Nuclear fuel pellet. (Source: © frog / stock.adobe.com)

Nuclear fuel, usually uranium dioxide, sintered into a ceramic cylinder. Each type of reactor has its own type of fuel. The fuel pellet contains natural uranium combined with the fissile uranium isotope ²³⁵U. The concentration of fissile ²³⁵U is called enrichment, and for pressure water-cooled reactors it is usually 3—5 percent. A mixture of uranium and plutonium, called MOX, is often used in fast neutron reactors. The most commonly used form is uranium oxide, but attractive alternatives to it could be carbide, nitride, or metal. Additives, like the burnable reactor poison gadolinium, can be added to the pellet during fabrication. The pellets are usually stacked inside a cladding tube to form a fuel rod.