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Why must a nuclear reactor remove heat even after fission has stopped?

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Labeled diagram showing the process of a nuclear power plant with a pressurized-water reactor. Water circulates through the reactor core, heats to produce steam in a steam generator, which drives a turbine connected to an electric generator. Steam is then cooled in a condenser using water from a cooling tower, which releases nonradioactive water vapor. Control rods, pumps, and transmission lines are also indicated. — nuclear power Schematic diagram of a nuclear power plant using a pressurized-water reactor.

Even after a nuclear reactor has been shut down and the fission chain reaction has ceased, the reactor core continues to generate heat due to the radioactive decay of fission products. These fission fragments, created during the fission process, remain in highly excited, unstable states and continue to emit energy as they transition toward stability.1

This "decay heat" is significant and must be continuously removed to prevent the fuel from overheating and melting. If cooling systems fail to dissipate this heat, the resulting temperature buildup can lead to a meltdown of the reactor fuel, as seen in historical accidents like Fukushima. Consequently, reactor designs must incorporate reliable cooling systems—such as emergency core-cooling systems—that remain operational under both normal and abnormal conditions to ensure the safety of the facility and the surrounding environment.2134