The tsunami destroyed the diesel generators that provide power to drive the pumps that circulate the water coolant through the reactor that removes decay heat.

FORBES: Explainer: What Caused The Incident At Fukushima-Daiichi

In a normally functioning plant, coolant water is circulated out of the reactors and chilled.

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But they do not have to use coolant fluids like water that must operate at high pressures in order to achieve high temperatures.

FORBES: Explainer: What Caused The Incident At Fukushima-Daiichi