What is the typical magnitude and decay behavior of decay heat immediately after reactor shutdown in a modern PWR?

What is the typical magnitude and decay behavior of decay heat immediately after reactor shutdown in a modern PWR?

• A. 0%
• B. 0.1%
• C. 6-7% of full power immediately; decays to about 1% within hours and to a fraction after days
• D. 50%

Answer: (C)

Decay heat after shutdown comes from the radioactive decay of fission products left in the fuel. Right after shutdown the heat from these decays is large because short‑lived isotopes, produced in big quantities, release a lot of energy quickly. The immediate heat level is about 6–7% of the reactor’s full power. As these short‑lived isotopes decay away, the heat output falls rapidly and is around 1% of full power within a few hours. After days, the remaining decay heat comes from longer‑lived isotopes and continues to fade, leaving only a very small fraction of the original heat. This is why reactors must be cooled after shutdown for an extended period.

Decay heat after shutdown comes from the radioactive decay of fission products left in the fuel. Right after shutdown the heat from these decays is large because short‑lived isotopes, produced in big quantities, release a lot of energy quickly. The immediate heat level is about 6–7% of the reactor’s full power. As these short‑lived isotopes decay away, the heat output falls rapidly and is around 1% of full power within a few hours. After days, the remaining decay heat comes from longer‑lived isotopes and continues to fade, leaving only a very small fraction of the original heat. This is why reactors must be cooled after shutdown for an extended period.