America's worst was in 1979 at Three Mile Island. 100,000 people were evacuated as a meltdown of the reactor core approached, the feared China syndrome of critical fuel boring itself under ground. In that case that was averted but it may be the eventual fate of Chernobyl, which exploded in 1986. They encased the melting radioactive core in a concrete tomb which simply isn't lasting. That concrete is falling away on a daily basis in large chunks.
Birds fly through that containment.
There are areas where people can walk through without bending and with arms outstretched. Every time it falls down, it brings radiation with it. The concrete sarcophagus is fundamentally flawed. Agricultural land the size of the Netherlands is still contaminated, but even that is said to have been eclipsed by a 1950s explosion, again in the Soviet Union. 400,000 people were contaminated, with little or no medical attention.