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.



                                   Image from the New York Times of Three Mile Island

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