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Current and future nuclear power reactors and plants 187

been designed, FBRs are more efficient breeders. It is also notable that FBRs utilize
thesamefuel, UO 2 -PuO 2 , which has been used in some of the current nuclear power
reactors.
The Th 232 -U 233 cycle is of interest due to the abundance of thorium in the earth’s
crust is between three to five times that of uranium. There are large thorium deposits
in some countries such as India, Brazil, Australia, and United States. The U 238 -Pu 239
cycle is the most effective as far as fast neutrons are concerned. For Pu 239 , the number
of emittedneutronsina fission reaction per absorbed neutronsis greater, when fissionis
induced by fast neutrons rather than thermal neutrons. The additional neutrons emitted
can be utilized for transform more U 238 nuclides to Pu 239 . Hence, the fast-breeder reac-
tors are based on U 238 -Pu 239 in which Pu 239 and U 238 in the core undergo fission. In a
two-region reactor, the U 238 nuclides in the core and in the blanket are transmuted to
Pu 239 . The blanket surrounds the reactor core and is the region containing the fertile
nuclides.
Even though U 235 is used as the primary fissile nuclide in the fuel of nuclear reac-
tors, the fuel is designed in various geometrical configurations and chemical forms. In
terms of geometrical configuration, nuclear fuels have been designed in the form of
cylindrical pellets, annular pellets, pebbles, plates, and TRISO pellets. In the vast
majority of current nuclear power reactors cylindrical pellets are used, but in AGRs
annular pellets are used. In terms of the chemical structure, nuclear fuels can be clas-
sified into four categories: (1) metallic fuels, (2) ceramic fuels, (3) hydride fuels, and
(4) composite fuels.

4.5.2 Metallic fuels

Uranium, plutonium, and thorium are the most common metallic fuels. These metallic
fuels have high thermal conductivity, high fissile atom density, good neutron econ-
omy, and good fabrication. On the other hand, metallic fuels suffer from irradiation
instability, poor mechanical properties and corrosion resistance, especially, at high
temperatures, when exposed to air or water. The exposure of the metallic fuels to a
neutron flux results in fuel swelling and has a negative effect on the thermal conduc-
tivity. In addition, metallic fuels suffer from low melting points, and, furthermore, the
fuel undergoes a phase change. A phase transformation results in a volume change in
the fuel. Consequently, the use of metallic uranium fuel is limited to temperatures
below 660°C.
To improve these undesirable characteristics of the metallic uranium, uranium
alloys such as uranium-aluminum, uranium-magnesium, and uranium-molybdenum
have been developed. Metallic fuels have been used in some power reactors and
research reactors with relatively low operating temperatures.
For use in high-temperature applications, a potential fuel must have a high melting
point, high thermal conductivity, and good irradiation and mechanical stability. These
requirements eliminate the use of metallic fuels mainly due to their low melting points
and high irradiation creep and swelling rates. On the other hand, ceramic fuels have
promising properties, which has made these fuels as the fuels of choice for the current
nuclear power reactors and suitable candidates for high-temperature applications.