as it were a lamp, and it fell upon the third part of the rivers, and
upon the fountains of waters; and the name of the star is called Wormwood: and
the third part of the waters became wormwood; and many men died of the waters,
because they were made bitter. -- Revelation 8:10-11
The story appears to
have originated - or at least spread to the West - with a New York Times
article by Serge Schmemann (Chernobyl Fallout: Apocalyptic Tale, July
25, 1986) in which an unnamed "prominent Russian writer" was quoted
as claiming the Ukrainian word for wormwood was chernobyl.
Computer
Virus
The CIH computer virus was given the name
"Chernobyl Virus" by many in the media, since the v1.2 variant
activates on April 26 of each year.
From
Wikipedia, the free encyclopedia.
On October 10, 1957,
the graphite core of a British nuclear reactor at the Windscale site, near Sellafield, caught fire, releasing substantial
amounts of radioactive
contamination into the surrounding area. It was considered the worst
nuclear accident in the world until it was dwarfed by the Chernobyl accident
in 1987.
The reactor
After the end of the Second World War, in 1946, in spite of the
participation of many British scientists in the Manhattan Project, the United States of
America closed its nuclear program to all other countries. The
British government embarked on a race to build the "British Bomb" as
quickly as possible. For a full nuclear weapons program, it is necessary to be
able to produce plutonium in quantity. The
British government therefore built, as quickly as possible, a nuclear reactor. The Windscale site was chosen
because of the ready availability of cooling water, the pre-existing buildings,
and the distance from large population centers should an accident occur.
Two reactors were
built at the Windscale site, to the same basic design, called "Windscale
Pile no. 1" and "Windscale Pile no. 2". The reactors were graphite-moderated and air-cooled. The air was
filtered (to remove stray radioactive materials) and then released through
large smokestacks. The reactors had horizontal channels through which cans of
unenriched uranium and lithium could be passed to expose them to neutron radiation and produce plutonium and tritium, respectively.
When the reactors
were built, little was known about the behaviour of graphite when exposed to
neutron radiation. Both reactors experienced unplanned rises in core
temperature, which were ultimately determined to be caused by sudden releases
of Wigner energy. This energy is stored as
dislocations in the crystal structure of graphite, caused by exposure to neutron
radiation. When enough accumulates, it is suddenly released. As a safety
measure, it was necessary to anneal the graphite before
enough Wigner energy accumulated to spontaneously release. The annealing
process was simple: the temperature was allowed to rise to a point where the
graphite was hot enough for the Wigner energy to be released. Annealing
succeeded in preventing the buildup of Wigner energy, but it was poorly
understood and each annealing cycle was different. The annealing cycles were
also growing more difficult; many of the later cycles had to be repeated.
Because they were
built hastily and during a time when little was known about reactor design, the
reactors had a number of dangerous features. Since graphite is flammable in
air, and air was being fed to the reactors constantly for cooling purposes,
fire was a constant danger. Since the cooling air was vented to the atmosphere,
any radioactive material released by the core had only to slip through the
filters to be released into the countryside. Since the annealing phases were
not planned, there were not enough thermocouples to monitor the core temperature
accurately. What thermocouples there were were placed in the zones of maximum
temperature under normal operation of the reactor, but these were not
necessarily the hottest zones while annealing was going on. The fuel was
uranium metal, rather than uranium dioxide, so its melting point was lower.
The accident
On October 7,
operators began an annealing cycle for Windscale Pile no. 1 by shutting off the
cooling systems and setting the reactor to low power. The temperature sensors
indicated a falling (rather than rising) temperature. Although the operators
did not realize, the temperature was indeed rising, but in a part of the pile
not measured by the thermocouples. The next day, to carry out the annealing,
the operators increased the power to the reactor. The reactor temperature
increased further until the reactor was hot enough to catch fire. It is not
known exactly what started the fire. It may have been simple overheating, or a
can of fuel or lithium may have burst. All that was visible on the instruments
was a small variability in temperature.
