What really caused the Chernobyl accident?
Four reactors had been completed on the site, and reactors #5 and 6 were under construction. Reactor #4 had been operating for two years and was undergoing maintenance checks and monitoring of the procedures to be used in case of emergency. The monitoring procedure was faulty and in the process of testing, there was an explosion and meltdown at reactor #4 that spewed over 200 times the amount of radiation released at Hiroshima and Nagasaki. The accident happened about 1 a.m. the morning of April 26, 1986.
Source: Friends of Chernobyl Centers