Fission products

The lighter nuclei left when a heavy nucleus splits, usually two per fission, spread across many elements such as caesium, strontium, iodine and xenon. Most are radioactive, and their decay is what produces decay heat after a reactor shuts down. They supply most of the radioactivity of spent fuel for its first few hundred years; caesium-137 and strontium-90, each with a half-life of about 30 years, dominate that period. Some, such as xenon-135, absorb neutrons strongly and affect how quickly a reactor can change power. Keeping fission products contained, inside the fuel, its cladding and the containment building, is most of what reactor safety is about.

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