We continue to review some of the most important materials in heat treatment and metallurgy.
Uranium (chemical symbol: U)
Uranium (Fig. 1) is a dense, silvery-white metal that is extremely harmful to humans due to both its chemical toxicity and natural radioactivity. Even limited exposure has been known to increase the risk of getting a variety of cancers. Uranium is rather ductile and malleable but will tarnish in air, creating a dark oxide layer (UO2).
Fine uranium powder is pyrophoric, meaning it will spontaneously ignite when exposed to air. Elemental uranium is highly reactive and will react with most nonmetallic elements and many of their compounds. Since all of uranium’s isotopes are radioactive, they are constantly experiencing radioactive decay. While uranium is often associated with high radioactivity, it is actually not very radioactive compared to some other elements. Uranium-238 has a half-life of 4.5 billion years, and uranium-235 has a half-life of 700 million years.
While uranium oxide did see use in ancient times for its yellow tint in ceramic glazes, it wasn’t formally discovered until 1789. German chemist Martin Heinrich Klaproth made his discovery when he was studying the mineral pitchblende (Fig. 2), which was thought at the time to be a zinc/iron ore. After dissolving the mineral in nitric acid and subsequently adding potash (potassium salt) to the precipitate that remained, he experienced a reaction that wasn’t consistent with reactions of other known elements. While Klaproth had actually discovered uranium oxide, he named the new element after the recently discovered planet of Uranus, which itself was named after the Greek god of the sky. It wasn’t until 1841 that a French chemist named Eugène-Melchior Péligot had isolated pure uranium from its oxide.
Uranium’s primary use in the world is as a source of nuclear fuel, which is harnessed to generate electrical power and create nuclear weapons. In fact, uranium is so effective as a fuel for creating energy that 1 kilogram of uranium can produce as much energy as 1,500,000 kilograms (1,500 tons) of coal. While 99% of naturally occurring uranium is in the isotope of U-238, it’s less common isotope, uranium-235, is considered fissile, meaning it’s nucleus can be split by thermal neutrons.
Nuclear engineers will enrich U-238 with a composition of 3-5% U-235 to make it more efficient. This enriched uranium will power nuclear reactors in cities as well as in military applications, such as powering nuclear submarines and aircraft carriers. A byproduct of the process for processing uranium in power plants will yield depleted uranium (0.2% U-235, which is half as radioactive), which sees use as effective tank armor and in ammunitions (Fig. 3).
Here are a few important facts about uranium.
- Atomic number: 92
- Atomic weight: 238.02891
- Melting point: 1408 K (1135°C or 2075°F)
- Boiling point: 4404 K (4131°C or 7468°F)
- Density: 18.95 grams per cubic centimeter
- Phase at room temperature: Solid
- Element classification: Metal
- Period number: 7
- Group number: none
- Group name: Actinide
- Electron configuration: [Rn] 5f36d17s2