In perhaps the blackest of ironies in history, the world’s worst nuclear catastrophe was the result of an unnecessary safety test. On the night of 25 April 1986, reactor No. 4 at the Chornobyl power plant was due to be shut down for regular maintenance. Workers decided to use the opportunity to see if, in the event of a shutdown, enough electricity remained in the grid to power the reactor core cooling systems, and turned off the emergency cooling mechanism. For various reasons, including a design flaw, operational errors and flouted safety procedures, the result was a power surge, a steam explosion and a full-blown nuclear explosion. At 1.26am on 26 April 1986, the reactor blew its 500-tonne top and spewed tonnes of radioactive material mainly over Belarus and Ukraine. Some material also wafted over Sweden, whose scientists were the first to alert the world.
The ensuing evacuation and clean-up cost billions of dollars and was a contributing factor in the collapse of the Soviet Union.