Uranium:

The  primary  nuclear  reactor  fuel  materials  used  presently  are  the  parts  thorium and uranium. Uranium has played the main role for purpose of both usability and availability. It could be used in the form of pure metal, as a constituent of an alloy, carbide, or as an oxide or other suitable compound. By metallic uranium was used as a fuel in previously reactors, its poor mechanical properties and great susceptibility to radiation damage excludes its use for commercial power reactors presently. The  source  material  for  uranium is  uranium  ore,  that  after  mining is concentrated in a "mill" and shipped as an impure form of the oxide U₃O₈  (yellow cake). The material is after that shipped to a materials plant where it is converted to uranium dioxide (UO₂), a ceramic that is the most general fuel material used in commercial power reactors. The UO₂ is  established  into  pellets  and clad  along with  zircaloy  (water-cooled  reactors)  or  stainless  steel  (fast sodium-cooled reactors) to form fuel elements. The cladding protects the fuel from attack through the coolant, avoids the escape of fission products, and gives geometrical integrity.

Oxide fuels have elaborates extremely satisfactory high-temperature, dimensional, and radiation stability sad chemical  compatibility  along with  cladding  metals  and  coolant  in  light-water  reactor service. Under the much more severe conditions in a fast reactor, therefore, even inert UO₂ starts to respond to its environment in a manner in which is frequently detrimental to fuel performance. Uranium dioxide is almost completely used in light-water-moderated reactors (LWR). Mixed oxides of plutonium and uranium are used in liquid-metal fast breeder reactors (LMFBR).

The  main drawbacks  of oxide fuels which have prompted the investigation of other fuel materials are their low uranium density and low thermal  conductivity which decreases along with increasing temperatures. The low density of uranium atoms in UO₂ needs a larger core for a given amount of fissile species than if a fuel of higher uranium density were used.  An increase in reactor size along with no increase in power raises the capital cost of the reactor. Poor thermal conductivity means in which the centerline temperature of the fuel and the temperature  difference among the middle and the surface of the fuel rod must be extremely large for enough fission heat be extracted from a unit of fuel to make electric power production economical. Alternatively,  central  fuel  temperatures  close  to  the  melting  point  have  a  beneficial  fission  product scouring effect on the fuel.