The thermal effects of different types of void volumes within a fuel pin, such as porosity, central void, and the gas gap between fuel and clad are examined. A new general relation for the dependence of thermal conductivity on the closed porosity is deduced. For given gas contents and specific ranges of pore temperatures and sizes, a simple approximate equation is set up, which is in good agreement with experimentally obtained results. A central void is very effective in reducing the maximum temperature; and its use implies a considerable increase in linear pin power. In-pile migration of the porosity in the hotter regions of oxide fuel pins forms or increases the central void and densifies the hotter region. This effect is calculated in a two-zone porosity model. For a uniform gap between fuel and clad, the temperature drop is calculated as a function of gap width, linear pin power, inner temperature of clad, inside radius of clad, emissivities of surfaces, and types of gas within the gap, such as noble gases and mixtures of He with gaseous fission products.