Fukushima happened in a highly regulated (and correspondingly expensive) environment. Improving the regulation doesn't necessarily just mean adding more of it or tightening targets far beyond any useful range. Also, arguably the evacuations as a response to Fukushima caused more harm than the radiation would have, an example of paranoia as opposed to effective regulation.

(Chernobyl also happened because of paranoia but that was largely the Soviet's obsession with saving face to the point of hiding a design flaw from the operators).

Though, I will push back on the idea that this is exclusively why it's so expensive: Nuclear has plenty of other issues with cost beyond excessive safety targets.

The human impact of those is a fraction of fossil-fuels. So it’s not fair to judge nuclear for being expensive if we don’t hold other fuels to similarly rigorous standards