What is decay heat in a nuclear reactor context?

• A. Heat generated by ongoing fission during reactor operation.
• B. Heat generated by the decay of fission products after reactor shutdown.
• C. Heat from corrosion products in the cooling system.
• D. Heat produced by neutron capture in the moderator.

Answer: (B)

Decay heat is the heat released by the radioactive decay of fission fragments that were produced during reactor operation. Even after the reactor is shut down and fission stops, those fragments continue to decay and emit radiation, converting that decay energy into heat in the fuel and surrounding structures. This heat fades over time as the fission products decay away, so the cooling system must continue to remove heat after shutdown to prevent overheating. The amount and rate of decay heat depend on how much fuel was burned and the specific mix of fission products formed. The other options describe heat sources active during operation, from corrosion, or from neutron capture, none of which account for the sustained heat after shutdown from decay of fission products.