There's a good argument that these are still system failures. If a single individual can intentionally blow up a nuclear reactor, that's a massive point of failure. Humans are often irrational and prone to anger, depression, malice, and greed. Any of those impulses could be motivation for destructive action. Nuclear submarines avoid this by requiring multiple personnel to simultaneously coordinate in activating a launch.

On Chernobyl, my take away from the excellent Netflix mini-series was that there was a massive cultural failure, where everyone abdicated responsibility, and there no ability for employees to counter poor decisions from superiors. Culture and organizational design are also important components of complex systems