Any kind of byproduct that is radioactive. The main one is spent fuel. Nuclear power plants take a fuel (typically enriched uranium). When this fuel us used (fission) other radioactive elements are produced until what you have is a mixture that can no longer undergo fission anymore, but is still radioactive.
This waste is either recycled, or if that’s deemed to expensive, put in a barrel, allowed to cool off in a vat of water, then stuck somewhere relatively safe for the next thousand years (e.g. concrete bunker underground).
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