Nuclear waste is the radioactive waste left over from nuclear reactors, nuclear research projects, and nuclear bomb production. Nuclear waste is divided into low, medium, and high-level waste by the amount of radioactivity the waste produces. Although nuclear waste can be very dangerous, and should not be handled by anyone without professional training, it is not necessarily more dangerous than chemical poisons such as ricin and botulin.

The largest source of nuclear waste is naturally occurring radioactive material (NORM). All substances are slightly radioactive from the decay of naturally occurring isotopes such as carbon-14, potassium-40, uranium-238 and thorium-232. If the radioactive elements are concentrated by natural processes or human industry, they may become concentrated enough to be treated as nuclear waste. Nuclear waste from NORM is not usually highly radioactive, and might be safe to hold, although it may produce dangerous levels of radioactive radon gas.

The other primary source of nuclear waste is human-built nuclear reactors. After an atom fissions, the two pieces of the nucleus may themselves be radioactive, and may take thousands of years to decay; these pieces are known as fission products. Other sources of nuclear waste include the breeding of new nuclear fuel from uranium, neutron activation of materials in the reactor, and leftover depleted uranium from the enrichment process.

After a nuclear fuel bundle has been removed from a reactor, it still contains most of its original uranium; nuclear fuel is mostly uranium-238, which does not fission easily. Mixed in with the uranium are transuranic elements, produced by neutrons reacting with the uranium, and fission products. This radioactive cocktail can be separated through a process called nuclear reprocessing, which sorts the chemicals inside the spent fuel.

The fission products produced from nuclear reactors are highly radioactive and will remain dangerous for thousands of years; they have no further use, and can be permanently disposed of. Although fission products are the most dangerous form of waste, a nuclear reactor doesn't produce much of them; a 1 GW nuclear reactor only produces around 3,600 pounds (1,500 kilograms) of fission products per year. The transuranic elements are also highly radioactive, but many of them can be used as fresh nuclear fuel; the remainder can be safely fissioned by particle accelerators.