@miriam98 - Yes, control rods are definitely the fail safe mechanism, I would agree. There are other cases where they don’t function properly, like in the event of an earthquake.
The earthquake and subsequent tsunami that hit Japan in 2011 is one such example. It caused damage to their reactors. Even though the nuclear power control rods were lowered into the reactor, the temperature kept climbing, to the point where eventually some of the radiation seeped into the nearby waters and poisoned the sea life.
That was a terrible situation, almost another Three Mile Island. It’s one more reminder that nuclear power, while generally safe, can be a nightmare in the event of a natural disaster.