Without an active removal of decay heat, the reactor was adding heat to the water faster than it was taking it out, and the temperature was rising.

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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

The enormous heat generated by the radioactive fuel in the reactor core needs a constant flow of cold water.

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The efficiency of his design is further improved by using the moving piston to drive water through a special steam nozzle, which increases the effectiveness of the heat transfer between the inside and outside of the reactor.

ECONOMIST: Nuclear power

Once the reactor has shut down, there remains "decay heat" from traces of other radioactive isotopes.

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There are many nontrivial ancillary challenges too, especially the secondary procedure of actually getting the energy out of the reactor into a useful form, i.e. heat to drive steam-turbines as with all other kinds of thermal power stations, to generate electricity.

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Thermoelectric coupling uses heat from the decay of a radioactive fuel (as opposed to the full-scale nuclear fission which powers a reactor on Earth) as its energy source.

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