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Chernobyl: 31

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The Chernobyl
Disaster
Katherine Culp
Erin Meyer
http://mt.sopris.net/mpc/military/v/reactor.chernobyl.jpg
The Irony
 “This accident [Three Mile
Island] could only happen in a
capitalistic society where they
put profits ahead of safety.”
 Professor Alexandrov,
president of the Soviet
Academy of Sciences
 “Good Evening Comrades.
All of you know that there has
been a incredible misfortune –
the accident at the Chernobyl
nuclear plant. It has painfully
affected the Soviet people
and shocked the international
community. For the first time,
we confront the real force of
nuclear energy, out of
control.”
 Soviet President Mikhail
Gorbachev
Background: How a Nuclear
Reactor Works
 Uranium-235 can easily undergo induced
nuclear fission; free neutron + U-235 пѓ fission
 Plutonium-239 can also be used
 Reactor operates at critical state – one neutron
ejected from each fission пѓ another fission
 Enriched uranium pellets arranged in long rods
 Bundle of rods submerged in coolant (water)
 Bundle must be at supercritical state
Nuclear Reactors
 Supercritical state – if left alone, uranium
bundles would overheat and melt
 Control rods – control rate of reaction
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Made of neutron-absorbing material
Can be raised or lowered into bundle
Raising пѓ more heat generated
Lowering пѓ less heat
Completely lowering пѓ shut down reaction
Nuclear Reactors…
 Bundle – heats water to form steam
 Steam drives a steam turbine
 Turbine spins a generator to produce
power
 Containment vessel – prevents leakage
of radioactive material
RBMK Reactor – Reactor
Bolshoy Moshchnosty
Kanalny
http://classroom.psu.ac.th/users/ssmarn/pplant/Nuclear/NPP8_files/image002.gif
RBMK Reactors
 Only in Russia
 Produce power and plutonium for military use
 Design Flaws
 In order to remove the fuel rods for reprocessing
and obtaining plutonium, a crane is needed.
However, this makes the reactor too tall for a
containment shell, which would keep radioactive
material from spreading in case of an accident.
 Carbon moderator used instead of water. Carbon
moderators have a tendency to catch fire.
Moderators are used to slow down rapid neutrons.
Positive Void Coefficient
 Positive void coefficient – if the water in the reactor boils,
steam bubbles are produced. In other reactors, excess
steam pockets reduce nuclear activity and slow down the
nuclear reaction. But in RBMK reactors, the nuclear
reaction speeds up.
 With a positive coefficient, a reactor is unstable at low power
and is susceptible to a rapid power surge that is
uncontrollable.
 In reactors with positive void coefficients, the moderator and
coolant are in separate circuits or are different materials.
However, RBMK reactors have no modifications to prevent
the rapid, uncontrollable power surge that can be produced
by a positive void coefficient.
 There are 13 RBMK reactors in the world still operating
today.
Construction Flaws
 Gross deviations were present in the construction of
the RBMK reactors at the Chernobyl plant.
 The foundation was not poured properly, so there were
gaps in the foundation which could allow for leakage.
 Roof placement did not conform to design
specifications.
 There was also damage to the waterproofing, which
could allow for radioactive contamination of
groundwater. The damage was backfilled instead of
repaired.
An Experiment
 Reactor 4 was shut down for routine maintenance, and
workers decided to run a test to determine if, during a
shutdown, enough electrical power could operate the
emergency equipment and core cooling pumps until
the diesel supply returned.
 Communication between group in charge of test and
group operating the nuclear reactor broke down.
 The emergency core cooling system was turned off;
reactor operated at ВЅ power.
 The reactor stabilized at 1,000 MW, but operator error
caused the power to drop to 30 MW, a power at which
the positive void coefficient can be a problem.
April 26, 1986
 Operators stabilized the system by withdrawing almost all the
control rods. A minimum of 30 control rods is required; only 6-8
were left in place. If a power surge were to take place, operators
would only have 20 seconds to lower the rods and to shut down
the reactor
 Reactor became extremely unstable; operators had to make
adjustments every few seconds in order to keep the power
constant.
 Operators decided to reduce the flow of water so that they could
maintain the steam pressure. Turbine slowed пѓ pumps not
providing as much cooling for the reactor.
 More steam was created in the cooling channels пѓ power surge
about 100 times the normal power.
 Power surge пѓ steam explosion which blew off the top off of the
reactor. A second explosion took place a few seconds later; its
origin is not understood.
 Carbon moderator caught on fire; burned for nine days.
