In most reactor designs, as a safety measure, control rods are attached to the lifting machinery by electromagnets, rather than direct mechanical linkage. This means that automatically in the event of power failure, or if manually invoked due to failure of the lifting machinery, the control rods will fall, under gravity, fully into the pile to stop the reaction. A notable exception to this fail-safe mode of operation is the BWR which requires the hydraulical insertion of control rods in the event of an emergency shut-down, using water from a special tank that is under high nitrogen pressure.Did the earthquake damage the insertion mechanism or rupture the nitrogen pressure tank ? Or is it that even after full insertion of the control rods the beast needs continuous cooling ?
Anyone know ? Call me naive, but I assumed in the event of trouble you dropped the rods in (or in this case shoved them up) and that was that.
UPDATE - "is it that even after full insertion of the control rods the beast needs continuous cooling ?" is indeed the answer. As laid out in the post above, there's still residual heat - because nuclear reaction products are in the fuel. These typically have short half-lives, which means they give off a lot of heat as they (quickly) decay - but not quickly enough in the absence of cooling water flow.
The estimated death-toll seems to be rising rapidly. The images of the water rolling towards people and vehicles looked very nasty, and gave the impression of potential casualties in the thousands or tens of thousands. I've not been able to gather from the news how much time the people of Sendai and the north-east coast had to flee in between the earthquake and the tsunami.
Social solidarity seems to have been magnificent. I heard a TV report that supermarkets were cutting food prices, and couldn't but wonder if in similar case they'd be raised over here. I bet there's either none or very little looting. In the July 2007 UK floods, not only were many abandoned cars broken into, but when free packs of bottled water were left on the pavement for those who needed it there were reports of people loading hundreds of bottles into vans - presumably to sell.