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 Tomorrow is the 26th anniversary of one of the
most infamous nuclear accidents ever to occur - Chernobyl. It was the only
commercial nuclear power plant accident ever directly linked to
radiation-related fatalities, and is considered the worst nuclear disaster in
history. Although not a cheerful memory, to be sure, it is certainly one that
shouldn't be forgotten as we continue to use and develop nuclear power.
(Credit: Yahoo News) -->
The Incident
The Chernobyl nuclear power plant was located in northern
Ukraine near the city of Chernobyl. On April 25, 1986, reactor 4 of the four-reactor
plant was scheduled to be shut down for maintenance. Operators started a test
on reactor 4 during this time, resulting in an explosion of the reactor a day
later (April 26). During the subsequent fire, which burned for 10 days, an
estimated 7 to 8 tons of radioactive material was released into the atmosphere,
including plutonium, graphite, iodine-131, and cesium-137. Winds blew much of
this material into the neighboring country of Belarus and the surrounding
regions.
The Cause
The nuclear operators and lead engineer were largely at
fault for what happened at Chernobyl. The engineer decided to run an experiment to test
how long the reactor could be cooled using only residual steam and coasting turbines
to run the coolant pumps. To run these experiments, they had to bypass several
safety systems, which was in violation of their own protocols. To make matters
worse, the grid controller requested the reactor continue to supply power past
the scheduled shut down time, delaying the test and keeping the reactor running
at low power for nearly twelve hours. This low power operation eventually
caused a severe power drop, which (after a few bad decisions) resulted in a
chain of events leading to an uncontrolled power surge and explosion.
But the faulty design of the reactor is as much to blame for the
accident as its operators. The Chernobyl nuclear plant housed a
graphite-moderated nuclear power reactor design called RBMK (Reaktor Bolshoy
Moshchnosti Kanalniy, or "High Power Channel-type Reactor"). This design had
multiple flaws, not the least of which was its instability at low temperatures.
During the testing, this led operators to turn off cooling pumps in an effort
to raise the power back to higher levels. But the reactor had a large positive
void coefficient, which meant that in the absence of coolant, power would
increase (rather than decrease) as temperatures rose. The power increased so
rapidly that one operator quickly called for emergency shutdown by hitting the BRB (big red button).
(Don't push the big red button... Credit: RPGSobs)
But the graphite tipped control rods (another design
flaw) which were pushed back into the reactor actually further increased the
reaction. This led to an out-of-control cycle known as a feedback loop, which
ended in a massive power spike and the resulting steam explosion which blew
apart the reactor head.
Effects
One person died instantly as a result of the explosion,
while a second died shortly after due to injuries. The release of radiation
directly caused the deaths of 28 people due to acute radiation syndrome (ARS),
and a total of 134 cases were reported. The most significant effect for the
offsite population was the large proportion of childhood thyroid cancers
diagnosed since the accident, likely due to the intake of radioactive iodine
fallout. The radiation also killed plant life within a 10 km2 area
surrounding the site, giving it the name 'Red Forest' from the color of the
dead pine trees. The area (originally called Wormwood Forest) has since
recovered and even flourished due to the lack of human activity in the area.
Most of the radioactive material is concentrated in the soil.
(Aerial photo of the Chernobyl power plant after the accident. - Credit: AP Photo/Volodymir Repik) -->
A number of studies have been done on the health problems
caused by Chernobyl, although a lot of the information and conclusions are
clouded in uncertainty and laced with assumptions which could drastically
affect the numbers. Suffice to say that radiation is not a good thing.
Cleanup of the site has cost many millions of dollars and is
still continuing over 25 years later. Some engineers say the new containment
structure being erected will not be completed until near 2020.
Lessons Learned
The worst nuclear disaster in history is not likely to be
forgotten any time soon, especially by the voices that speak against nuclear
power. It is an important reminder of the reasons why there are established
protocols and safety systems, and why operator and engineering experience is so
important.
But beyond these fundamental lessons, Chernobyl has little
relevance to the nuclear industry. Realistically, the disaster was an
unfortunate consequence of a uniquely flawed design much different from
American reactors at that time and most modern reactor designs today. For
instance, all reactors now implement a self-regulating negative temperature
coefficient design, which would have prevented the extreme drops and spikes in
power and temperature. In addition, the reinforced concrete and steel
containment structures presently built for most reactors would have contained the
release of radioactive material into the atmosphere.
If anything, the Chernobyl disaster keeps me honest in my
perspective on nuclear power. Regardless of the inherent safety of nuclear
reactors today, the fact remains that there is enormous power behind these feats of
engineering and science - power that can have dire consequences if handled
foolishly.
References
Gerd
Ludwig Photography - Chernobyl Cleanup: No End In Sight
Nuclear
Fissionary - What Happened at Chernobyl
World Nuclear
Association - Chernobyl Accident 1986
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