Three main ceramic uranium fuels are UO2, UC, and UN (to a smaller extent U3Si and US), i. e. uranium sulfide. The operating experience with UO2 is the greatest even though UN and UC remain the best potential fuels for higher performance in the long term. Improvement in fuel performance and enhanced thermal efficiency require the fuel element temperature to be as high as possible. However, with metallic fuels, two main problems may occur: (a) central fuel melting and (b) excessive irradiation swelling and creep deformation due to irradiation instability at higher temperatures. In this regard, ceramic fuels have certain advan­tages over metallic uranium fuels: (a) higher permissible fuel and plant operating temperatures due to higher melting point, (b) good irradiation stability due to the absence of polymorphic phase transformation, and (c) high corrosion resistance to the environmental attack as a result of its chemical inertness and compatibility with cladding. The basic nuclear properties of competitive ceramic fuels are as follows:

(a) large number of fissile uranium (U235) atoms per unit volume of the fuel in order to avoid necessity for high enrichment, and (b) small neutron absorption cross section of the nonfissile components of the compound for preserving the neutron economy. The following sections discuss various aspects of uranium diox­ide, which is the mainstay of nuclear fuels used in current generation of power reactors. We will briefly discuss UN and UC.