The wiki article gives a decent overview: https://en.wikipedia.org/wiki/Olkiluoto_Nuclear_Power_Plant#... Though you will need to follow the references, look for STUK approving some methodology, then claiming its results are insufficient.

Umm, it's not the safety regulator's task to promise the results will be sufficient if you use a specific methodology: "Scrum methodology is fine, so the resulting software quality will be fine too."

There is much more to it than that in this case.