Fukushima. It was a Gen 1 plant which already has the issue that a thermal runaway is possible. There were other examples of this happening like TMI. The backup for Fukushima was onsite generators which were flooded and ultimately failed causing the meltdown.
The safety lessons we learned from all gen 1 reactors was to apply passive shutdown mechanism where if input power fails fission ultimately stops. That's not something that can be applied across the fleet because it requires more infrastructure and an almost complete redesign of the reactor's setup. Which is why these early reactors all have a potential risk of thermal runaway.
Edit: It looks like all gen Is have been decommissioned as of 2015, which is great. But we really should now be talking about decommissioning gen IIs and leaping forward to Gen IVs.
The safety lessons we learned from all gen 1 reactors was to apply passive shutdown mechanism where if input power fails fission ultimately stops. That's not something that can be applied across the fleet because it requires more infrastructure and an almost complete redesign of the reactor's setup. Which is why these early reactors all have a potential risk of thermal runaway.
Edit: It looks like all gen Is have been decommissioned as of 2015, which is great. But we really should now be talking about decommissioning gen IIs and leaping forward to Gen IVs.