What is decay heat and why must it be removed after reactor shutdown?

What is decay heat and why must it be removed after reactor shutdown?

• A. Decay heat is the residual heat from fission product decay after shutdown, and it must be removed to prevent fuel overheating.
• B. Decay heat is the heat produced by the reactor core during operation.
• C. Decay heat is the heat from environmental radiation cooling.
• D. Decay heat is the heat of water freezing.

Answer: (C)

Decay heat is the heat produced by the radioactive decay of fission products left in the fuel after the reactor is shut down. Even though fission stops, the unstable fragments created during operation continue to release energy as they decay, which translates into heat inside the core. This heat is present immediately after shutdown at a significant level and then declines as the fission products decay away. If this heat isn’t removed, the fuel and cladding can overheat, the coolant could boil, and safety margins could be compromised. That’s why the reactor’s cooling systems stay active to remove decay heat until it has decayed to safe levels. In practice, decay heat is a few percent of full power right after shutdown and decreases over hours to days.

Decay heat is the heat produced by the radioactive decay of fission products left in the fuel after the reactor is shut down. Even though fission stops, the unstable fragments created during operation continue to release energy as they decay, which translates into heat inside the core. This heat is present immediately after shutdown at a significant level and then declines as the fission products decay away. If this heat isn’t removed, the fuel and cladding can overheat, the coolant could boil, and safety margins could be compromised. That’s why the reactor’s cooling systems stay active to remove decay heat until it has decayed to safe levels. In practice, decay heat is a few percent of full power right after shutdown and decreases over hours to days.