Actinides in the environment
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The actinide series is a group of chemical elements with atomic numbers ranging from 89 to 102, 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. 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. However, certain isotopes of elements up to californium (atomic number 98) still have practical applications which take advantage of their radioactive properties.
While all actinides are radioactive, actinides and actinide compounds comprise a significant portion of the Earth's crust. 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. The global production of uranium in 2021 was over six million tons, with Australia having been the leading supplier. 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.
The primary hazard associated with actinides is their radioactivity, though they may also cause heavy metal poisoning if absorbed into the bloodstream. 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. Inhaled actinide compounds, however, will be more damaging as they remain in the lungs and irradiate lung tissue.