For fission reactors, the fuel (typically based on uranium) is usually based on the metal oxide; the oxides are used rather than metals themselves because the oxide melting point is very high compared to the metal and because it cannot burn, being already in the oxidized state.
UOX (Uranium Oxide):
Uranium dioxide is a black semiconducting solid. It can be made by reacting uranyl nitride with a base (ammonia) to form a solid (ammonium uranate).
MOX (Mixed Oxide):
Mixed oxide, or MOX fuel, is a blend of plutonium and natural or depleted uranium which behaves similarly (though not identically) to the enriched uranium for which most nuclear reactors were designed. MOX fuel is an alternative to low enriched uranium (LEU) fuel used in the light water reactors which predominate nuclear power generation. Metal Fuel:
Metal fuels have the advantage of much higher heat conductivity than metal oxide (MOX) fuels but cannot survive equally high temperatures. Metal fuels have the potential for the highest fissile atom density, Metal fuels are normally alloyed, but some metal fuels have been made of pure uranium metal.
In a fast neutron reactor, the minor actinides produced by neutron capture of uranium and plutonium can be used as fuel. Metal actinide used is typically an alloy of zirconium, uranium, plutonium and minor actinides. It can be made inherently safe as thermal expansion of the metal alloy will increase neutron leakage.
Molten plutonium, alloyed with other metals to lower its melting point and encapsulated in tantalum.