Theactinide series is a group ofchemical elements withatomic numbers ranging from 89 to 102,[note 1] including notable elements such asuranium andplutonium. Thenuclides (orisotopes)thorium-232,uranium-235, anduranium-238 occurprimordially, while trace quantities ofactinium,protactinium,neptunium, and plutonium exist as a result ofradioactive decay and (in the case of neptunium and plutonium)neutron capture of uranium.[note 2] These elements are far more radioactive than the naturally occurring thorium and uranium, and thus have much shorterhalf-lives. Elements with atomic numbers greater than 94 do not exist naturally on Earth, and must be produced in anuclear reactor.[2] However, certain isotopes of elements up tocalifornium (atomic number 98) still have practical applications which take advantage of their radioactive properties.[3][4]
While all actinides are radioactive, actinides and actinide compounds comprise a significant portion of the Earth's crust.[5] There is enough thorium and uranium to be commercially mined, with thorium having a concentration in the Earth's crust about four times that of uranium.[6] The global production of uranium in 2021 was over six million tons, withAustralia having been the leading supplier.[7] Thorium is extracted as a byproduct oftitanium,zirconium,tin, and rare earths frommonazite, from which thorium is often a waste product. Despite its greater abundance in the Earth's crust, the low demand for thorium in comparison to other metals extracted alongside thorium has led to a global surplus.[8]
The primary hazard associated with actinides is their radioactivity, though they may also cause heavy metal poisoning if absorbed into the bloodstream.[9] Generally, ingested insoluble actinide compounds, such asuranium dioxide andmixed oxide (MOX) fuel, will pass through the digestive tract with little effect since they have long half-lives, and cannot dissolve and be absorbed into the bloodstream.[10] Inhaled actinide compounds, however, will be more damaging as they remain in thelungs and irradiate lung tissue.
Actinium can be found naturally in traces in uranium ore as227Ac, anα andβ emitter with half-life 21.773 years. Uranium ore contains about 0.2 mg of actinium per ton of uranium. It is more commonly made in milligram amounts by neutron irradiation of226Ra in a nuclear reactor. Natural actinium almost exclusively consists of one isotope,227Ac, with only minute traces of other shorter-livedisotopes (225Ac and228Ac) occurring in other decay chains.[11]
InIndia, a large amount ofthorium ore can be found in the form ofmonazite inplacer deposits of the Western and Eastern coastaldunesands, particularly in theTamil Nadu coastal areas. The residents of this area are exposed to a naturally occurring radiation dose ten times higher than the worldwide average.[12]
Thorium is found at low levels in most rocks andsoils, where it is about three times more abundant than uranium and about as abundant aslead. On average, soil commonly contains around 6 parts per million (ppm) thorium.[13] Thorium occurs in severalminerals; the most common is the rare earth-thorium-phosphate mineralmonazite, which contains up to 12% thorium oxide. Several countries have substantial deposits.232Th decays very slowly (itshalf-life is about three times theage of the Earth). Otherisotopes of thorium occur in the thorium and uranium decay chains. These are shorter-lived and hence much more radioactive than232Th, though on a mass basis they are negligible.
Thorium has been linked tolivercancer. In the past,thoria (thorium dioxide) was used as acontrast agent for medical X-ray radiography but its use has been discontinued. It was sold under the nameThorotrast.
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Protactinium-231 occurs naturally in uranium ores such as pitchblende, to the extent of 3 ppm in some ores. Protactinium is naturally present in soil, rock, surface water, groundwater, plants and animals in very low concentrations (on the order of 1 ppt or 0.1picocuries per gram (pCi/g).
Uranium is a natural metal which is widely found. It is present in almost all soils and it is more plentiful thanantimony,beryllium,cadmium,gold,mercury,silver, ortungsten, and is about as abundant asarsenic ormolybdenum. Significant concentrations of uranium occur in some substances such asphosphate rock deposits, and minerals such aslignite, and monazite sands in uranium-richores (it is recovered commercially from these sources).
