Back end
The back end of the of the nuclear fuel cycle, mostly spent fuel rods, often contains fission products that emit beta and gamma radiation, and may contain actinides that emit alpha particles, such as 234U, 237Np, 238Pu and 241Am, and even sometimes some neutron emitters such as Cf. These isotopes are formed in nuclear reactors.
It's important to distinguish the processing of uranium to make fuel from the reprocessing of used fuel. Used fuel contains the highly radioactive products of fission (see High Level Waste below). Many of these are neutron absorbers called neutron poisons in this context. These eventually build up to a level where they absorb so many neutrons that the chain reaction stops, even with the control rods completely removed. At that point the fuel has to be replaced in the reactor with fresh fuel, even though there is still a substantial quantity of 235U and plutonium present. Currently, in the USA, this used fuel is stored. In other countries (the UK, France, and Japan in particular) the fuel is reprocessed to remove the fission products, and the fuel can then be re-used. The reprocessing process involves handling highly radioactive materials, and the fission products removed from the fuel are a concentrated form of High Level Waste as are the chemicals used in the process.