Hindsight is 20/20 and all. How many other problems were identified at Fukushima and other reactors throughout Japan that were considered to be equally or more

Hindsight is 20/20 and all. How many other problems were identified at Fukushima and other reactors throughout Japan that were considered to be equally or more risky? If you cannot answer this question then how can you conclude that this was "preventable" in any useful sense?