There were multitudes of design flaws.

* building reactors in areas with severe earthquakes and tsunamis was the main design flaw

* flood protection too low, even though this was known

* backup-generators not safe against flooding

* loss of outside electricity over a long period of time -> powerlines were not working, other sources of electricity on the grid were offline due to earthquake

* too long time to restore electricity, we are talking about many MWs for cooling and other purposes -> generators were difficult to bring in with further problems making them work

* various damage due to earthquake on reactors, buildings and infrastructure around the plant

* too much spent fuel in pools, required large amount of electricity for cooling

* spent fuel pools high in the buildings, hard to reach

* no cooling capability in case of days-long loss of electricity -> then needs to be cooled with seawater pumped with vehicles with concrete pumps (which were flown in from remote places, even from the US) -> caused structural damages to buildings and spread radiation

* no idea what to do with the contaminated cooling water

* no protection in main buildings against the explosions that happened

* no technology existed for a decade or more to deal with molten cores

* ineffective security/safety process -> the plant had checks a few months before the accident with no consequences

* unwillingness to invest major amounts of money into upgrades

* life extension for outdated reactors, due to economical pressure

* no independent controlling instances for the nuclear industry