Chernobyl: Facts About the
Nuclear Disaster
by Marc Lallanilla | September 25, 2013 07:09pm ET
The expolsion at the Chernobyl Nuclear
Power Plant is among the worst nuclear
accidents the world has ever witnessed.
Credit: USSR
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In the early morning hours of April 26, 1986, the Chernobyl nuclear power plant in
Ukraine (formerly part of the Soviet Union) exploded, creating what has been described
as the worst nuclear disaster the world has ever seen.
Even after many years of scientific research and government investigation, there are
still many unanswered questions about the Chernobyl accident — especially regarding
the long-term health impacts that the massive radiation leak will have on those who
were exposed.
Where is Chernobyl?
Located about 81 miles (130 km) north of the city of Kiev, Ukraine, and about 12 miles
(20 km) south of the border with Belarus, the four reactors at the Chernobyl Nuclear
Power Plant were designed and built during the 1970s and 1980s. A manmade
reservoir, roughly 8.5 square miles (22 sq. km) in size and fed by the Pripyat River, was
created to provide cooling water for the reactor.
The nearest town to the power plant was the newly built city of Pripyat, which housed
almost 50,000 people in 1986, according to the World Nuclear Association. A smaller
town, Chornobyl, was home to about 12,000 residents. The remainder of the region was
primarily farms and woodland.
The Chernobyl plant used four Soviet-designed RBMK-1000 nuclear reactors — a
design that's now universally recognized as inherently flawed. RBMK reactors use
enriched U-235 uranium fuel to heat water, creating steam that drives the reactors'
turbines and generates electricity.
In most nuclear reactors, where water is used as a coolant and to moderate the
reactivity of the nuclear core, as the core heats up and produces more steam, the
increase in steam bubbles or "voids" in the water reduces the reactivity in the nuclear
core. This is an important safety feature found in most reactors built in the United States
and other Western nations.
But not in the RBMK-1000, which used graphite to moderate the core's reactivity and to
keep a continuous nuclear reaction occurring in the core. As the nuclear core heated
and produced more steam bubbles, the core became more reactive, not less, creating a
positive-feedback loop that engineers refer to as a "positive-void coefficient."
What happened?
The day before the Chernobyl nuclear disaster, plant operators were preparing for a
one-time shutdown to perform routine maintenance on reactor number 4. In violation of
safety regulations, operators disabled plant equipment including the automatic
shutdown mechanisms, according to the U.N. Scientific Committee on the Effects of
Atomic Radiation (UNSCEAR).
At 1:23 a.m. on April 26, when extremely hot nuclear fuel rods were lowered into cooling
water, an immense amount of steam was created, which — because of the RBMK
reactors' design flaws — created more reactivity in the nuclear core of reactor number
4. The resultant power surge caused an immense explosion that detached the 1,000-ton
plate covering the reactor core, releasing radiation into the atmosphere and cutting off
the flow of coolant into the reactor.
A few seconds later, a second explosion of even greater power than the first blew the
reactor building apart and spewed burning graphite and other parts of the reactor core
around the plant, starting a number of intense fires around the damaged reactor and
reactor number 3, which was still operating at the time of the explosions.
Radioactive fallout
The explosions killed two plant workers, who were the first of several workers to die
within hours of the accident. For the next several days, as emergency crews tried
desperately to contain the fires and radiation leaks, the death toll climbed as plant
workers succumbed to acute radiation sickness.
Most of the radiation released from the failed nuclear reactor was from iodine-131,
cesium-134 and cesium-137. Iodine-131 has a relatively short half-life of eight days,
according to UNSCEAR, but is rapidly ingested through the air and tends to localize in
the thyroid gland. Cesium isotopes have longer half-lives (cesium-137 has a half-life of
30 years) and are a concern for years after their release into the environment.
On April 27, the residents of Pripyat were evacuated — about 36 hours after the
accident had occurred. By that time, many were already complaining about vomiting,
headaches and other signs of radiation sickness. Officials eventually closed off an 18mile (30 km) area around the plant; residents were told they would be able to return
after a few days, so many left their personal belongings and valuables behind.
