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The actinide series is a group of chemical elements with atomic numbers ranging from 89 to 102,[note 1] including notable elements such as uranium and plutonium. The nuclides (or isotopes) thorium-232, uranium-235, and uranium-238 occur primordially, while trace quantities of actinium, protactinium, neptunium, and plutonium exist as a result of radioactive 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 shorter half-lives. Elements with atomic numbers greater than 94 do not exist naturally on Earth, and must be produced in a nuclear reactor.[2] However, certain isotopes of elements up to californium (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, with Australia having been the leading supplier.[7] Thorium is extracted as a byproduct of titanium, zirconium, tin, and rare earths from monazite, 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 as uranium dioxide and mixed 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 the lungs and irradiate lung tissue.
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