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Oxide ceramics play a critical role in nuclear energy applications. For example, uranium dioxide (UO2) and uranium–plutonium mixed oxide (MOX) are widely used as nuclear fuels in light water reactors (LWRs) to produce about 15% of the electricity in the world. Other oxide ceramics such as pyrochlores have been extensively studied as waste forms for the immobilization of nuclear waste. Similar to other materials, radiation induces complex microstructural evolution in oxide ceramics. In nuclear fuels, nuclear fission produces not only point defects and defect clusters but also many fission products such as xenon (Xe) and krypton (Kr). The evolution of these defects and fission products can form many extended defects such as dislocation loops, gas bubbles, and metallic precipitates. In turn, many physical properties such as thermal conductivity and mechanical strength degrade significantly. Pyrochlores are complex oxides and usually contain more than two types of cations (e.g., A and B cations in A2B2O7). Under radiation, not only interstitials and vacancies are produced, but also cation antisite defects, in which some A and B cations exchange their positions. The accumulation of antisite defects can lead to amorphization and eventually degradation of the structural stability. Therefore, understanding the detailed information of radiation effects in these oxides is important for improving their performance for nuclear applications. Radiation damage is a multi-timescale and multilengthscale problem. The timescale spans from picosecond to year, and the lengthscale spans from angstrom to meter. Therefore, understanding the radiation effects requires a multiscale approach, both experimentally and computationally. In particular, the information at the atomistic and mesoscale levels is crucial for us to understand the detailed defect and microstructural evolution under radiation. In this topic, five articles have been selected to show some recent progress in using advanced characterization and modeling approaches to understand the complex radiation effects in oxide ceramics with atomisticand meso-scale resolution. In the article by Desgranges et al., the authors used Raman spectroscopy to try to understand the defect structures in alpha particle irradiated UO2. Compared with pristine UO2, three new peaks, namely U1, U2, and U3, appeared in the Raman spectrum of the irradiated UO2. The appearance of new Raman peaks could be induced by resonant Raman, formation of new molecular entities, or breakdown in symmetry. The authors argued that the last one could be the predominant mechanism for inducing these new Raman peaks. To support this argument, the authors compared the Raman spectrum of irradiated UO2 with that of U4O9, which has a known crystal structure. The three peaks also appear in the spectrum of U4O9, although some peaks have much stronger intensities than those in the irradiated UO2. U4O9 contains some cuboctahedral oxygen interstitial clusters that have been identified in neutron diffraction experiments previously, and the charge state of some uranium ions is found to change from 4+ to 5+. Accordingly, the coordination polyhedrons of uranium ions change from a cube for U to a square antiprism for U, which can lead to high U2 and U3 peaks. The authors also discussed that doping of trivalent (3+) cations can form octahedral polyhedrons, leading to the appearance of U1 and U2 peaks. Therefore, the authors proposed that radiation may change the charge state of U to U and U, and their corresponding coordination polyhedrons lead to the new peaks in Raman spectrum. Xian-Ming ‘‘David’’ Bai is the guest editor for the Nuclear Materials Committee of the TMS Structural Materials Division, and coordinated the topic Radiation Effects in Oxide Ceramics and Novel LWR Fuels in this issue. JOM, Vol. 66, No. 12, 2014

Recent Progress in Using Advanced Characterization and Modeling Approaches to Study Radiation Effects in Oxide Ceramics