Nuclear waste is the radioactive waste produced by nuclear reactors, or left over from research projects, medical uses, and the manufacture of nuclear weapons. Methods of classification can vary somewhat from country to country, but two categories — high-level waste (HLW), and low-level waste (LLW) — are generally recognized, based on radioactivity, source and half-life. These materials are potentially dangerous because they emit ionizing radiation, which can damage or kill cells, produce mutations and birth defects, and cause cancer. The degree of risk depends upon the level, and type, of radioactivity, with some materials being very hazardous in raw form, and others posing little threat in most circumstances. The safe disposal of nuclear waste is an ongoing and controversial issue.
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.
Disposal
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 still contains most of its original uranium, but mixed with highly radioactive fission products. This radioactive cocktail can be separated through a process called nuclear reprocessing, which sorts the elements in the spent fuel, and allows useful fuel to be recovered and reused. This process still leaves behind some highly radioactive waste that needs to be disposed of, but the quantity is very much smaller. As of 2013, reprocessing is not practiced in the USA, but is carried out in the UK and in France.
Low-level waste is stored in special containers that, where necessary, have a degree of shielding appropriate to the level, and type, of radioactivity. Substances that emit alpha radiation do not require shielding, as they are only dangerous if swallowed or inhaled. Gamma radiation and neutron emitters, however, require significant shielding. As with high-level waste, the materials are buried underground, but usually relatively near the surface.
The Risk Posed by Nuclear Waste
Nuclear waste is potentially dangerous because it emits types of radiation that have enough energy to ionize atoms, meaning that the atoms become electrically charged due, for example, to electrons being removed. In the human body, these charged atoms can interact with other atoms, causing chemical changes in cells and in DNA. High levels of ionizing radiation kill cells and can cause immediate life-threatening effects, while lower levels can cause genetic damage and cancer. To put things in perspective, however, nuclear waste is not necessarily more dangerous than chemical poisons, which are produced in much larger quantities. It has been estimated that public exposure to cancer-causing agents from coal-fired power stations is much greater than from nuclear waste, due to chemicals, and natural radioactive elements, released into the atmosphere from the burning of coal.
By Michael Anissimov