Chernobyl's operators conducted an unsafe test

Engineers at Chernobyl in 1986 were concerned about excess heat generated by the nuclear decay of the reactor fuel, which had the potential to damage the reactor in the event of an emergency shutdown. If electrical power was lost, water pumps would no longer be able to cool the system adequately. To deal with this potential problem, Chernobyl's operators designed a test where they would lower the output of reactor number four and try to get power from a turbine to operate the coolant water pumps. Unfortunately, errors in the experiment's design caused the power level of the reactor to drop far lower than planned, and the engineers' attempts to compensate for the problem caused further instability and ultimately an explosion in the reactor.