Molten Nuclear Fuel Structure Could Help Prevent Meltdowns

Scientists have for the first time obtained X-ray structures that reveal the dynamics of the nuclear fuel uranium dioxide as it transitions from a solid to its molten state (Science 2014, DOI: 10.1126/science.1259709). Researchers have previously relied on computer models to investigate the behavior of molten UO₂, but the descriptions haven’t been consistent. This relative dearth of knowledge has hindered the ability to predict properties important for reactor safety, such as viscosity and compressibility of the material. Lawrie B. Skinner at Argonne National Laboratory and colleagues used synchrotron X-ray diffraction studies to probe UO₂ beads heated by a laser. Solid UO₂ forms crystals made of UO₈ cubic polyhedra. The group found that as the solid heats up to 2,670 K it undergoes a so-called lambda transition in which oxygen becomes increasingly disordered and the material’s heat capacity and thermal conductivity rise sharply. When the solid actually melts, at 3,270 K, the number of oxygen neighbors close to each uranium drops, forming structures composed largely of UO₆ and UO₇.