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- NEW
YORK - How do you go
about cleaning up an
invisible, odorless mess? That's the tricky task of
workers who come in
to decontaminate a plant following a nuclear accident.
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- If radiation strikes a person,
it can cause damage to
tissues and cells
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- Much of the process requires
technology such as instruments
to measure radiation levels,
lead-protected gear and sometimes even robots
to travel into zones with
unknown degrees of contamination.
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- "Once the reaction stops,
it's mostly the by-products
you have to worry about," explained
Peter Parker, a professor at Yale
University's Nuclear Structure
Laboratory.
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- Judging from early figures released from the Tokaimura
uranium
processing plant outside of Tokyo, experts in the U.S. predict
the
damage may not require extensive cleaning.
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- On Thursday contamination
levels around the plant were
reported to have been from 10,000 to
20,000 times the normal levels. By
Friday those levels had apparently
dropped low enough that Japanese officials
lifted an advisory that had
ordered 310,000 area residents to stay indoors.
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- "If radiation levels were
high and then went back
down, that implies you had a big blast of
radiation and less contamination,"
said Brian Rothman, a radiation
clean-up specialist at Rothman Radiological
Services in New Mexico.
"It's like if someone had turned on a giant
x-ray machine and
zapped everything around."
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- A Self-Sustaining Chain
Reaction
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- When a nuclear reaction spins out of control, there are
two things that clean-up crews need to worry about: radiation and the
by-products
of the uncontrolled nuclear reaction.
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- Radiation and radiated metals
are not the only dangers
associated with a nuclear accident
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- A nuclear reaction in
a uranium plant produces energy
by combining high amounts of reactive
uranium. The proximity of the uranium
atoms triggers the atoms to
release their nuclei, which then become neutrons.
Those released
neutrons act like tiny cleavers and split other uranium
atoms, which,
in turn, release even more neutrons in a process known as
fission.
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- If fission is not contained and controlled in a reactor,
those
atom-splitting neutrons can spread from the reaction in the form
of
radiation. If radiation strikes a person,it can cause damage to tissues
and cells.
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- If the radiation reaches any metals in the surrounding
area, it
reacts with the metal and makes it "hot." That means
that
atoms in the metal absorb an extra neutron and then become unstable.
An
unstable atom is dangerous because it releases continuous energy in
the
form of gamma rays or alpha or beta particles. Radioactive metals also
remain hot long after the reaction, itself, is extinguished.
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- "Being near
radioactive metal like that is like
sitting on an x-ray machine that's
always on," said Rothman. The only
way to eliminate the hazard, he
explained, is to remove any metal in the
vicinity of the
reaction.
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- But radiation and radiated metals are not the only dangers
associated with a nuclear accident. Even more damaging are the amounts
of radioactive materials that spew out from a nuclear reaction. During
fission, as uranium atoms are split apart, uranium atoms are converted
into a range of radioactive materials such as plutonium, krypton, iodine,
xenon, cesium and strontium. All of these elements then escape into the
atmosphere and contaminate their surroundings.
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- Decontamination
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- "The good news is there
was only about 35 pounds
of material involved," said David Lochman
at the Union of Concerned
Scientists. That means, he explained, that
radiation from the Japanese
reaction probably only reached metals
inside the facility. By comparison,
nearly 100 tons of uranium reacted
out of control at the Chernobyl plant
in the Ukraine in 1986.
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- Less severely
contaminated material can simply be wiped
down. 'You literally use soap
and water'
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- At Chernobyl the range of contamination was horrifyingly
vast.
As the massive explosion burned for days, plumes of radioactive gasses
and tiny particles rose into the sky and were carried literally around
the world by winds. Soils, waters, clothes, and houses were contaminated
to an extent that a complete clean up was virtually impossible (it
continues
today).
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- Contamination following the explosion in Japan was less
severe since the explosion was smaller and because it happened on a rainy
day.
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- "Water helps to dilute those particles," said
Rothman. "So at some points you're not even going to be able to
measure
it."
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- Even in a diluted state, however, contamination can be
dangerous. Lochbaum estimates that if people were exposed to some of the
higher contamination levels reported around the Japanese plant for 90
minutes
or more, they stand a one in 300 chance of getting
radiation-induced cancer.
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- To reduce that risk, workers will need to take readings
of all materials in the vicinity of the plant and then clean infected
areas.
That work may require a robot to go in first and scout out just
how high
radiation levels are in certain places and then gauge how long
people can
safely work in the area. Any severely contaminated material
is then removed.
According to Rothman, less severely contaminated
material can simply be
wiped down. "You literally use soap and
water," he said.
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- Although it may sound like a straight-forward job,
Lochbaum
cautions a proper cleaningtakes time.
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- "It's not going to be a
huge undertaking,"
he said. "But clean up should be a slow
and deliberate process that
could take a year or longer."
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