Decay heat

The heat produced by the radioactive decay of fission fragments inside reactor fuel, which continues long after the chain reaction itself has been shut down. Immediately after shutdown it is about 6.5% of the power the reactor was running at, falling to roughly 1 to 1.5% after an hour, about 0.4% after a day and 0.2% after a week; the exact figures depend on how long the reactor ran and on its fuel. For a large reactor producing 3,000 MW of heat, that is about 195 MW at shutdown and still about 12 MW a day later. Losing the ability to remove decay heat, not a runaway chain reaction, caused the fuel damage at Three Mile Island and Fukushima Daiichi. Decay heat scales with reactor power, while the surface through which heat can leak away shrinks more slowly than the volume as a reactor gets smaller, so a very small reactor can plausibly shed its decay heat to its surroundings by conduction and natural airflow. Large reactors can also remove it passively for a stated period, but only through dedicated systems such as raised water tanks and natural circulation loops.

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