Seawater contains about 3.3 parts per billion of uranium by weight[14] as uranium (VI) forms solublecarbonate complexes. Extraction of uranium from seawater has been considered as a means of obtaining the element. Because of the very low specific activity of uranium the chemical effects of it upon living things can often outweigh the effects of its radioactivity. Additional uranium has been added to the environment in some locations, from thenuclear fuel cycle and the use ofdepleted uranium in munitions.
Like plutonium,neptunium has a high affinity for soil.[15] However, it is relatively mobile over the long term, and diffusion of neptunium-237 in groundwater is a major issue in designing adeep geological repository for permanent storage ofspent nuclear fuel.237Np has a half-life of 2.144 million years and is therefore a long-term problem; but its half-life is still much shorter than those ofuranium-238,uranium-235, oruranium-236, and237Np therefore has higherspecific activity than those nuclides. It is used only to makeplutonium-238 when bombarded with neutrons in a lab.
Plutonium in the environment has several sources. These include:
Plutonium, like other actinides, readily forms aplutoniumdioxide (plutonyl) core (PuO2). In the environment, thisplutonyl core readily complexes withcarbonate as well as other oxygenmoieties (OH−, NO2−, NO3−, and SO42−) to form charged complexes which can be readily mobile with low affinities to soil.
PuO2 formed from neutralizing highly acidic nitric acid solutions tends to form polymeric PuO2 which is resistant to complexation. Plutonium also readily shifts valences between the +3, +4, +5 and +6 states. It is common for some fraction of plutonium in solution to exist in all of these states in equilibrium.
Plutonium is known to bind to soil particles very strongly; see above[where?] for an X-ray spectroscopic study of plutonium in soil andconcrete. Whilecaesium has very different chemistry from the actinides, it is well known that both caesium and many actinides bind strongly to the minerals in soil. It has been possible to use134Cs-labeled soil to study the migration of Pu and Cs is soils. It has been shown thatcolloidal transport processes control the migration of Cs (and will control the migration of Pu) in the soil at theWaste Isolation Pilot Plant.[16]
Americium often enters landfills from discardedsmoke detectors. The rules for the disposal of smoke detectors are very relaxed in most municipalities. For instance, in theUK it is permissible to dispose of a smoke detector containing americium by placing it in the dustbin with normal household rubbish, but each dustbin worth of rubbish is limited[clarification needed][by law?] to only containing one smoke detector. The manufacture of products containing americium (such as smoke detectors) as well as nuclear reactors and explosions may also release the americium into the environment.[17]
In 1999, a truck transporting 900 smoke detectors inFrance was reported to have caught fire; it is claimed that this led to a release of americium into the environment.[18] In the U.S., the "Radioactive Boy Scout"David Hahn was able to buy thousands of smoke detectors at remainder prices and concentrate the americium from them.
There have been cases of humans being exposed to americium. The worst case was that ofHarold McCluskey, who was exposed to an extremely high dose ofamericium-241 after an accident involving aglove box. He was subsequently treated withchelation therapy. It is likely that the medical care which he was given saved his life; despite similar biodistribution and toxicity to plutonium, the two radioactive elements have different solution-state chemistries.[19] Americium is stable in the +3oxidation state, while the +4 oxidation state ofplutonium can form in the human body.[20]
The most common isotope americium-241 decays (half-life 432 years) to neptunium-237 which has a much longerhalf-life, so in the long term, the issues discussed above for neptunium apply.[21]
Americium released into the environment tends to remain in soil and water at relatively shallow depths and may be taken up by animals and plants during growth; shellfish such asshrimp take up americium-241 in their shells, and parts ofgrain plants can become contaminated by exposure.[22] In a 2021 paper, J.D. Chaplin et al. reported advances in thediffusive gradients in thin films technique, which have provided a method to measure labile bioavailableamericium in soils, as well as in freshwater and seawater.[23]
Atmosphericcurium compounds are poorly soluble in common solvents and mostly adhere to soil particles. Soil analysis revealed about 4,000 times higher concentration of curium in the sandy soil particles than in water present in the soil pores. An even higher ratio of about 18,000 was measured inloam soils.[24]
Californium is fairly insoluble in water, but it adheres well to ordinary soil, and concentrations of it in the soil can be 500 times higher than in the water surrounding the soil particles.[25]
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