"Bolted" wrote in message
...

The reactors shut down properly immediately the primary shockwaves from
the earthquake was
detected.

Properly? Doesn't seem to be working that properly now, unless you
really shut your eyes and ears and mind at tsunami +x hours (where x
varies by reactor).

They shut down, they would have gone critical a few minutes after the
tsunami if they had not.
The loss of cooling is the problem, it boils all the water out because the
decay side of things will be generating a couple of megawatts of heat for
weeks after shutdown.
Without cooling, the core melts, becomes compact enough to become critical
and the whole thing goes Chernobyl.
What they need to do now is hang onto the short half life stuff for as long
as possible to keep it away from people.
This is proving difficult as they are just pumping water through the
reactors and have run out of room to store it.
Normally they reuse the stuff so don't have storage problems.