Initial Release of
Radiation
 When Reactor 4 went supercritical and
exploded there was a �mechanical discharge of
dispersed radioactive fuel’, meaning that a
plume of radioactive elements was released
into the atmosphere. This release was made
up of isotopes that closely corresponded to the
fission products of spent fuel, including 89,90S,
131I, and 134,137Cs, which made up the largest
part of the initial emissions.
The Days After the
Accident
 From April 26th to May 1st the rate of
radioactive element release decreased
because the core was covered with boron,
lead, and sand in an attempt to extinguish the
fire that had begun during the explosion.
 This covering of the core, however, led to
temperatures in the core increasing and to
release of more radiation in the form of 131I.
Estimates indicate that 50% of the core’s 131I
was released.
Soviet Disclosure
 The Soviets did not initially inform other nations
of the disaster. The first indication European
nations had that a large-scale nuclear disaster
had taken place was when nuclear plants in
Scandinavia began to register unusually high
levels of background radiation. The Soviets
denied all knowledge about the accident for
several days. Full disclosure was not made
until August of 1986
Soviet Evacuation
 The Soviet authorities did not evacuate
the villages that surrounded Chernobyl
until April 27. At that time they also
provided iodine supplements for the
evacuated residents. These
supplements are supposed to help
prevent thyroid cancer by filling the
thyroid with nonradioactive iodine so it
doesn’t take up the radioactive forms.
Soviet Response to
Chernobyl
 The Soviet authorities sent in cleanup crews to
supplement the workers already in Chernobyl.
Of those who were sent in to contain the fires
and radiation damage, 29 died from intense ОІ
burns. There were also reports of 237 cases of
acute radiation sickness in the surrounding
areas. Later research has questioned these
numbers, and there is some evidence that the
Soviets grossly underestimated the number of
people made ill by the explosion and the
subsequent release of radioactive isotopes.
Effects on the World
 Chernobyl released radiation high into the
atmosphere, where it eventually was carried
across Europe. The amount of radioactive
elements varied country by country, according
to their proximity to the Ukraine and also
meteorological patterns. Countries that
received a lot of rainfall in the days after the
explosion tended to have higher levels of
radiation due to the elements being sent out in
�fine aerosol form’ that was best carried by a
liquid medium.
Long-term effects
 Using risk models, researchers have
estimated that the risk of developing
cancer is doubled in those who were
within a thirty-mile radius of Chernobyl.
 The modeled risks for those in Europe
are considered statistically insignificant.
Health Effects
 Researchers in 1996 indicated that, out of 235 villagers
in the area of Chernobyl at the time of the explosion,
35 had died of cancer. Many survivors suffer from
possibly radiation-influenced conditions such as goiter
and diabetes.
 A Russian Health Ministry report from the same time
indicates that half of the children in the areas irradiated
by the fallout show signs of radiation sickness,
including anemia, kidney failure, and lymph gland
inflammation.
 A Ukrainian biologist’s study indicates that 8500 of
those who went in to clean up the radiation hot spots
had died by the mid 1990s.
The Chernobyl Legacy
 After covering Reactor 4 with a cement
sarcophagus that was intended to seal up the
radioactive elements, the Soviets continued to
operate the other reactors at Chernobyl.
 By the mid-’90s the sarcophagus was
crumbling and international workers had to go
into Chernobyl and reinforce it.
 The last reactor was shut down in late 2000.
 People are still not allowed to live in an
eighteen mile radius from Chernobyl.
Sources
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Anspaugh, Lynn R., et al. “The Global Impact of the Chernobyl Reactor Accident.”
Science, 242(Dec., 1988), 1513-1519.
ApSimon, Helen, et. al. “Analysis of the Dispersal and Deposition of Radionuclides from
Chernobyl Across Europe.” Proceedings of the Royal Society of London. 425(Oct,
1989), 365-405
Brain, Marshall. “How Nuclear Power Works.”
http://science.howstuffworks.com/nuclear-power.htm
“Chernobyl: Assessment of Radiological and Health Impact.” Nuclear Energy
Agency. http://www.nea.fr/html/rp/chernobyl/c01.html
“Chernobyl Nuclear Disaster.” http://www.chernobyl.co.uk
Dahlburg, John-Thor. “Study Finds Chernobyl Radiation Worse than Originally
Reported.” Los Angeles Times. April 14, 1992, pg. 2
“Fire near Chernobyl reported out.” CNN.com, April 1996.
http://www.cnn.com/WORLD/9604/23/chernobyl.htm
McCarthy, John. “Chernobyl.” http://wwwformal.stanford.edu/imc/progress/chernobyl.html
“The RMBK Reactor” World Nuclear Association. http://www.worldnuclear.org/info/inf31.htm
Wilson, Richard. “A Visit to Chernobyl.” Science, 236(June, 1987), 1636-1640.
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