How does decay heat change with time after shutdown?

• A. It remains constant.
• B. It oscillates with time.
• C. It decreases with time.
• D. It increases as the fuel cools.

Answer: (C)

Decay heat after shutdown comes from the radioactive decay of fission products left in the fuel. The heat generation rate is set by the activities of many isotopes, each with its own decay constant. Right after shutdown, short-lived isotopes dominate, producing a relatively high decay heat. As time passes, those short-lived isotopes decay away, and the remaining longer-lived isotopes contribute less heat, so the total decay heat steadily falls. In other words, the decay heat decreases with time, eventually approaching very small levels as the isotopes continue to decay. This is why cooling systems must handle decay heat even after the reactor is shut down.