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Understanding the Complete Meltdown at Fukushima Unit 1
from Nature News
Last week, workers entered the stricken unit 1 reactor at the Fukushima Daiichi nuclear plant and began work to further stabilize it. One of their first tasks was to recalibrate some of the sensors on the reactor, so that engineers had a better sense of how it was doing. That recalibration has led to a startling revelation: virtually all of the fuel inside the unit 1 reactor appears to have "melted down."
Press reports on the meltdown have variously described it as a setback and or admission by the Tokyo Electric Power Company (TEPCO) that things are worse than they thought...
During normal operation, the core of a nuclear power reactor like unit 1 consists of long narrow tubes of a zirconium alloy filled with uranium fuel pellets. Tubes are bundled together into "assemblies" which in turn make up the core of the reactor. When it's humming along at full tilt, the core boils water that is used to turn turbines in the adjacent building. After the earthquake and tsunami on 11 March, the water stopped circulating and the core heated up.