Various construction changes could have prevented this from happening:
- the whole power plan being built higher up or further inland
-> this would likely be quite a bit more expensive due to land availability & cooling water management when not on sea level & next to the sea
- the emergency generators being built higher up or protected from a tsunami by other means (watertight bunker ?)
-> of course this requires the plan cooling systems & the necessary wiring itself working after surviving a massive earthquake & being flooded
An inland power plant - while quite wasteful in an island country - would be protected from tsunamis & certainly doable. On the other hand, I do wonder how would high concrete cooling towers handle strong earthquakes ? A lot of small cooling towers might have ti be used, like in Palo Verde nuclear generating station in Arizona.
Otherwise a bizzare case could still happen, with a meltdown possibly happening due to your cooling towers falling over & their cooling capacity being lost.