What does the term ‘decay heat’ refer to in nuclear reactors?

The term ‘decay heat’ refers specifically to the residual heat produced by radioactive isotopes after a nuclear reactor has been shut down. When a reactor operates, it generates energy through fission reactions, but even after these reactions cease, the reactor still emits heat due to the decay of radioactive materials formed during fission. This residual heat stems from the decay of fission products and actinides that are still active after the chain reaction stops.

Decay heat is a critical consideration for reactor safety because it necessitates continued cooling of the reactor core even when the reactor is not operational. If this heat is not adequately managed, it could lead to overheating, and potentially, a meltdown or release of radioactive materials.

While other options pertain to various aspects of nuclear reactor operations, they do not capture the specific meaning of decay heat. The heat generated during the fission process occurs actively during operation, not after shutdown. Similarly, the heat needed for steam generation pertains to the energy required to produce steam for power generation, and heat from active cooling systems involves mechanisms used to manage temperatures in the reactor; both concepts are distinct from the persistent heat produced by decay.