1986: Design flaws, compounded by human errors, cause Soviet engineers to lose control of a reaction at the Chernobyl nuclear power plant. A partial meltdown occurs. Many die. Many more suffer. The final count of victims may not be over yet.

When someone says “nuclear disaster” you don’t think Three Mile Island. You probably don’t think Windscale Fire. You probably don’t even think Hiroshima. You think Chernobyl.

Ironically the disaster that has become synonymous with the dangers of nuclear energy was caused in part by a safety test. The power-regulating system and emergency safety system of the fourth reactor at Chernobyl in Ukraine (then part of the old Soviet Union) were shut down for the test on April 25. Most of the control rods (the reactor components that stop nuclear fission from cascading out of control) were withdrawn from the nuclear core, while engineers allowed the reactor to operate at 7 percent power.