Fukushima was not a case of unknown unknowns. That style of reactor requires active cooling to prevent meltdown, and the cooling failed. The consequences of the cooling system failing were known, as were the vulnerabilities of the cooling system. They just didn’t add enough safeguards/didn’t account for the backup failing. There’s an argument that you can’t add enough safeguards to account for all possible ways an active cooling system could fail, which is perfectly reasonable/a good argument. Thats’s why next gen nuclear reactor designs don’t require active cooling.

People citing Chernobyl and Fukushima strikes me as rhe equivalent to people citing the Hindenburg when talking about flying; I’m pretty sure both reactor designs could be thought of as the design equivalent to the hydrogen blimp. People’s minds are stuck on an image of a blimp on fire when thinking about flying even though modern passenger jets exist.