The earthquake triggered automatic core shutdown.
- ... That happened in seconds.
-- Load dropped as the national grid fell apart.
--- Turbines auto-shut down simultaneously with the rising (huge) seismic acceleration profile.
---- External power (diesel) engaged to power pumps and fans.
----- Tsunami crashed through and whacked 'em.
------ Battery power engaged for critical pumps.
------- Waste-heat sent to circulating pond.
-------- Pressure builds as ponds are clogged.
--------- Overpressure limits of ultra steels approached.
---------- Gas vented - contains mucho hydrogen from cladding thermolysis.
----------- Blam, it blows, due to some sparking something-or-another.
------------ But, exterior walls built for explosive event, panels go flying, superstructure intact.
------------- Core now entrained by supercritical water/vapor.
-------------- Heat conduction substantially rises, so long as the amount of water in core doesn't exceed mechanical strength of containment.
--------------- Cooling by sea-water spray engaged (oh, the cleanup will be expensive!)...
---------------- pressure is unstable but capped, radiation dropping.


Read that paragraph again if you don't see the glass-half-full view: the over-engineering of the core, the backup network of systems worked, the measured release of supercritical water reduced containment criticality, the building engineering yielded gracefully to second-to-worst case containment conditions. It is highly likely that the seawater spray will be viewed as permanently damaging the reactor, but it has resolved the heat-dumping problem.

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