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Molten uranium dioxide structure and dynamics

Science  21 Nov 2014:
Vol. 346, Issue 6212, pp. 984-987
DOI: 10.1126/science.1259709

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Abstract

Uranium dioxide (UO2) is the major nuclear fuel component of fission power reactors. A key concern during severe accidents is the melting and leakage of radioactive UO2 as it corrodes through its zirconium cladding and steel containment. Yet, the very high temperatures (>3140 kelvin) and chemical reactivity of molten UO2 have prevented structural studies. In this work, we combine laser heating, sample levitation, and synchrotron x-rays to obtain pair distribution function measurements of hot solid and molten UO2. The hot solid shows a substantial increase in oxygen disorder around the lambda transition (2670 K) but negligible U-O coordination change. On melting, the average U-O coordination drops from 8 to 6.7 ± 0.5. Molecular dynamics models refined to this structure predict higher U-U mobility than 8-coordinated melts.

Containing the nuclear elephant's foot

Molten nuclear fuel composed of large amounts of uranium dioxide is extremely dangerous. Liquid UO2 has a high melting temperature and is very reactive, making it difficult to find a suitable sample container within which to study it. Skinner et al. bypassed the container and used instead a laser to heat beads of UO2 levitated in a synchrotron x-ray beam with inert gas. They found an unexpected increase in the fluidity of molten nuclear fuel caused by a fall in the number of oxygen atoms surrounding each uranium cation. These findings are important when considering how to contain nuclear fuel during an accident.

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