On September 29th, 1957, the plant's cooling system failed, triggering an explosion which catapulted a cloud of radioactive waste into the atmosphere. When it fell back to Earth, it contaminated 20,000 square km. There were evacuations, sure, but not everyone. At least a few people in the area were pressed into service, cleaning villages and burning crops to try to decontaminate the area. Decades later, the region is still a hotbed for cancer, "enjoying" a diagnosis rate five times higher than the national average.
The Soviets covered it up, because of course they would, but interestingly, that's only half the reason almost no one's heard about this. It turns out the CIA also helped cover up the disaster for decades. They had two very good reasons for this. First, they wouldn't be abiding by their remit if they weren't being colossal dickbags to someone, and second, they didn't want to cause any panic about the safety of nuclear energy in their own country.