It is one of only two classified as a level 7 event on the International Nuclear Event Scale, the other being the recent Fukushima I nuclear incident, which was set in motion by a massive earthquake and subsequent tsunami.
What set the Chernobyl disaster in motion?
Answer: A systems test.
On April 26, 1986, the operating crew planned to test whether the Reactor No. 4 turbines could produce enough energy to keep the coolant pumps running until the emergency diesel generator was activated in case of an external power loss.
During the test, power surged unexpectedly, causing an explosion and driving temperatures in the reactor to more than 2,000 degrees Celsius—melting the fuel rods, igniting the reactor’s graphite covering, and releasing a cloud of radiation into the atmosphere.