If the heat produced by the decay of radioactive fission products AFTER a reactor has been shut down is not continually removed, this heat could cause a failure of the system that is designed to contain the fission products.

The fission products generated in the reactor core are highly radioactive; thus, releasing significant amounts of them to the environment could be quite harmful.

Great care has been taken to prevent such a release through the defense-in-depth approach used in the design of nuclear power plants.