Hot Rock, Boil Water, Make Steam
From a current nuclear operator and former Navy Nuclear Machinist Mate about the Japanese nuke plant issue.
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What’s going on is known as a LOCA—loss of cooling accident. Though the reactor is safely shut down it continues to produce decay heat from residual fission reactions that can take days to weeks to dissipate depending on how long they were at full power prior to shutdown. The plant would have an emergency cooling backup system that should take over, but if both systems were physically damaged from the quake then not enough coolant is being circulated to keep the core cool.