Editing Radioactive waste (section)

== Nature and significance ==
A quantity of radioactive waste typically consists of a number of [[radionuclides]], which are unstable isotopes of elements that undergo [[radioactive decay|decay]] and thereby emit [[ionizing radiation]], which is harmful to humans and the environment. Different isotopes emit different types and levels of radiation, which last for different periods of time.

=== Physics ===
{{Main article|Fission product yield}}
{{See also|Radioactive decay}}
{{Medium-lived fission products}}
{{Long-lived fission products}}
The radioactivity of all radioactive waste weakens with time. All [[radionuclide]]s contained in the waste have a [[half-life]]—the time it takes for half of the atoms to decay into another [[nuclide]]. Eventually, all radioactive waste decays into non-radioactive elements (i.e., [[stable nuclide]]s). Since radioactive decay follows the half-life rule, the rate of decay is inversely proportional to the duration of decay. In other words, the radiation from a long-lived isotope like [[iodine-129]] will be much less intense than that of a short-lived isotope like [[iodine-131]].<ref>{{cite web |date=28 March 2011 |title=What about Iodine-129 – Half-Life is 15 Million Years |url=http://www.nuc.berkeley.edu/node/2115 |url-status=dead |archive-url=https://web.archive.org/web/20130513121013/http://www.nuc.berkeley.edu/node/2115 |archive-date=13 May 2013 |access-date=1 December 2012 |work=Berkeley Radiological Air and Water Monitoring Forum |publisher=University of California |language=en-us |publication-place=Berkeley, California}}</ref> The two tables show some of the major radioisotopes, their half-lives, and their [[fission product yield|radiation yield]] as a proportion of the yield of fission of uranium-235.

The energy and the type of the [[ionizing radiation]] emitted by a radioactive substance are also important factors in determining its threat to humans.<ref>{{cite book |title=Introduction to Radiological Physics and Radiation Dosimetry |last=Attix |first=Frank |year=1986 |publisher=Wiley-VCH |location=New York |isbn=978-0-471-01146-0 |pages=2–15,468,474 |url=https://books.google.com/books?id=PL8971RdEfoC}}</ref> The chemical properties of the radioactive [[chemical element|element]] will determine how mobile the substance is and how likely it is to spread into the environment and [[contaminate]] humans.<ref>{{cite book |last=Anderson |first=Mary |author-link=Mary P. Anderson |title=Applied Groundwater Modeling |author2=Woessner |first2=William |publisher=Academic Press Incorporated |year=1992 |isbn=0-12-059485-4 |location=San Diego, California |pages=325–327 |language=en-us}}</ref> This is further complicated by the fact that many radioisotopes do not decay immediately to a stable state but rather to radioactive [[decay product]]s within a [[decay chain]] before ultimately reaching a stable state.

=== Pharmacokinetics ===
Exposure to radioactive waste may cause health impacts due to ionizing radiation exposure. In humans, a dose of 1 [[sievert]] carries a 5.5% risk of developing cancer,<ref name="ICRP103">{{cite journal |title=The 2007 Recommendations of the International Commission on Radiological Protection |journal=Annals of the ICRP |year=2007 |volume=37 |series=ICRP publication 103 |issue=2–4 |url=http://www.icrp.org/publication.asp?id=ICRP%20Publication%20103 |isbn=978-0-7020-3048-2 |url-status=live |archive-url=https://web.archive.org/web/20121116084754/http://www.icrp.org/publication.asp?id=ICRP+Publication+103 |archive-date=2012-11-16}}</ref> and regulatory agencies [[linear no-threshold model|assume the risk is linearly proportional to dose]] even for low doses. Ionizing radiation can cause deletions in chromosomes.<ref>Gofman, John W. ''Radiation and human health''. San Francisco, California: Sierra Club Books, 1981, p. 787.</ref> If a developing organism such as a [[fetus]] is irradiated, it is possible a [[birth defect]] may be induced, but it is unlikely this defect will be in a [[gamete]] or a gamete-forming [[cell (biology)|cell]]. The incidence of radiation-induced mutations in humans is small, as in most mammals, because of natural cellular-repair mechanisms, many just now coming to light. These mechanisms range from DNA, [[mRNA]] and protein repair, to internal lysosomic digestion of defective proteins, and even induced cell suicide—apoptosis<ref>Sancar, A. et al ''Molecular mechanisms of mammalian DNA repair and the DNA damage checkpoints''. Washington, D.C.: National Institutes of Health PubMed.gov, 2004.</ref>

Depending on the decay mode and the [[pharmacokinetics]] [[radiopharmacology|of an element]] (how the body processes it and how quickly), the threat due to exposure to a given activity of a [[radioisotope]] will differ. For instance, [[iodine-131]] is a short-lived [[beta decay|beta]] and [[gamma decay|gamma]] emitter, but because it concentrates in the [[thyroid]] gland, it is more able to cause injury than [[caesium]]-137 which, being [[water soluble]], is rapidly excreted through urine. In a similar way, the [[alpha decay|alpha]] emitting actinides and [[radium]] are considered very harmful as they tend to have long [[Biological half-life|biological half-lives]] and their radiation has a high [[relative biological effectiveness]], making it far more damaging to tissues per amount of energy deposited. Because of such differences, the rules determining biological injury differ widely according to the radioisotope, time of exposure, and sometimes also the nature of the chemical compound which contains the radioisotope.