More common than gold, uranium is abundant on Earth. But less than 1% of the naturally occurring element is useful for nuclear power.
Uranium was discovered in 1789 by Martin Klaproth, a German chemist. It is a radioactive element found naturally in the environment in very small amounts — scientists call these trace amounts — in rock, soil,watersystems, such as streams, on plants and as dust in the air.
Uranium has the same silver-white-gray finish as other heavy metals, such as lead, cadmium or tungsten. And it, too, is very dense.
TheInternational Atomic Energy Agency(IAEA) calculates that a 10-centimeter (3.93 inch) cube of uranium weighs about 20 kilograms (44 pounds).
When uranium is not of the natural variety, it can be either "enriched" or "depleted."
Enriched uranium is used as fuel for nuclear power plants and the nuclear reactors that run naval ships and submarines. It can also be used innuclear weapons.
Depleted uranium is used for radiation shielding or as projectiles in armor-piercing weapons.
There are three naturally occurring uranium isotopes: U-234, U-235 and U-238.
An isotope is a type of atom that differs from the original chemical element in the number of neutrons it has. The number of protons and electrons remain the same.
In uranium isotopes, the number after the "U" relates to the combined number of protons and neutrons. For example, there are 92 protons in uranium and 143 neutrons in the U-235 isotope: so 92+143 = 235.
U-235 is the most sought uranium isotope, used in bothnuclear reactorsand nuclear weapons. However, naturally occurring uranium only consists of 0.72% U-235, so it must be artificially "enriched" to be useful.
Like sifting dirt for gold, the U-235 is extracted and retained, while the rest of the uranium is set aside. The process is repeated until predominantly U-235 remains.
Uranium enriched to 3.67% U-235 is sufficient for use in nuclear power.
Uranium enriched to 90% or more is considered to have reached "nuclear threshold capability" — suitable for use in a weapon.
You can probably find it in the soil in your backyard. The IAEA says the average concentration of natural uranium in soil is two parts per million, which equals about 0.0002%. That's what you might call a trace amount.
As dust in the air, uranium can settle on the surface water of rivers, streams and lakes and then find its way to the bottom of the water, where it mixes with natural uranium already there.
Livestock consume uranium when they feed on grass, but it is quickly shed again through urine and feces.
Uranium can also get into groundwater. The World Health Organization (WHO) says, "intake through drinking water is normally extremely low."
But that depends on the concentration of uranium in the water you drink. The IAEA says in some parts of the world concentrations can be very high and that this results in much higher intakes of uranium from drinking water than from food. For example, consumption of uranium in parts of Finland can be tens of micrograms per day.
Whether that amount is carcinogenic for humans is unclear. The WHO says there is insufficient data from human and other animal laboratory studies.
Uranium can be life-threatening, but that, too, depends on your exposure to it.
If you're exposed to uranium in the environment, outside your body — what experts call external exposure — the risk to your health is low.
Uranium decays as alpha particles, and the skin can block such particles.
Alpha particles are relatively slow and heavy compared with other forms of nuclearradiationand are unable to penetrate very deeply or through matter.
If, however, a person ingests a high concentration of uranium, it can causecancer, affecting bones or the liver. And if a person inhales a large concentration of uranium, the alpha particles are likely to cause lung cancer.
Uranium is also a toxic chemical, so any form of consumption will severely damage a person's organs, particularly the kidneys.
Some of the uranium in the environment runs off from industrial works. Some of it gets into the atmosphere through mining.
The majority of the world's production ofuranium from minesis from Kazakhstan, Canada and Australia. But it is also mined in Niger and Namibia.
This story was updated on June 23, 2025, with a new section on uranium enrichment.