The accident happened when the engineer test function of the cooling system. And cooling of the reactor core for nuclear emergency But the test system delay and test engineers by the night shift.But the automation does not work. Heat up the reactor core, resulting in melting 4 nuclear number and explode.To the west of the Soviet Union, Eastern Europe, covering Western Europe, northern Europe, in Ukrainian, Belarusian and Russian immigrants have a population of more than 336.000 people leave the area as an emergency.