Among the various thermal properties, thermal conductivity is the most useful property for the nuclear scientist. It is the ability of the material to transfer heat from a region of high temperature to a region of low temperature. In normal conditions, thermal conductivity and linear power determine the peak fuel oper­ating temperature. Under the accident conditions, the thermal conductivity of the fuel determines the maximum permissible linear rating, vmax, if central melting is to be avoided [10, 38]. The thermal conductivity, k, allows the determination of centre temperature of fuel, Tc, when the surface temperature Ts, is known by using the conductivity integral,

For ceramic oxide systems, two contributions are used to describe the behavior of thermal conductivity with temperature: (i) the phonon-phonon interaction and (ii) the density of defects (phonon scattering centers) in the lattice [91]. For temperatures below 1,900 K, the contribution of the free electrons to the thermal conductivity can be neglected.