Three Mile Island
On the 28th
of March, 1979, Three Mile Island plant experienced either a
mechanical or electrical failure that stopped the pumps from
sending water to the steam generators. As a result, the
pressure in the nuclear part of the system rapidly increased.
In response, a relief valve opened. Once the pressure returned
to normal levels, the valve should have closed but it remained
stuck open. The control room however, indicated to the
operators that the valve was closed; leaving the staff unaware
of the draining water. When the alarm system went off, the
operators, still unaware of the leak, initialed a series of
actions that further reduced the amount of cooling water.
Without the coolant, the reactor began to over heat. It got to
the point where the containers for the nuclear fuel ruptured
and the pellets began to melt. Thankfully the building
remained intact and the majority of the radioactive material
was contained.
Chernobyl
The Chernobyl plant consisted of 4 reactors and capable of
producing a total of 4,000 megawatts (MW) of power. On 25
April 1986, a group of engineers decided to test whether or
not the reactor’s turbine could run the emergency water pumps
with just its inertial power. The first step of their
experiment was to disconnect the reactor’s emergency safety
systems and power regulating system. They then ran the reactor
at a power level low enough to cause the reaction to become
unstable. In an attempt to get the reaction stabilized, the
engineers started to remove the control rods and the power
level rose to 200 MW. In the early morning on the 26th, the
fearless engineers continued the experiment by shutting down
the turbine’s engine. Without the cool water, the power level
of the reactor went out of control. In an attempt to stop the
coming meltdown, the engineers reinserted all of the control
rods. However, their control rods had a fatal design flaw:
graphite tips. As discussed on another page, graphite is a
moderator. Consequently, the graphite tips set off the
chemical explosion that released more than 50 tons of
radioactive material.
Image from The Daily Telegraph of Chernobyl