On October 10, air
samplers about a kilometer away detected a rise (to 10 times the usual value)
of radioactivity in the air. Operators tried to examine the pile with a remote
scanner but it had jammed. Operators donned protective gear and tried to exmine
the reactor, only to discover that it was red hot; the fuel rods had melted and
the reactor had been ablaze for nearly two days.
The fire carried a
great deal of radioactive material out of the pile; the filters could remove
only a small fraction of this material, and the rest was carried out the
smokestack to fall as radioactive
contamination on the surrounding countryside.
Operators were unsure
what to do about the fire. Men were sent with sticks to push fuel cans out of
the reactor in the hopes that this would help cool it. The reactor was flushed
with carbon dioxide, but
the temperature was so high this proved ineffective.
On the morning of
Friday October 11, temperatures were rising 20 degrees per minute, and
operators decided to try showering the pile with water. While this might extinguish
the fire, it might also evolve hydrogen and acetylene gases, which could then lead to an explosion. In the event, the gamble paid off;
the fire came under control, although a cloud of radioactive steam was released
to drift across all of England
and over to Europe.
The aftermath
The fire itself released
an estimated 20000 curies (700 terabecquerels) of radioactive material into the
nearby countryside. Of particular concern was the radioactive isotope iodine-131, which has a half-life of only 8 days but is taken up by the
human body and stored in the thyroid. As a result,
consumption of iodine-131 often leads to cancer of the thyroid.
No one was evacuated
from the surrounding area, but there was concern that milk
might be dangerously contaminated. Milk from about 500km2 of nearby
countryside was destroyed (by dumping in local rivers) for about a month.
The reactor was
unsalvageable; all the fuel rods that could be removed were, and the reactor
was buried in concrete. Approximately 6700 fire-damaged fuel elements and 1700
fire-damaged isotope canisters remain in the pile. The damaged reactor core is
still heated by continuing nuclear reactions. Windscale Pile no. 2, though
undamaged by the fire, was considered too unsafe for continued use. It was shut
down shortly afterward. No air-cooled reactors have been built since.
The Windscale site
was decontaminated and
is still in use; several more modern nuclear reactors are there now. To avoid
bad associations, the UKAEA renamed the site Sellafield.
In the 1990s, the UKAEA began
plans to decommission (disassemble and clean up) both piles; the
decommissioning is now partially complete. Plans are being explored to safely
remove the fire-damaged core, which is still radioactive and could burst into
flame.
Kyiv, in Ukrainian; Киев,
Kiev, in Russian) is the capital and largest city of Ukraine, located in the north central part of
the country on the Dnieper river.
As of 2003, Kiev officially had 2,642,486 inhabitants,
although the large number of unregistered migrants would probably raise this figure to
about 4 million.
Administratively, Kiev is a national-level
subordinated municipality,
independent from surrounding Kyivs'ka oblast'.
However, the governing bodies of that oblast' are situated in the city.
Kiev is an
important industrial, scientific, educational and cultural center of Eastern Europe. It is home to many high-tech
industries, higher education
institutions, world-famous museums and art
institutions.
The city has an
extensive infrastructure and highly-developed system of public transport, including metro
system.
Historically, Kiev is one of the most ancient (probably
1550 years old) and important cities of the region, the center of Rus
civilization, survivor of numerous wars, purges
and genocides. Many historical and architectural landmarks are preserved or
reconstructed in the city.
Chernobyl accident
From
Wikipedia, the free encyclopedia.
The Chernobyl Nuclear power plant
The Chernobyl
accident which occurred on 26 April 1986
at the Chernobyl nuclear power plant in Ukraine (then part of the Soviet Union) is widely regarded as the worst in
the history of nuclear power
generation. It produced a plume of radioactive debris that drifted over parts of
the western USSR, Eastern Europe, and Scandinavia. Large areas of the Ukrainian, Belarusian, and Russian republics of the USSR were contaminated, resulting
in the evacuation and resettlement of roughly 200,000 people. About 60% of
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 separate countries of Russia,
Ukraine and Belarus have been burdened with continuing and
substantial costs for decontamination and health care because of the Chernobyl accident.
