Radioactive waste is a term used for certain substances that are radioactive and that are no longer useful. Most radioactive waste is a by-product of operating nuclear power plants. Certain treatments in hospitals use radioactive substances. Scientists doing certain kinds of research use radioactive substances. Nuclear weapons also produce radioactive waste. Low-level radioactive wastes are radioactive gases, solids, or liquids that give off small amounts of ionizing radiation. High-level radioactive wastes give off large amounts of ionizing radiation.