The night of April 26th 1986, the reactor number 4 of Chernobyl nuclear power exploded during a routine maintenance. The beginning of disaster that killed thousands of people.
This is the chronology of the tragic accident:

April 25th, 01:00 AM - The reactor power was reduced to allow an experiment.
April 25th, 02:00 PM - The emergency core cooling system was disconnected, this breached the security system.
April 25th, 11:00 PM - The reactor power went down to 700 mw.
April 26th, 00:28 AM - An operator error caused a strong decline of power, down to 30 mw. The reactor reached a instability condition.
April 26th, 01:23:04 AM - The operator closed the emergency turbine valve, the last emergency system that had to save the reactor.
April 26th, 01:23:31 AM - The core reactivity began to increase. The control bars didn't able to balance the raising.
April 26th, 01:23:43 AM - The core temperature rised irreversibly.
April 26th, 01:23:44 AM - The reactor power rised from 200 to 100,000 mw in 40 seconds. The reactor exploded.
April 26th, 01:23:45 AM - The explosion destroyed the high part of the walls and the building roof.

The radioactive cloud caused by the explosion contaminated 150,000 square kilometers around the central, while the wind pushed it to Europe. The emission of radioactive particles continued for several days, only in November, the exploded reactor was sealed in a concrete box, where there still are 180 tons of uranium. The worst nuclear disaster of the story. In the atmosphere were put about 45 millions of curies xenon 133; 7 millions of curies iodine 131; a million of curies di cesium 134 e 137. The release of radionuclides into the atmosphere continued increasingly until the 10th of may, then it decreased.