The plant
The Chernobyl station
is situated at the settlement of Pripyat, Ukraine, 11 miles (18 km) northwest of the city
of Chernobyl, 10 miles (16 km) from the border of
Ukraine and Belarus, and 70 miles (110 km) north of Kiev.
The station consisted of four reactors, each capable of
producing 1 GW of electric power (3.2
gigawatts of thermal power), and the four together produced about 10 percent 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 No. 6, also capable of producing 1 gigawatt each)
were under construction at the time of the accident.
The four plants were
designed as a type of reactor called RBMK-1000.
The accident
On Saturday, April 26, 1986
at 1:23:58 am local time, the fourth reactor of the Chernobyl power plant - known as
Chernobyl-4 - suffered a catastrophic steam explosion that resulted in a fire, a
series of additional explosions, and a nuclear meltdown.
Causes
The catastrophe is
attributed to a flawed reactor design and mistakes made by the plant operators,
who violated procedures intended to ensure safe operation of the plant. As at Three Mile Island, a secondary factor
contributing to the accident was the fact that plant operators were
insufficiently trained and unfamiliar with many characteristics of the reactor.
Several procedural
irregularities contributed to the cause of the accident. One was insufficient
communication between the safety officers and the operators in charge of an
experiment being run that night. Moreover, because of insufficient training,
the operators had only a very imperfect understanding of how the reactor worked
under the low amount of reactivity caused by the experiment. Many of the
reactor's engineering features, such as the dangerously large positive void
coefficient (see below), were virtually treated as military secrets, and the
operators were unaware of them. In addition, several safety systems were
bypassed and ignored in order to conduct the experiment.
Events
The reactor was
undergoing an experiment to test the electrical backup supply which allows the
reactor to run safely during a power loss. The power output of the reactor was
reduced from its normal capacity of 3.2 to 1 GW in order to conduct the test at
a safer, low power. The actual power output fell to 30 MW, however, allowing
the concentration of the neutron absorbing fission product xenon-135
to rise; this product is typically consumed in a reactor under higher power
conditions. As the operators attempted to restore the power to the desired 1
GW, the concentration of Xenon-135 limited the power output to around 200 MW.
In order to overcome the neutron absorption of the Xenon-135, the control rods were pulled out of the reactor
farther than normally allowed under safety regulations.
As the coolant flow
was decreased, the coolant was heated rapidly so that much of it began to boil.
As the coolant heated, pockets of steam formed in the coolant lines. The
particular design of the RBMK graphite moderated reactor
at Chernobyl
has a large positive void coefficient,
which means that the power of the reactor increases rapidly in the absence of
the coolant. As the power output rapidly increased the operators attempted to shut
down the reactor by ordering an emergency manual "scram"--or rapid
full insertion of the control rods. However due to the slow speeds of the
control rod insertion mechanism, the graphite tips of the rods and the
temporary displacement of coolant the scram actually caused the reaction rate
to increase. Within seconds 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, displacing and destroying the
reactor lid, rupturing the coolant tubes and then blowing a hole in the roof.
When outside air contacted the graphite moderator of the core, the graphite began
to burn. The fire dispersed most of the radioactive
contamination.
To reduce costs, and
because of its large size, the reactor only had partial containment. This
allowed the radioactive contaminants to escape into the atmosphere after the
steam explosion burst the primary pressure vessel. 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.
Immediate crisis management
In order to try to
limit the scale of the disaster, the Soviet government immediately sent in
workers to try to clean up. Firefighters were sent in to try to extinguish the
fires, although they were not told how dangerously radioactive the smoke was.
