The composition, atomic structure, and microstruc­ture of an irradiated fuel are complex and evolve during irradiation. Because the thermal conductivity depends on each of these characteristics, a large number of parameters are required to determine its value. At the mesoscopic scale, the main relevant parameters are the microstructure (porosity, grains size, fission gas bubbles, and plutonium or additives distribution) and the eventual precipitates. The para­meters acting at the microscopic (atomic) scale are plutonium and the additives that are dissolved in the lattice, the soluble and insoluble fission products (fission products) and fission gases dispersed as atoms, radiation damage (interstitials, vacancies), stoichiometry, and fission density. A scheme sum­marizing some of these parameters is presented in Figure 3. The radiation effects in UO2 are described with more detail in Chapter 2.18, Radiation Effects in UO2.