Uranium is a naturally radioactive heavy metal that has often been perceived as potentially dangerous. However, we cannot maintain our standard of living without it.
In its pure form, uranium (U) is a silvery metal almost twice as dense as lead. It originated in exploding stars billions of years ago and heat from its radioactive decay is believed to have kept the Earth’s core in molten state. Uranium occurs most commonly in nature in oxide form as pitchblende. Natural uranium consists of three isotopes, U-238, U-235 and U-234, all of which are radioactive but only U-235 is capable of sustaining a spontaneous fission reaction when the critical mass is reached. During groundwater transport, uranium preferentially adheres to soil particles, giving a concentration typically about 35 times higher than that in pore water. Although uranium can bioconcentrate in some biota, it is not known to biomagnify in terrestrial or aquatic food chains.
Uranium occurs in trace amounts in everything we encounter in our daily life, including our own body. The top 1 m of soil in a typical suburban garden in the UK has been reported to contain 2 kg U.
Uranium is less dangerous and more useful than often believed
The hazard of uranium is twofold: 1) the metal itself is toxic, and 2) its natural decay gives rise to ionization radiation, mainly in the form of alpha particles, and unstable intermediate daughter products such as radium and radon. Uranium is a health hazard only if it is taken into the body by ingestion or inhalation. Once ingested most uranium is excreted within a few days while a small fraction (0.2-5%) absorbed into the bloodstream is deposited preferentially in the bone (~22%, where it can remain for years) and the kidney (~12%, where it is discharged in days). Similarly, only a small portion of inhaled uranium usually penetrates to the lung’s alveolar region, where it may be retained for many years.