In the next months, many "liquidators" - members of the army and
other workers - were sent in as cleanup staff, and again, most were not told
anything about the danger. Effective protective gear was unavailable. The worst
of the radioactive debris was collected inside what was left of the reactor, and
a large steel sarcophagus was hastily erected to seal off the reactor and its
contents.
Immediate
results
203 people were
hospitalized immediately, of whom 31 died (28 of them died from acute radiation
exposure). 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. 135,000
people were evacuated from the area, including 50,000 from the nearby town of Pripyat, Ukraine. Health officials have
predicted that over the next 70 years there will be a 2% increase in cancer
rates in much of the population which was exposed to the 5-12 (depending on
source) EBq of radioactive contamination
released from the reactor. An additional 10 individuals have already died of
cancer as a result of the accident.
In January 1993,
the IAEA issued a revised analysis of the Chernobyl accident,
attributing the main root cause to the reactor's design and not to operator
error. The IAEA's 1986 analysis had cited the operators' actions
as the principal cause of the accident.
Soviet scientists have reported that the
Chernobyl Unit 4 reactor contained about 190 metric tons of uranium dioxide
fuel and fission products. Estimates of the amount of this material that
escaped range from 13 percent to 30 percent.
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
percent of the contamination that fell on the former Soviet
Union. But a large area in the Russian Federation
south of Bryansk was also contaminated, as were parts of
northwestern Ukraine.
Chernobyl was a secret disaster at first. The
initial evidence that a major nuclear accident had occurred came not from
Soviet sources, but from Sweden, where on April 27 workers at a nuclear power plant were
found to have radioactive particles on their clothes. It was Sweden's search for the source of radioactivity,
after they had determined there was no leak at the Swedish plant, that led to
the first hint of a nuclear problem in the Soviet Union.
Short-term
impact
Workers and liquidators
Workers involved in
the recovery and cleanup after the accident received high doses of radiation.
In most cases, these workers were not equipped with individual dosimeters to measure the amount of radiation
received, so experts can only estimate their doses. Even where dosimeters were
used, dosimetric procedures varied. Some workers are thought to have been given
more accurate estimated doses than others. According to Soviet estimates,
between 300,000 and 600,000 people were involved in the cleanup of the 30 km evacuation zone around
the reactor, but many of them entered the zone two years after the accident.
(Estimates of the number of "liquidators" - workers brought into the
area for accident management and recovery work - vary; the World Health
Organization, for example, puts the figure at about 800,000, also Russia lists as
liquidators some people who did not work in contaminated areas.) In the first
year after the accident, the number of cleanup workers in the zone was
estimated to be 211,000, and these workers received an estimated average dose
of 165 millisievert (16.5 rem).
Civilians
Some children in the
contaminated areas were exposed to high thyroid doses up to 50 gray (Gy) because of an
intake of radioactive iodine, a relatively
short-lived isotope, from contaminated local milk. Several studies have found
that the incidence of thyroid cancer
among children in Belarus, Ukraine and Russia has risen sharply. The IAEA
notes "1800 documented cases of thyroid cancer in children who were
between 0 and 14 years of age when the accident occurred, which is far higher
than normal" but fails to note the expected rate. The childhood thyroid
cancers that have appeared are of a large and aggressive type, and if detected
early, can be treated. Treatment entails surgery followed by iodine-131 therapy
for any metastases. To date, such treatment appears to
have been successful in all diagnosed cases.
Longer-term
impact
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 and mushrooms and enter the local food
supply. Recent tests have shown that caesium-137 levels in trees of the area
are continuing to rise. The main source of elimination is predicted to be
natural decay of caesium-137 to stable barium-137, since runoff by rain and groundwater has been
demonstrated to be negligible.
Global impact
The IAEA notes that,
while the Chernobyl accident released as much as
400 times the radioactive contamination of the Hiroshima bomb, it was 100 to 1000 times less
than the contamination caused by atmospheric nuclear weapons testing in the
mid-20th century. One can conclude that while the Chernobyl accident was a local disaster, it
was not a global one.
