Advances in nuclear power reactors include the use of mixed oxide fuel, containing uranium and plutonium oxides. The high-temperature behaviour and structure of PuO_2–x above 1,800 K remain largely unexplored, and these conditions must be considered for reactor design and planning for the mitigation of severe accidents. Here, we measure the atomic structure of PuO_2–x through the melting transition up to 3,000 ± 50 K using X-ray scattering of aerodynamically levitated and laser-beam-heated samples, with O/Pu ranging from 1.57 to 1.76. Liquid structural models consistent with the X-ray data are developed using machine-learned interatomic potentials and density functional theory. Molten PuO_1.76 contains some degree of covalent Pu–O bonding, signalled by the degeneracy of Pu 5f and O 2p orbitals. The liquid is isomorphous with molten CeO_1.75, demonstrating the latter as a non-radioactive, non-toxic, structural surrogate when differences in the oxidation potentials of Pu and Ce are accounted for. These characterizations provide essential constraints for modelling pertinent to reactor safety design.

核反应堆的进步包括使用含有铀和钚氧化物的混合氧化物燃料。 PuO _{2– x}在 1,800 K 以上的高温行为和结构在很大程度上尚未被探索，反应堆设计和规划必须考虑这些条件，以减轻严重事故。在这里，我们使用空气动力悬浮和激光束加热样品的 X 射线散射，测量了 PuO _{2– x}通过高达 3,000 ± 50 K 的熔化转变的原子结构，O/Pu 范围为 1.57 至 1.76。使用机器学习的原子间势和密度泛函理论开发了与 X 射线数据一致的液体结构模型。熔融的 PuO _{1.76含有一定程度的共价 Pu-O 键合，由 Pu 5} f和 O 2 p轨道的简并性表明。该液体与熔融的 CeO _1.75是同晶的，当考虑到 Pu 和 Ce 的氧化电位差异时，表明后者是一种非放射性、无毒的结构替代物。这些特征为与反应堆安全设计相关的建模提供了基本约束。