Fuel containing fissile elements used in nuclear reactors. Uranium, plutonium, or thorium are used. Some fast neutron reactors can burn long-lived actinides recovered from spent fuel. The fissile material may be bound into a solid matrix in the form of an oxide, carbide, nitride, or as a metal fuel. The most commonly used type of fuel is uranium dioxide enriched with 3—5 percent of the fissile isotope ²³⁵U. The fuel is usually in the form of ceramic pellets. These are placed in a cylindrical cladding to form a fuel rod. A bundle of many fuel rods forms a fuel assembly. In some types of reactors, the fuel takes the form of spheres (e.g. High Temperature Gas-cooled Reactor) or the fuel is dissolved in molten salt which also serves as a coolant (Molten Salt Reactor).