Impact on the natural world
According to reports
from Soviet scientists at the First International Conference on the Biological
and Radiological Aspects of the Chernobyl Accident (September 1990),
fallout levels in the 10 km
zone around the plant were as high as 4.81 GBq/m². The so-called "red forest" of
pine trees killed by heavy radioactive fallout lies immediately behind the
reactor complex within the 10
km zone. The "red forest" covered some 4 km²
and only pine trees died while birch and aspen survived. The "red
forest" is so named because it was reported by evacuees that in the days
following the accident the trees glowed red, apparently due to heavy
radioactive fallout.
Evacuation
Soviet authorities
started evacuating people from the area around Chernobyl within 36 hours of the accident. By
May
1986, about a month later, all those living within a 30 km (18 mile) radius of the
plant-- about 116,000 people-- had been relocated.
According to reports
from Soviet scientists, 28,000 km² (10,800 mile²) were contaminated by caesium-137 to levels greater than 185 kBq/m².
Roughly 830,000 people lived in this area. About 10,500 km ² (4,000 mile²)
were contaminated by caesium-137 to levels greater than 555 kBq/m². Of this
total, roughly 7,000 km² (2,700 square miles) lie in Belarus, 2,000 km² (800
square miles) in the Russian Federation and 1,500 km² (580 square miles) in
Ukraine. About 250,000 people lived in this area. These reported data were
corroborated by the International
Chernobyl Project.
Comparison with other disasters
The Chernobyl accident was a unique event, on a
scale by itself. It was the first time in the history of commercial nuclear
electricity generation that radiation-related fatalities occurred. (note: an
accident at the Japanese Tokaimura nuclear fuel reprocessing plant on September
30, 1999 resulted in the radiation related death of one worker on December 22
of that same year)
Long-term effects on civilians
Epidemiological
studies have been hampered in the former Soviet Union
by a lack of funds, an infrastructure with little or no experience in chronic
disease epidemiology, poor communication facilities and
an immediate public health problem with many dimensions. Emphasis has been
placed on screening rather than on well-designed epidemiological studies.
International efforts to organize epidemiological studies have been slowed by
some of the same factors, especially the lack of a suitable scientific
infrastructure.
An increased
incidence of thyroid cancer among children in areas of Belarus, Ukraine
and Russia affected by the Chernobyl accident has been firmly established as a result
of screening programs and, in the case of Belarus, an established cancer registry. The findings of most
epidemiological studies must be considered interim, say experts, as analysis of
the health effects of the accident is an ongoing process.
The activities
undertaken by Belarus and Ukraine in response to the accident--remediation of
the environment, evacuation and resettlement, development of noncontaminated
food sources and food distribution channels, and public health measures-- have
overburdened the governments of those countries. International agencies and
foreign governments have provided extensive logistic and humanitarian
assistance. In addition, the work of the European Commission
and World Health Organization in strengthening the epidemiological research
infrastructure in Russia, Ukraine and Belarus is laying the basis for
major advances in these countries' ability to carry out epidemiological studies
of all kinds.
Wildlife
In marked contrast to the human
cost, the evacuation of the area surrounding the plant has created a lush and
unique wildlife refuge. It is unknown whether fallout contamination will have
any long-term adverse affect on the flora and fauna of the region, as plants
and animals have significantly different and varying radiologic tolerance
compared with humans. However, it seems that the biodiversity around the
massive radiation spill has increased due to the removal of human influence
(see the first hand account of the wildlife preserve . There are
reports of mutations in some plants in the area, leading to unsubstantiated
tales of a "forest of wonders" containing many strangely mutated
plants. Also, the area is reported to be silent, suggesting that birds have not
yet recolonised it.
Chernobyl after the
accident
The trouble at the Chernobyl plant itself
did not end with the disaster in Reactor No. 4. 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 No. 2 in 1991; the authorities
subsequently declared the reactor damaged beyond repair and had it taken
offline. Reactor No. 1 was decomissioned 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. In November 2000, Ukrainian President Leonid Kuchma personally turned off the switch
to Reactor No. 3 in
an official ceremony, effectively shutting down the entire plant.
