When a nuclear power plant has a failure as catastrophic as that in Chernobyl it is active (radioactive) for a _very_ long time, with no way of shutting it down. The immediate site of the power plant will be uninhabitable for about 20,000 years, however, the wider area might become safe to live in in just a couple of hundred years. While radiation levels decrease in general, there have also been measurements of increasing radiation in 2021.
Uninhabitable for humans, but it's great for wildlife (who don't care if they get cancer later in life, or don't live long enough to get it). And no, AFAIK no three-eyed fish has been sighted yet...
