The reactors at Fukushima were GE BWR(boiling water reactors) technology from the late 1950s-early 1960s. I’m not familiar with GE reactors, only the Westinghouse PWR (presurized water reactors) and those from the late 1960s-early 1970s.
The PWR have a failsafe mechnaism that immediately drops rods into the reactor to stop the fissioning process whenever an earthquake is detedted. I do no know whether there is a similair falisafe mechanism on BWRs. Do you happen to know if Fukushima was equipped with such a system?
And do you happen to know if Fukushima was licensed to operate by the AEC (predessesor of the NRC). Many overseas nuclear power plants are licensed by our regulatory agency - many people don’t know that fact.
The difference between the PWR and the BWR is that control rods drop from the top of the PWR.
Control rods are inserted from the bottom up on a BWR using gas (nitrogen) driven hydraulics.
A couple of GE engineers in the early stages of designing the BWR quit in protest claiming it was a extremely poor method of inserting control rods in an emergency (versus gravity fed in the PWR along with all the necessary ‘holes’ needs for the control rods to pass through on the bottom of a RPV). The engineers went on to be anti-nuke activists.
If I can find a picture to post of the bottom of an actual BWR during manufacturing or being installed, you will see that it is an accident waiting to happen as it looks like a sieve and each of the many ‘holes’ requires a seal that could fail and leak.