On Saturday April 26, 1986 at 1:23:58 a.m. reactor 4 suffered a catastrophic steam explosion that resulted in a fire, a series of additional explosions, and a nuclear meltdown. The accident can be thought of as an extreme version of the SL-1 accident where the core of a reactor was destroyed (killing three men) spreading radioactivity through the inside of the building that SL-1 was in. The "liquidators" received high doses of radiation. According to Soviet estimates, between 300,000 and 600,000 liquidators were involved in the cleanup of the 30-km evacuation zone around the reactor, but many of them entered the zone two years after the accident.