Fission-Informed Partial/Local High-Entropy Design in Nuclear Ceramics: Opportunities and Constraints

High-entropy ceramics are generally designed by distributing several principal cations over one or more crystallographic sublattices. Nuclear fuel ceramics develop a different form of chemical complexity: fission, neutron capture, radioactive decay, redox changes, temperature gradients, and diffusion produce a phase-partitioned UO2 microstructure containing dissolved fission products, gas bubbles, noble-metal precipitates, complex oxides, and chemically modified interfaces. Here we use this irradiation-driven partitioning to motivate partial/local high-entropy (PLHE) design, in which deliberately introduced compositional complexity is confined to a function-controlling sublattice, grain-boundary region, interface, or secondary phase while the primary matrix is retained. Irradiated UO2 is treated as a non-equilibrium heterogeneous state rather than a high-entropy phase. PLHE is defined by resolved spatial or crystallographic confinement, local compositional/site evidence, a specified performance bottleneck, and comparison with a matched lower-complexity reference. The resulting framework treats configurational entropy as one screening metric within broader thermodynamic and kinetic constraints. Waste-form ceramics provide the most direct near-term test bed, whereas fuel-matrix and in-fuel concepts require additional thermal, neutronic, redox, and irradiation screening.