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,
means it is aging badly, and if it collapses, another cloud of radioactive dust
could be released. A number of plans have been discussed for building a more
permanent enclosure, but all of the plans so far have been too costly and
dangerous to be attempted.
Chernobyl in the popular consciousness
The Chernobyl accident riveted international
attention. Around the world, people read the story and were profoundly
affected. As a result, "Chernobyl"
has entered the public consciousness in a number of different ways.
Political
Outcome
The Chernobyl accident was clearly a major
disaster, and it received worldwide media attention. Public awareness of the
risks of nuclear power increased signficantly. Organizations, both pro- and
anti-nuclear, have made great efforts to sway public opinion. Casualty figures,
reactor safety estimates, and estimates of the risks associated to other
reactors differ greatly depending on which position is favored by the author of
any given document. For example, the UN scientific committee on the effects of
radiation has publicly criticized the UN office on humanitarian affairs with
respect to some of its publications. The true facts of the affair are therefore
rather difficult to uncover.
Chernobyl and the
Bible
Because of a controversial translation of "chernobyl"
as wormwood, an urban myth started among English-speaking people
(for evident reasons, these people have been assumed to be Christians) that the Chernobyl accident was
mentioned in the Bible:
And the third angel sounded, and there fell a great star from heaven,
burning as it were a lamp, and it fell upon the third part of the rivers, and
upon the fountains of waters; and the name of the star is called Wormwood: and
the third part of the waters became wormwood; and many men died of the waters,
because they were made bitter. -- Revelation 8:10-11
The story appears to
have originated - or at least spread to the West - with a New York Times
article by Serge Schmemann (Chernobyl Fallout: Apocalyptic Tale, July
25, 1986) in which an unnamed "prominent Russian writer" was quoted
as claiming the Ukrainian word for wormwood was chernobyl.
Computer
Virus
The CIH computer virus was given the name
"Chernobyl Virus" by many in the media, since the v1.2 variant
activates on April 26 of each year.
From
Wikipedia, the free encyclopedia.
On October 10, 1957,
the graphite core of a British nuclear reactor at the Windscale site, near Sellafield, caught fire, releasing substantial
amounts of radioactive
contamination into the surrounding area. It was considered the worst
nuclear accident in the world until it was dwarfed by the Chernobyl accident
in 1987.
The reactor
After the end of the Second World War, in 1946, in spite of the
participation of many British scientists in the Manhattan Project, the United States of
America closed its nuclear program to all other countries. The
British government embarked on a race to build the "British Bomb" as
quickly as possible. For a full nuclear weapons program, it is necessary to be
able to produce plutonium in quantity. The
British government therefore built, as quickly as possible, a nuclear reactor. The Windscale site was chosen
because of the ready availability of cooling water, the pre-existing buildings,
and the distance from large population centers should an accident occur.
Two reactors were
built at the Windscale site, to the same basic design, called "Windscale
Pile no. 1" and "Windscale Pile no. 2". The reactors were graphite-moderated and air-cooled. The air was
filtered (to remove stray radioactive materials) and then released through
large smokestacks. The reactors had horizontal channels through which cans of
unenriched uranium and lithium could be passed to expose them to neutron radiation and produce plutonium and tritium, respectively.
When the reactors
were built, little was known about the behaviour of graphite when exposed to
neutron radiation. Both reactors experienced unplanned rises in core
temperature, which were ultimately determined to be caused by sudden releases
of Wigner energy. This energy is stored as
dislocations in the crystal structure of graphite, caused by exposure to neutron
radiation. When enough accumulates, it is suddenly released. As a safety
measure, it was necessary to anneal the graphite before
enough Wigner energy accumulated to spontaneously release. The annealing
process was simple: the temperature was allowed to rise to a point where the
graphite was hot enough for the Wigner energy to be released. Annealing
succeeded in preventing the buildup of Wigner energy, but it was poorly
understood and each annealing cycle was different. The annealing cycles were
also growing more difficult; many of the later cycles had to be repeated.
