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190 Managing Global Warming

4.5.3.1 Oxide fuels

Oxides of uranium, thorium, and plutonium have been used as nuclear fuels (i.e., UO 2 ,
PuO 2 , and MOX) have good corrosion resistance, high melting point, and excellent
mechanical and irradiation stability. As such, UO 2 , PuO 2 , and MOX have been used
as fuels in nuclear power reactors such as PWRs, BWRs, and CANDU reactors. In
addition, oxide fuels are chemically compatible with the cladding (sheath) materials
and water, which is used as the coolant in these reactors. On the other hand, the
disadvantages of oxide fuels include low uranium-atom density, low thermal conduc-
tivity, and poor thermal-shock resistance.
Also, it should be noted that many physical properties of nuclear fuels have
relatively large uncertainties and depend on many factors including porosity,
manufacturing process, stoichiometric composition, and irradiation with neutron flux
(see Fig. 4.50).
Currently, there is an interest in using thorium-based fuels in nuclear reactors.
Thorium is widely distributed in nature and is approximately three times as abundant
as uranium. However, ThO 2 does not have any fissile elements to fission with thermal
neutrons. Consequently, ThO 2 must be used in combination with a “driver” fuel
(e.g., UO 2 , UC, or PuO 2 ), which has U 235 as its initial fissile element. The presence
of a “driver” fuel such as UO 2 in a nuclear reactor core results in the production of
enough neutrons, which in turn start the thorium cycle.

Fig. 4.50 Uncertainty in thermal conductivity of UO 2 [1].