= Solution
<Mixed oxide fuel> begins as a specified mixture of uranium and plutonium oxides. A fuel pin develops strong radial <temperature> gradients during operation. The components and oxygen need not remain spatially uniform: their <chemical potentials> depend on <temperature>, local composition and <oxygen-to-metal ratio>.
<Actinide redistribution in oxide fuel> can involve thermally driven solid-state <diffusion> and evaporation–condensation transport across pores or a central cavity. Oxygen redistributes as well, changing local oxidation states and the relative thermodynamic tendencies of uranium- and plutonium-bearing species. Pore migration, grain growth and central-void formation couple compositional redistribution to fuel restructuring. Differential local burnup also changes composition over time.
The direction and magnitude of plutonium migration cannot be inferred from a <temperature> gradient alone. In particular irradiated fast-reactor pins, enrichment around a central cavity has been observed, but the result depends on stoichiometry and operating history. <M. Bober> and colleagues investigated thermal-diffusion and evaporation–condensation effects using autoradiographs of sections of fuel irradiated in RAPSODIE.<Plutonium redistribution in fast reactor mixed oxide fuel pins by M. Bober and others>[Abstract: irradiated-pin autoradiographs, oxygen redistribution, and dependence on stoichiometric state]{ref}
Redistribution changes the radial <heat generation rate> and material properties. Concentrating more fissile material near an already hot region can raise local heat generation; changes in <thermal conductivity> and local melting behavior modify the <temperature> response. Gas release, pore formation and fuel–cladding contact can also change thermal resistance and mechanical loading. A spatially averaged composition can therefore underestimate a local <temperature> or chemical interaction.
Segregated <fission products> add another concern: mobile species can accumulate at colder interfaces or react with the cladding, while insoluble products form separate phases. Fuel performance must therefore track composition, <temperature> and <microstructure> together, rather than treating segregation as merely a cosmetic change in an otherwise fixed fuel.
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