The Chernobyl Disaster
A Day that Shocked
the World and
Scarred it for Years to
The Chernobyl Nuclear Power
Located 11 miles north of the city of Chernobyl
Plant consisted of 4 reactors each producing
1,000 MW of electricity
Produced 10% of Ukraine’s electricity
Construction began in the 1970’s
Reactor #4 was completed in 1983
At the time of the accident, reactors #5 and #6
progress of being built.
Events Leading to the
4 was to be shutdown for routine
maintenance on April 25,1986.
4 was a RBMK (an acronym
derived from the Russian initials for
heterogeneous, water-graphite channeltype reactor), which uses graphite as a "moderator," i.e., to slow down neutrons
and thereby sustain the nuclear chain
Events Leading to the
Saturday April 26, 1986
During the shutdown a test was to be run to
see if the slowing turbine could provide enough
electrical power to run the main core cooling
water circulating pumps.
Reactor #4 was undergoing a test to test the
backup power supply in case of a power loss.
The power fell too low, allowing the
concentration of Xenon 135 to rise.
Power dropped to 30MWt
The workers continued the test and in
order to control the rising levels of Xenon
135 the control rods were pulled out.
Power was brought back to 200MWt
Although operation below 700MWt was
dangerous they continued the test.
The experiment involved shutting
down the coolant pumps, which caused
the coolant to rapidly heat up and boil.
Pockets of steam formed in the coolant
lines. When the coolant expanded in
this particular design, the power level
The automatic control rod system was
able to counteract the rise of power for
The automatic control system only had
control of 12 rods though since nearly
all the rods had been removed
All control rods were ordered to be
inserted. Doing this increased
reactivity at the bottom of the core.
This was due to the flawed graphite-tip
design that introduced moderating
graphite before neutron absorbing
Power increased to over 530MWt within 3
With this power spike fuel rods began to
fracture, blocking the control rods at 1/3
As the rods were inserted, they became
deformed and stuck. The reaction could
not be stopped.
The rods melted and the steam pressure
caused an explosion, which blew a hole in
By some estimations reactor power
jumped to around 30,000 MWt.
This caused the 2000-ton upper plate
to blow off, sending it through the roof
of the reactor building.
To save money the reactor was
constructed with only partial
containment which allowed the
radiation to escape, 13% - 30% of the
One fourth of the release of radioactive material
occurred within the first day; the remaining 3/4
was released in the following two weeks.
The final solution was found in starving the
graphite fire of oxygen.
To cool the reactor under the debris and prevent
a meltdown through the reactor floor which
could have contaminated the water table 50 feet
below, Soviet engineers pumped in liquid
After the Accident
Since then, the RBMK reactors in Russia and
Lithuania have undergone modifications
recommended by the International Atomic Energy
This includes speeding up the control-rod insertion
time by about a third, to 12 seconds, and using
uranium of a slightly higher enrichment in the core,
which essentially means the reactor doesn’t have to
be driven as hard to spin the turbines.
Nuclear experts say the...
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