Amorphous form of boron. (Source: © Björn Wylezich / stock.adobe.com)

A chemical element with atomic number 5. Depending on its crystalline form, it is either a dark metalloid or brown powder. As it is not created by stellar nucleosynthesis, it is a low-abundant element in the solar system. Boron constitutes about 0.001 percent by weight of the Earth’s crust. It occurs mostly in compounds, e.g., borax (sodium borate), ulexite (sodium-calcium borate), or colemanite (calcium borate), and is mined for wide commercial use.

In nuclear power plants, boron is used for its ability to absorb neutrons. Boron carbide is a compound of control rods or shut-off rods. Boric acid is added to the water during fuel handling in water-cooled reactors.

Boron is a possible fuel for thermonuclear fusion. The proton-boron fusion demands very high ignition temperatures (around 1.4 billion kelvin) but produces no neutrons that could activate the reactor chamber. This reaction was first demonstrated in 2023 in an LHD stellarator.