But this is exactly the point. Catastrophes like Fukushima are practically always a combination of individual deficiencies in design, process, and operator errors. Designs can be improved, processes adapted, and people trained better, but that will not prevent accidents from happening. This is mostly because human imagination is limited and humans are fallible, and what‘s not covered by the previous two is lack of knowledge & understanding.
Said in other words, if you wait long enough, a catastrophe is inevitable. And history, both old and recent, has told us that the time you‘ll have to wait is much shorter that you‘d think.
I work in aerospace operations and every freakin‘ day things go different than planned and anomalies happen. In „my“ „industry“, we try to prepare for off-nominal situations and that buffers the effects, but you can only do so much and you end up in contingencies very often. You can also easily see when a new player enters the stage as they very quickly (should) learn that you‘ll have to react and adapt your plans very often and tone down any promises …
Long story short, whatever means you put up to prevent catastrophic events, they will never be enough. Then the question of cost arises, which is undoubtedly extremely high for nuclear events, especially in such a densely populated and small country like Germany, and you’ll quickly realise that you probably do not want to take that risk even if probability is very low.
And finally, we have yet to find a working way to handle our nuclear waste for the next 10k-100k years. (I am aware of the options but obviously we are not there yet and it‘s unclear if we ever reach the state of „acceptable solution“ instead of pushing the issue to generations to come.)