= Solution
<plutonium> introduces \b[<decay heat>, self-irradiation and composition-sensitive thermochemical behaviour], as well as demanding radiological handling and material accountancy. <isotopic composition> affects heat and <neutron> production, so total <plutonium> mass alone does not specify the handling environment. The elemental metal also has inconvenient allotropic and thermal behaviour; using an appropriate stable ceramic fuel form avoids treating bare metal as a straightforward substitute for <uranium> oxide.
In <mixed oxide fuel>, <plutonium dioxide> and <uranium dioxide> form the engineered ceramic system. Important challenges are composition uniformity, <thermal conductivity> and melting margin, oxygen chemistry, restructuring and <fission gas> behaviour, and compatibility with <nuclear fuel cladding>. A local composition variation can produce a local power or <temperature> variation. During service, <actinide> redistribution can change the initial spatial distribution; its direction cannot be inferred from the <temperature> gradient alone. Autoradiographic examination of irradiated mixed-oxide pins directly investigated the competing transport effects.<Plutonium redistribution in fast reactor mixed oxide fuel pins by M. Bober and others>[Journal of Nuclear Materials 47, 187–197, abstract: irradiated-pin observations, thermal diffusion, evaporation–condensation and oxygen chemistry]{ref}
Mitigation is to use a characterised fuel form and isotopic inventory, qualify composition and <microstructure> uniformity, retain suitable thermal margins, and establish irradiation performance and interface compatibility. <radioactive material confinement>, remote or protected handling and waste management address the accompanying contamination and exposure problems. These are \b[materials requirements and verification objectives]; no <plutonium> preparation, separation or fuel-manufacturing procedure is needed to answer them.
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