Reaction kinetics and chemical thermodynamics of nuclear materials

The present chapter covers fundamental concepts of chemical kinetics and thermodynamics that are key to the understanding and prediction of nuclear fuel chemistry at each stage of the nuclear fuel cycle and illustrates these with concrete examples applied to nuclear fuel materials. The basic concepts of chemical kinetics, including reaction rates and the temperature dependence of rate constants, are first introduced. The fundamentals of classical and statistical thermodynamics are then treated. The molar thermodynamic functions of solids and gases (i.e., enthalpy of formation, entropy, and heat capacity) are also defined, and methods used for their theoretical and experimental determination are mentioned. Furthermore, the basics of phase diagrams are explained, and key thermodynamic parameters that govern the fuel fabrication and in-reactor behavior are presented: melting transition, fuel stoichiometry, oxygen potential, and vapor pressure. The chemical state of fission products are also treated, including the derivation and application of Ellingham diagrams. Finally, some key concepts of solution thermodynamics are introduced, which are relevant to the understanding of the chemical state and transport behavior of actinides and fission products in the environment during the storage of spent fuel and geological disposal. The basics of redox reactions, hydrolysis/complexation, and solubility/precipitation are presented, and the derivation of Pourbaix and speciation diagrams is explained.