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1.17.1 Introduction

Nuclear fuels and structural materials are complex systems that have been very difficult to understand and model despite decades of concerted effort. Even single actinide oxide or metallic alloy fuel forms have yet to be accurately, fully represented. The problem is compounded in fuels with multiple actinides such as the transuranic (TRU) fuels envisioned for consuming long-lived isotopes in thermal or fast reactors. More­over, a fuel that has experienced significant burnup becomes a very complex, multicomponent, multi­phase system containing more than 60 elements. Thus, in an operating reactor the nuclear fuel is a high-temperature system that is continuously changing as fission products are created and actinides consumed and is also experiencing temperature and composi­tion gradients while simultaneously subjected to a severe radiation field. Although structural materials for nuclear reactors are certainly complex systems that benefit from thermochemical insight, the emphasis and examples in this chapter focus on fuel materials for the reasons noted above. The higher temperatures of fuels quickly drive them to the thermochemical equi­librium state, at least locally, and their compositional complexity benefits from computational thermo­chemical analysis. Related information on thermody­namic models of alloys can be found in Chapter 2.01, The Actinides Elements: Properties and Character­istics; Chapter 2.07, Zirconium Alloys: Properties and Characteristics; Chapter 2.08, Nickel Alloys: Properties and Characteristics; Chapter 2.09, Properties of Austenitic Steels for Nuclear Reactor Applications; Chapter 1.18, Radiation-Induced Segregation; and Chapter 3.01, Metal Fuel.

A major issue for nuclear fuels is that the original fuel material, whether the fluorite-structure phase for oxide fuels or the alloy for metallic fuels, has variable initial composition and also dissolves significant bred actinides and fission products. Thus, the fuel phase is a complex system even before irradiation and becomes significantly more complex as other elements are generated and dissolve in the crystal structure. Com­pounding the complexity is that, after significant burnup, sufficient concentrations of fission products are formed to produce secondary phases, for example, the five-metal white phase (molybdenum, rhodium, palladium, ruthenium, and technicium) and perovskite phases in oxide fuels as described in detail in Chapter 2.20, Fission Product Chemistry in Oxide Fuels. Thus, any chemical thermodynamic representation of the fuel must include models for the nonstoichiome­try in the fuel phase, dissolution of other elements, and formation of secondary, equally complex phases.

Dealing with the daunting problem of modeling nuclear fuels begins with developing a chemical thermodynamic (or thermochemical) understanding of the material system. Equilibrium thermodynamic states are inherently time independent, with the equilibrium state being that of the lowest total energy. Therefore, issues such as kinetics and mass transport are not directly considered. Although the chemical kinetics of interactions are important, they are often less so in the fuel undergoing burnup (fis­sioning) because of the high temperatures involved and resulting rapid kinetics and can often be neglected on the time scales involved for the fuel in reactor. The time dependence of mass transport, however, does influence fuel behavior as evidenced by the significant compositional gradients found in high burnup fuel whether metal or oxide, and most notably by attack of the clad by fission products and oxidation by species released from oxide fuel.

Although the equilibrium state provides no infor­mation on diffusion or vapor phase transport, it does provide source and sink terms for these phenomena. Thus, the calculation of local equilibrium within fuel volume elements can in principle provide activity/ vapor pressure values useful in codes for computing mass flux. Thermochemically derived properties of fuel phases also provide inherent thermal conductiv­ity, source terms for grain growth, potential corrosion mechanisms, and gas species pressures, all important for fuel processing and in-reactor behavior. Thermo­chemical insights can therefore provide support for modeling species and thermal transport in fuels.