A tiny fuel pellet the size of a fingernail. Just 7 grams. Yet it holds as much energy as a full ton of coal. That is uranium — the heaviest element found in nature in any real quantity. With a density of 19.1 g/cm³, it is 70% heavier than lead. Atomic number 92.
Uranium has been quietly decaying for 4.5 billion years — roughly the age of Earth itself. Heat from that decay still warms our planet's core. But uranium's biggest role today is energy. Over 440 reactors worldwide run on uranium fuel, generating about 10% of the world's electricity.
Natural uranium is a mix of isotopes. 99.27% is uranium-238, and only 0.72% is uranium-235 — the one capable of chain fission. Reactor fuel is enriched to 3-5% U-235. The IAEA strictly monitors this process worldwide.
Uranium remains a key resource for low-carbon electricity. Countries like France get over 70% of their power from nuclear plants. New reactor designs promise safer, more efficient use of uranium — and some can even burn spent fuel. The debate continues, but uranium's energy density is unmatched by any fossil fuel.
Uranium is both radioactive and chemically toxic. Alpha particles cannot penetrate skin, but inhaling uranium dust is extremely dangerous — it settles in the lungs and irradiates tissue from inside. As a heavy metal, uranium damages kidneys. Enriched uranium at critical mass (52 kg for U-235) can trigger an uncontrolled chain reaction. Working with uranium requires protective clothing, respirators, and dosimeters at all times.
Uranium is 70% heavier than lead. A 10 cm cube of uranium weighs 19.1 kg — about the same as two full buckets of water.
Uranium-238 has a half-life of 4.468 billion years — nearly the exact age of Earth. Geologists use this to date the oldest rocks on the planet.
A single uranium fuel pellet (7 g) releases as much energy as one ton of coal. One kilogram of U-235 replaces 2,700 tons of coal.
Radioactive decay of uranium and thorium deep underground generates about half of Earth's internal heat. Without it, tectonic plates would stop moving.
In 1789, Martin Klaproth named his new element after the planet Uranus, discovered just 8 years earlier. Nobody imagined the power hidden inside this metal.
Uranium glass was a hit in the 1800s. Uranium oxide gave dishes a vivid green color that glows under ultraviolet light. Collectors still hunt for these pieces.
| Isotope | Mass (u) | Abundance | Half-life | Decay |
|---|---|---|---|---|
234U☢ | 234.040952 | trace | 2.455×10⁵ years | α |
235U☢ | 235.043930 | 0.72% | 7.04×10⁸ years | α |
238U☢ | 238.050788 | 99.27% | 4.468×10⁹ years | α |
Analysis of pitchblende