Radioactive isotopes are often used as tracers. They can help to detect leaks or monitor the uptake of pharmaceuticals. Materials contaminated with radionuclides become radioactive waste.

Every human activity produces waste, some of which may be hazardous to people and the environment because of its toxicity or radioactivity. Radioactive waste represents only a small fraction of the total waste generated by human activities and must be carefully managed and safely isolated from people and the environment. Unlike waste containing persistent toxic substances, however, the radioactivity of nuclear waste decreases naturally with time as its radionuclides decay. Depending on the radionuclides present, some radioactive waste becomes sufficiently low in activity to be cleared from regulatory control, while long-lived waste may require isolation for very long periods.

Radioactive waste is not produced only by the nuclear power industry. Medicine, research, industry and other applications of radioactive materials also generate radioactive waste. Consequently, radioactive waste has to be safely managed even in countries that do not operate nuclear power plants.

Large volumes of waste are generated by uranium mining and processing, mainly as waste rock and mill tailings. These materials contain naturally occurring radionuclides and require appropriate management.

The largest volumes of waste associated with the nuclear fuel cycle arise from uranium mining and milling. Mining produces waste rock, while the processing of uranium ore produces fine-grained residues known as mill tailings. Although most of the uranium is removed, the tailings retain most of the original radioactivity of the ore, particularly from long-lived uranium decay products such as thorium-230 and radium-226. They must therefore be managed to limit the release of radon and the contamination of surface water and groundwater. Depending on national regulations, uranium mining and milling residues may be managed separately from conventional radioactive waste.

Nuclear power plant operation produces mainly low-level waste (LLW) and intermediate-level waste (ILW). It includes used filters and ion-exchange resins, contaminated tools and protective clothing, sludges and other materials arising from the treatment of reactor coolant and plant systems. During decommissioning, additional radioactive waste is produced from contaminated or neutron-activated components, including steel structures, piping and concrete.

Spent nuclear fuel is highly radioactive and generates decay heat. In countries where it is designated as waste, it is classified as high-level waste (HLW) and requires cooling, shielding and careful long-term management. In other countries, spent fuel is reprocessed to recover uranium and plutonium for further use.

A typical 1,000 MW(e) light-water reactor discharges about 20—30 tonnes of spent nuclear fuel each year, corresponding to roughly 10 m³ of fuel assemblies. If the spent fuel is reprocessed, most of its uranium and plutonium can be separated for potential reuse, leaving approximately 2—3 m³ of vitrified high-level waste. The plant also produces larger volumes of low- and intermediate-level waste, although modern waste treatment and volume-reduction methods can substantially reduce the amount requiring disposal.

For comparison, a coal-fired power plant producing a similar amount of electricity may generate hundreds of thousands of tonnes of ash each year, in addition to millions of tonnes of carbon dioxide.

Worldwide, nuclear power plants have discharged several hundred thousand tonnes of spent fuel since commercial nuclear power generation began, while much larger volumes of less radioactive LLW and ILW have been generated, treated and in many cases already disposed of. Despite their relatively small volume, spent fuel and HLW contain the great majority of the radioactivity and therefore require the most demanding long-term management.

Medicine is an important source of radioactive waste outside the nuclear power industry. Radioisotopes are widely used in various diagnostic and therapeutic procedures.

Radioactive waste is also generated outside the nuclear power industry. An important source is health care, where radionuclides are widely used for diagnosis and treatment. Depending on the application, radioactive waste may include contaminated gloves, syringes and other materials, as well as sealed radioactive sources used in radiotherapy and other medical equipment. Much medical radioactive waste contains short-lived radionuclides and can be stored until its activity has decayed sufficiently for clearance.

Radioactive materials are also used in industry, agriculture, scientific research and education. These activities generate various types and quantities of radioactive waste, including contaminated laboratory materials and disused sealed radioactive sources. Countries therefore need appropriate systems for radioactive waste management regardless of whether they operate nuclear power plants.