Because they were
built hastily and during a time when little was known about reactor design, the
reactors had a number of dangerous features. Since graphite is flammable in
air, and air was being fed to the reactors constantly for cooling purposes,
fire was a constant danger. Since the cooling air was vented to the atmosphere,
any radioactive material released by the core had only to slip through the
filters to be released into the countryside. Since the annealing phases were
not planned, there were not enough thermocouples to monitor the core temperature
accurately. What thermocouples there were were placed in the zones of maximum
temperature under normal operation of the reactor, but these were not
necessarily the hottest zones while annealing was going on. The fuel was
uranium metal, rather than uranium dioxide, so its melting point was lower.
The accident
On October 7,
operators began an annealing cycle for Windscale Pile no. 1 by shutting off the
cooling systems and setting the reactor to low power. The temperature sensors
indicated a falling (rather than rising) temperature. Although the operators
did not realize, the temperature was indeed rising, but in a part of the pile
not measured by the thermocouples. The next day, to carry out the annealing,
the operators increased the power to the reactor. The reactor temperature
increased further until the reactor was hot enough to catch fire. It is not
known exactly what started the fire. It may have been simple overheating, or a
can of fuel or lithium may have burst. All that was visible on the instruments
was a small variability in temperature.
On October 10, air
samplers about a kilometer away detected a rise (to 10 times the usual value)
of radioactivity in the air. Operators tried to examine the pile with a remote
scanner but it had jammed. Operators donned protective gear and tried to exmine
the reactor, only to discover that it was red hot; the fuel rods had melted and
the reactor had been ablaze for nearly two days.
The fire carried a
great deal of radioactive material out of the pile; the filters could remove
only a small fraction of this material, and the rest was carried out the
smokestack to fall as radioactive
contamination on the surrounding countryside.
Operators were unsure
what to do about the fire. Men were sent with sticks to push fuel cans out of
the reactor in the hopes that this would help cool it. The reactor was flushed
with carbon dioxide, but
the temperature was so high this proved ineffective.
On the morning of
Friday October 11, temperatures were rising 20 degrees per minute, and
operators decided to try showering the pile with water. While this might extinguish
the fire, it might also evolve hydrogen and acetylene gases, which could then lead to an explosion. In the event, the gamble paid off;
the fire came under control, although a cloud of radioactive steam was released
to drift across all of England
and over to Europe.
The aftermath
The fire itself released
an estimated 20000 curies (700 terabecquerels) of radioactive material into the
nearby countryside. Of particular concern was the radioactive isotope iodine-131, which has a half-life of only 8 days but is taken up by the
human body and stored in the thyroid. As a result,
consumption of iodine-131 often leads to cancer of the thyroid.
No one was evacuated
from the surrounding area, but there was concern that milk
might be dangerously contaminated. Milk from about 500km2 of nearby
countryside was destroyed (by dumping in local rivers) for about a month.
The reactor was
unsalvageable; all the fuel rods that could be removed were, and the reactor
was buried in concrete. Approximately 6700 fire-damaged fuel elements and 1700
fire-damaged isotope canisters remain in the pile. The damaged reactor core is
still heated by continuing nuclear reactions. Windscale Pile no. 2, though
undamaged by the fire, was considered too unsafe for continued use. It was shut
down shortly afterward. No air-cooled reactors have been built since.
The Windscale site
was decontaminated and
is still in use; several more modern nuclear reactors are there now. To avoid
bad associations, the UKAEA renamed the site Sellafield.
In the 1990s, the UKAEA began
plans to decommission (disassemble and clean up) both piles; the
decommissioning is now partially complete. Plans are being explored to safely
remove the fire-damaged core, which is still radioactive and could burst into
flame.
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