Health effects
Twenty-eight of the workers at Chernobyl died in the four months following the accident,
according to the U.S. Nuclear Regulatory Commission (NRC), including some heroic
workers who knew they were exposing themselves to deadly levels of radiation in order
to secure the facility from further radiation leaks.
The prevailing winds at the time of the accident were from the south and east, so much
of the radiation plume traveled northwest toward Belarus. Nonetheless, Soviet
authorities were slow to release information about the severity of the disaster to the
outside world. But when radiation alarms began to go off at a nuclear plant in Sweden,
authorities were forced to reveal the full extent of the crisis.
Within three months of the Chernobyl accident, a total of 31 people died from radiation
exposure or other direct effects of the disaster, according to the NRC, UNSCEAR and
other sources. More than 6,000 cases of thyroid cancer may eventually be linked to
radiation exposure in Ukraine, Belarus and Russia, though the precise number of cases
that are directly caused by the Chernobyl accident is difficult (if not impossible) to
ascertain.
Surprisingly, the overall rate of cancer deaths and other health effects related to
Chernobyl's radiation leak is lower than was initially feared. "The majority of the five
million residents living in contaminated areas … received very small radiation doses
comparable to natural background levels (0.1 rem per year)," according to an NRC
report. "Today the available evidence does not strongly connect the accident to
radiation-induced increases of leukemia or solid cancer, other than thyroid cancer."
Some experts have claimed that unsubstantiated fear of radiation poisoning led to
greater suffering than the actual disaster. For example, many doctors throughout
Eastern Europe and the Soviet Union advised pregnant women to undergo abortions to
avoid bearing children with birth defects or other disorders, though the actual level of
radiation exposure these women experienced were too low to cause any problems.
Even the United Nations published a report on the effects of the Chernobyl accident that
was so "full of unsubstantiated statements that have no support in scientific
assessments," according to the chairman of UNSCEAR, that it was eventually
dismissed by most authorities.
Dead forest at the Chernobyl site.
Credit: dreamstime
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Environmental impacts
Shortly after the radiation leaks from
Chernobyl occurred, the trees in the
woodlands surrounding the plant were
killed by high levels of radiation. This region came to be known as the "Red Forest"
because the dead trees turned a bright ginger color. The trees were eventually
bulldozed and buried in trenches.
The damaged reactor was hastily sealed in a concrete sarcophagus intended to contain
the remaining radiation: How effective this sarcophagus has been — and will continue
to be into the future — is a subject of intense scientific debate. Plans to construct a
safer and more permanent containment structure around the reactor have yet to be
implemented.
Despite the contamination of the site — and the inherent risks in operating a reactor
with serious design flaws — the Chernobyl nuclear plant continued operation for many
years, until its last reactor was shut down in December of 2000. The plant, the ghost
towns of Pripyat and Chernobyl, and a large area surrounding the plant known as the
"zone of alienation" are largely off-limits to humans.
There are, however, exceptions: A few hundred former residents of the area have
returned to their former homes, despite the risks of radiation exposure. Scientists,
government officials and other personnel are allowed on the site for inspections and
other purposes. And in 2011,Ukraine opened up the area to tourists who want to see
firsthand the after-effects of the disaster.
Chernobyl today
The region today is widely known as one of the world's most unique wildlife sanctuaries.
Thriving populations of wolves, deer, lynx, beaver, eagles, boar, elk, bears and other
animals have been documented in the dense woodlands that now surround the silent
plant. Only a handful of radiation effects, such as stunted trees growing in the zone of
highest radiation and animals with high levels of cesium-137 in their bodies, are known
to occur. [Infographic: Chernobyl Nuclear Disaster 25 Years Later]
But that's not to suggest that the area has returned to normal, or will at any point in the
near future. Because of the long-lived radiation in the region surrounding the former
Chernobyl Nuclear Power Plant, the area won't be safe for human habitation for at least
20,000 years.