Nuclear waste is classified as high-level waste (HLW) or low-level waste (LLW) based on radioactivity, source, and half-life. HLW consists of spent fuel rods from nuclear reactors, while LLW includes materials that have come into contact with radioactive substances or have become radioactive through exposure to radiation. Disposal of nuclear waste is an ongoing and controversial issue, with deep underground storage being the preferred option. Nuclear waste emits ionizing radiation, which can damage or kill cells, cause mutations, birth defects, and cancer. However, nuclear waste isn’t necessarily more dangerous than chemical poisons, which are produced in much larger quantities.
Nuclear waste is radioactive waste produced by nuclear reactors or left over from research projects, medical uses and the manufacture of nuclear weapons. Classification methods may vary slightly from country to country, but two categories are generally recognised: high-level waste (HLW) and low-level waste (LLW), 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 on the level and type of radioactivity, with some materials being very dangerous in raw form and others posing little threat under most circumstances. Safe disposal of nuclear waste is an ongoing and controversial issue.
Types and sources
High-activity waste consists mainly of spent fuel rods from nuclear reactors. These power plants rely on nuclear fission to generate heat, and the fuel is turned into rods that can be moved in and out of the reactor core to control the process. After a while, the fission rate 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, essentially includes everything that does not fall into the category of high-level waste. It comes from a wide variety of sources. It consists of materials that have come into contact with radioactive substances, or have themselves become radioactive through exposure to some form of radiation, as well as small quantities of radioisotopes from research institutions and hospitals. Examples are the protective clothing worn by personnel working with radioactive materials and the syringes and needles used for injecting radioisotopes for medical purposes. It generally remains potentially dangerous for a few tens and a few hundred years.
Some countries have an intermediate category of waste. This is not recognized in the United States, but is used in Europe and includes discarded parts of nuclear reactors that have come into contact with nuclear fuel and materials from reactor decommissioning. Another category is the “mill steri”, which are the leftovers of the extraction of uranium from its ore. These are only mildly radioactive and are usually thought to pose more of a chemical risk than a radiological risk, as they often contain toxic heavy metals.
Disposal
High-activity waste is typically stored on-site at nuclear power plants until a suitable 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 preferred option for long-term disposal is deep underground, with the radioactive material encased in glass and closely monitored. Finding a suitable disposal site is, however, problematic, as plans to store highly radioactive waste at any given location tend to meet with fierce opposition. Spent fuel rods can also be stored above ground in large metal and concrete containers.
Another option for spent fuel rods is reprocessing. After a bundle of nuclear fuel 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 selects the elements in spent fuel and allows for the recovery and reuse of useful fuel. This process still leaves some highly radioactive waste that needs to be disposed of, but the amount is much less. As of 2013, reprocessing is not practiced in the US, but is done in the UK and France.
Low-level waste is stored in special containers which, where necessary, have a degree of shielding suitable for the level and type of radioactivity. Substances that emit alpha radiation do not require shielding, as they are only hazardous if ingested or inhaled. Gamma radiation and neutron emitters, however, require significant shielding. As with high-activity wastes, materials are buried, but usually relatively close to the surface.
The risk posed by nuclear waste
Nuclear waste is potentially dangerous because it emits types of radiation that have sufficient energy to ionize atoms, meaning that atoms become electrically charged due to, for example, the removal of electrons. In the human body, these charged atoms can interact with other atoms, causing chemical changes in cells and 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 into perspective, however, nuclear waste isn’t necessarily more dangerous than chemical poisons, which are produced in much larger quantities. It has been estimated that public exposure to carcinogens from coal-fired power plants is much greater than from nuclear waste, due to naturally occurring radioactive elements and chemicals released into the atmosphere by burning coal.
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