Plutonium dioxide (PuO2) is the most common form used as a reactor fuel. PuO2 is not used
alone as a reactor fuel; it is mixed with uranium dioxide. This mixture ranges from 20%
plutonium dioxide for fast reactor fuel to 3% to 5% for thermal reactors.
Plutonium-239 can serve as the fissile material in both thermal and fast reactors. In thermal
reactors, the plutonium-239 produced from uranium-238 can provide a partial replacement for
uranium-235. The use of plutonium-239 in fast reactors is much more economical, because
breeding takes place, which results in the production of more plutonium-239 than is consumed
The basic nuclear reactor fuel materials used today are the elements uranium and thorium.
Uranium has played the major role for reasons of both availability and usability. It can be used
in the form of pure metal, as a constituent of an alloy, or as an oxide, carbide, or other suitable
compound. Although metallic uranium was used as a fuel in early reactors, its poor mechanical
properties and great susceptibility to radiation damage excludes its use for commercial power
The source material for uranium is uranium ore, which after mining is
concentrated in a "mill" and shipped as an impure form of the oxide U3O8 (yellow cake). The
material is then shipped to a materials plant where it is converted to uranium dioxide (UO2), a
ceramic, which is the most common fuel material used in commercial power reactors. The UO2
is formed into pellets and clad with zircaloy (water-cooled reactors) or stainless steel (fast
sodium-cooled reactors) to form fuel elements. The cladding protects the fuel from attack by the
coolant, prevents the escape of fission products, and provides geometrical integrity.
Oxide fuels have demonstrated very satisfactory high-temperature, dimensional, and radiation
stability and chemical compatibility with cladding metals and coolant in light-water reactor
service. Under the much more severe conditions in a fast reactor, however, even inert UO2
begins to respond to its environment in a manner that is often detrimental to fuel performance.
Uranium dioxide is almost exclusively used in light-water-moderated reactors (LWR). Mixed
oxides of uranium and plutonium are used in liquid-metal fast breeder reactors (LMFBR).
The major disadvantages of oxide fuels that have prompted the investigation of other fuel
materials are their low uranium density and low thermal conductivity that decreases with
increasing temperatures. The low density of uranium atoms in UO2 requires a larger core for a
given amount of fissile species than if a fuel of higher uranium density were used. The increase
in reactor size with no increase in power raises the capital cost of the reactor. Poor thermal
conductivity means that the centerline temperature of the fuel and the temperature difference
between the center and the surface of the fuel rod must be very large for sufficient fission heat
be extracted from a unit of fuel to make electric power production economical. On the other
hand, central fuel temperatures close to the melting point have a beneficial fission product
scouring effect on the fuel.