The world's worst nuclear disaster began 40 years ago at 1:23 a.m. on April 26, 1986, when Unit 4 at the Chernobyl nuclear power generation facility experienced an explosion and meltdown. Ironically, the explosion was caused by a botched safety test.
The point of the test had been to see what would happen if the power plant lost its main electrical supply: Could spinning turbines generate enough power to run the coolant pumps until emergency backup diesel generators could kick in? The experiment had failed three times previously, but never as catastrophically as it did that night.
