[anigbrowl]

They were, but the actual damage done by the tsunami was to wreck the fuel tanks for the diesel backup generators, which were intended to keep the cooling systems operating if the grid power supply failed. The failure of the backup system led to failure of the cooling system, allowing the heat to build up inside the reactors, causing the pressure explosions around the reactors and seemingly inside one or two of them, if reports about cracked containment chambers are correct.

A seawall was supposed to handle any large waves, but among other problems, some fuel tanks for the diesel generators were immediately behind it - not really the ideal place to put any component of your backup system. There's a before & after photo here: http://everist.org/pics/Fukushima/Fukushima_fuel_tanks.jpg

The reactors themselves would have been hard to move, but would likely have been OK had the backup systems been more robust - and being external and modular, they could have been made more robust, and ought to have been.

Even if it were an all-or-nothing proposition, the additional data gathered in the years since the plants' construction was sufficient to justify a re-evaluation of risk; if the risk were sufficiently probable but impossible to mitigate, then decommissioning would be the appropriate response. Sure, that would be very expensive, but much less so than a post-disaster situation like this. Going ahead with a plan that's known to be flawed because you've already put a lot of money into it is known as a 'sunk costs fallacy'; the amount you've already spent has zero bearing on the future probability of failure, so if the latter is unacceptably high then the size of the former is no excuse for inaction.