Types and Sources
High-level waste consists mainly of spent fuel rods from nuclear reactors. These power plants rely on nuclear fission to generate heat, and the fuel is made into rods that can be moved in and out of the reactor core to control the process. After a time, the rate of fission in a rod will decrease to the point where it is no longer efficient, and the rod will be removed. The removed rods are known as spent fuel rods and are highly radioactive, containing a number of fission products — radioactive elements created by the fission process. These elements decay at different rates, and over time, the rods become less radioactive, but will remain potentially dangerous for many thousands of years.
Low-level waste, in the USA, includes essentially everything that does not come into the high-level waste category. It comes from a wide variety of sources. It consists of materials that have come into contact with radioactive substances, or which have become radioactive themselves due to exposure to some forms of radiation, as well as small quantities of radioisotopes from research establishments and hospitals. Examples are items of protective clothing worn by staff who work with radioactive materials, and syringes and needles used for the injection of radioisotopes for medical purposes. It typically remains potentially hazardous for between a few tens and a few hundreds of years.
Some countries have an intermediate category of waste. This is not recognized in the USA, but is used in Europe, and includes discarded parts of nuclear reactors that have come into contact with nuclear fuel, and materials resulting from the decommissioning of reactors. Another category is “mill tailings,” which are leftovers from the extraction of uranium from its ore. These are only slightly radioactive, and are usually thought to pose more of a chemical than a radiological hazard, as they often contain toxic heavy metals.
High-level waste is generally stored on site at nuclear power plants until a proper disposal site becomes available. During this initial storage period it is kept under at least 20 feet (6 meters) of water, which absorbs the radiation. The favored option for long-term disposal is deep under the ground, with the radioactive material encased in glass, and carefully monitored. Finding a suitable disposal site is, however, problematical, as plans to store highly radioactive waste at any given location tend to meet with fierce opposition. Spent fuel rods may also be stored above ground in large metal and concrete containers.
Another option for spent fuel rods is reprocessing. After a nuclear fuel bundle has been removed from a reactor, it