Most industrial accidents result from either a design flaw in the equipment or human error. The nuclear power plant disaster in Chernobyl, USSR (now Ukraine) in 1986 -- the worst of its kind in history -- has both to blame. By most accounts, the Soviet-built reactor, one of four at the Chernobyl plant, had inherent problems. The nuclear reaction it facilitated was hard to control and prone to unpredictable power surges, especially when the reactor was running at very low power outputs, such as during maintenance or testing.

The Chernobyl disaster began during a test of whether the reactor's emergency equipment and cooling system would function in the event of power loss to the station. However, insufficient safety precautions were taken. Experts argue that reactor operators should have been on high alert given the unpredictable nature of this type of reactor at low outputs. Instead, due to complacency and miscommunication, the reactor fell to an output so low that a power surge was practically inevitable. At the same time, plant operators removed increasing numbers of control rods, used to slow or stop nuclear fission in a reactor, virtually assuring a runaway reaction if a power surge occurred.

Poor decisions combined with a reduced flow of coolant water to the reactor (due, in part, to the test) and the disconnection of the emergency core-cooling system resulted in a power surge estimated at 100 times normal output. Graphite surrounding the fuel rods and at the tips of the control rods, which facilitated fission reactions during normal operation, speeded the out-of-control reaction to the point of catastrophe. The ensuing explosions destroyed the reactor core, blew off the roof, and, because there was no containment structure, spread radioactive debris across hundreds of kilometers. More than a hundred thousand people were evacuated. According to estimates, in the ensuing months several thousand people died from complications due to radiation exposure. More than 15 years later, survivors of the Chernobyl disaster continue to report poor physical and psychological health and higher than average rates of cancer.