Decay Heat

You shut down a nuclear reactor by inserting control rods. The chain reaction stops in seconds. But the reactor is still dangerously hot, because fission products accumulated during operation continue to undergo radioactive decay, releasing energy that has nothing to do with the chain reaction. This is decay heat, and it is what makes nuclear accidents possible.

Fission creates hundreds of radioactive isotopes. These products decay on their own schedules, emitting radiation that converts to heat. Immediately after shutdown, decay heat equals roughly 6-7% of the reactor's full thermal power. For a 3,000 MW thermal reactor, that is 180-210 MW of heat with no off switch. Decay heat drops to about 1% within a day and continues declining, but it remains significant for hours to days.

Size the danger. 6% of 3,000 MW = 180 MW. That is enough heat to boil thousands of gallons of water per minute. Without active cooling, fuel temperatures rise past the point where zirconium cladding reacts with steam, generating hydrogen and potentially leading to fuel damage.

Why can't you just let the heat dissipate passively?

The cooling imperative. 180 MW concentrated in a reactor vessel cannot dissipate through natural convection fast enough. Active cooling (pumps circulating water) is required for hours after shutdown. Fukushima's disaster occurred precisely because the tsunami destroyed the backup generators powering these cooling pumps.

Decay heat is why nuclear safety focuses on cooling systems, backup power, and passive safety designs that function without electricity.

---