The Heat That Won't Stop
When a nuclear reactor shuts down, the fission chain reaction halts within seconds as control rods absorb neutrons. But the fuel remains intensely radioactive — hundreds of different fission product isotopes are decaying simultaneously, releasing beta particles, gamma rays, and heat. This decay heat starts at about 6% of full power and drops following an inverse power law, but even 1% of a 3,000 MW reactor is 30 MW — enough to melt fuel if cooling is lost.
The Fukushima Lesson
The 2011 Fukushima Daiichi accident demonstrated the consequences of failing to remove decay heat. All three operating reactors shut down successfully during the earthquake. But when the tsunami destroyed backup generators, coolant pumps stopped. Within hours, decay heat boiled away the water covering the fuel. Without cooling, fuel temperatures exceeded 2,000°C, zirconium cladding reacted with steam to produce hydrogen, and three cores partially melted. Decay heat — not the chain reaction — caused the disaster.
Time Scales of Decay
Decay heat spans an enormous range of time scales. In the first seconds, short-lived isotopes like I-137 (half-life 24 seconds) dominate. Within hours, intermediate isotopes like I-131 (8 days) and Ba-140 (12.7 days) take over. After months, long-lived Cs-137 (30 years) and Sr-90 (29 years) sustain a low but persistent heat output that requires spent fuel cooling for years in storage pools before transfer to dry casks.
The ANS Standard Model
This simulation implements the ANS 5.1 standard approximation for decay heat following shutdown. Adjust reactor power, operating history, and time window to visualize the decay heat curve. Notice how longer operating history increases the long-lived fission product inventory, sustaining higher decay heat at late times. The integrated energy curve shows the total thermal energy that must be removed — a critical parameter for emergency cooling system design.