Why is uranium used to make atomic bombs?

The ability to store and release explosive energy makes uranium very useful, especially in the production of nuclear weapons.

Uranium is one of the heaviest natural elements. In its nucleus, there are 92 protons and a variable number of neutrons, ranging from 140 to 146. However, only a few combinations occur spontaneously, the most abundant being uranium-238 (92 protons and 146 neutrons). ) and uranium-235 (92 protons and 146 neutrons).

A block of uranium unearthed in the wild.

Uranium can only form in some extreme events, known as r-processes, which occur in intense cosmic explosions such as supernovas or neutron star collisions. From those explosions, it spread throughout the universe and became an important, albeit rare, component of the Earth. The presence of decaying uranium is exactly why our planet has heat inside.

That is the essential feature of uranium. Over time, it emits radiation in the form of helium atoms, turning into thorium. Almost all uranium isotopes (versions with a different number of neutrons) have very long half-lives (the time it takes for a sample to halve its uranium content). Uranium-238 has a half-life of up to 4.5 billion years.

Uranium has been used since Roman times as a yellow glaze in ceramics and glass. In 1789, German chemist Martin Heinrich Klaproth mixed nitric acid with a solid, then neutralized the solution with sodium hydroxide. This reaction produces a yellow substance that sinks to the bottom.

When heated with coal, it turned into a black powder that Kalproth mistook for pure uranium, but most likely an oxide. He named this new element after the planet Uranus, discovered just eight years ago by Willaim Herschel. It was not until 1841 that the first sample of pure Uranium was successfully isolated by the chemist Eugène-Melchior Péligot.

Today, uranium is no longer used in coloring glasses and enamels, instead its main application lies in radioactivity, discovered in 1896 by Henri Becquerel.

Four decades later, in 1934, a team of Italian physicists led by Enrico Fermi bombarded uranium with neutrons and found it emitted electrons and positrons (the antiparticles of electrons). Subsequent work by Otto Hahn, Fritz Strassmann, Lise Meitner and Otto Robert Frisch showed that uranium could decay to a lighter element and named the process nuclear fission. This is what makes uranium a "game changer".

One kilogram of uranium-235, if led through complete fission, can release chemical energy equivalent to burning 1.5 million kilograms of coal. That explosive ability to store and release energy has allowed the element to be used in power generation and in nuclear weapons such as the atomic bomb.

In nuclear power plants, the radioactive emission of fuel rods made of uranium heats a coolant, the heat generated then heats water in another container and turns it into steam. Steam pushes the generator's turbines to generate electricity, and importantly, the process does not produce greenhouse gas emissions.

In either case, natural uranium is not ideal in many reactors. More than 99.2% of uranium mined on Earth is uranium-238, while uranium-235 makes up only 0.711%. Uranium-235 makes a very good nuclear chain reaction, helping to maintain a stable reaction. However, we need to have enough of this isotope in the reactor's fuel rod. That's when uranium needs to be enriched, through isotope separation to increase the percentage of uranium-235.

The remainder of the enrichment process produces depleted uranium (with less uranium-235). It is used as a container for transporting radioactive materials, industrial radiography equipment, as well as military purposes such as armor plating and armor-piercing ammunition.