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Plutonium oxide melt structure and covalency

Nature Materials (2024)Cite this article

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Abstract

Advances in nuclear power reactors include the use of mixed oxide fuel, containing uranium and plutonium oxides. The high-temperature behaviour and structure of PuO2–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 PuO2–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 PuO1.76 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 CeO1.75, 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.

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Fig. 1: Aerodynamic levitation and laser-beam heating of PuO2–x high-temperature solids and melts.
Fig. 2: X-ray total scattering of high-temperature PuO2–x.
Fig. 3: Structure of molten PuO1.76.
Fig. 4: Structural surrogacy of CeO2–x for PuO2–x melts.

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Data availability

X-ray structure factor data for the PuO1.76 and CeO1.75 melts are provided in the Supplementary Information. All other relevant data are available from the corresponding author upon reasonable request.

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Acknowledgements

This work was supported by the US Department of Energy (DOE) through grants DE-SC0018601 and DE-SC0015241 and the Argonne Laboratory Directed Research and Development program. X-ray diffraction measurements were made at Sector 6-ID-D of the Advanced Photon Source, a US DOE Office of Science User Facility, operated by Argonne National Laboratory under contract no. DE-AC02-06CH11357. Computational modeling resources were provided by Bebop, a high-performance computing cluster operated by the Laboratory Computing Resource Center at Argonne National Laboratory. We gratefully acknowledge J. Vacca, L. Soderholm, W. VanWingeren and E. Schmidt for technical advice and safety management contributions that enabled the X-ray measurements; M. Schvaneveldt for assembling the sample chamber; A. Hebden for sample chamber design considerations; and S. Salbeck, Hadco Tool LLC, for advice and fabrication of the sample handling, nozzle housing and sample die components.

Author information

Authors and Affiliations

  1. Materials Development, Inc., Arlington Heights, IL, USA

    Stephen K. Wilke & Richard Weber

  2. X-Ray Science Division, Advanced Photon Source, Argonne National Laboratory, Lemont, IL, USA

    Stephen K. Wilke, Chris J. Benmore & Richard Weber

  3. ISIS Neutron & Muon Source, Rutherford Appleton Laboratory, Chilton, Didcot, UK

    Oliver L. G. Alderman

  4. Department of Chemical & Biomolecular Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, USA

    Ganesh Sivaraman

  5. Chemical and Fuel Cycle Technologies Division, Argonne National Laboratory, Lemont, IL, USA

    Matthew D. Ruehl, Krista L. Hawthorne & Mark A. Williamson

  6. Tamalonis Technologies, Highland Park, IL, USA

    Anthony Tamalonis

  7. Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM, USA

    David A. Andersson

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Contributions

C.J.B., O.L.G.A., M.A.W. and R.W. conceived the idea for X-ray diffraction measurements and developed the sample chamber with A.T. O.L.G.A. wrote the proposal for X-ray beamtime. S.K.W., C.J.B. and R.W. conducted the X-ray measurements. S.K.W. analysed the X-ray data and performed the EPSR measurements with guidance from C.J.B., O.L.G.A. and R.W. G.S. developed the GAP from DFT calculations and performed the MD simulations, under the guidance of D.A.A. M.D.R. and K.L.H. prepared the PuO2 samples and facilitated radioactive-sample handling under the guidance of M.A.W. S.K.W. prepared the manuscript draft, with revisions contributed by all authors.

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Correspondence to Stephen K. Wilke.

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Extended data

Extended Data Fig. 1 Fitting of Gaussian distributions to the total PDF for molten PuO1.76.

a, Fitting a single PuO distribution to the leading edge of the first peak, and a single PuPu distribution to the leading edge of the second peak. b, Optimized peak fitting for all pair correlations, guided by the bond distances and coordinations anticipated from f-PuO2 and α-Pu2O3 crystal structures.

Extended Data Fig. 2 Ce-O phase diagram.

Colored markers show oxygen gas partial pressure (pO2) isobars32. The redox trajectory for a sample heated under pure O2 is shaded in pink.

Extended Data Fig. 3 Faber-Ziman X-ray weighting factors.

a, PuO1.76. b, CeO1.75. From Eq. 2.

Extended Data Fig. 4 Examples of sample temperature during levitation and laser beam heating.

a, Emissivity-corrected temperature measurements (10 Hz acquisition) from the optical pyrometer for PuO2−x solid heated under 5% CO (Ar balance). Laser power was increased incrementally, interspersed with X-ray diffraction measurements taken while the sample was held isothermally. b, Zoomed-in view of a single isotherm from (a). For the time period 364–416 s, the temperature mean was 2140 K with a standard deviation of 71 K. c, Temperature of molten PuO1.57 during a different heating run than (a-b). X-ray measurements were analyzed for the time period 105–110 s, which had a temperature mean of 2730 K and a standard deviation of 2 K.

Extended Data Fig. 5 X-ray diffraction and Rietveld refinements for selected crystalline samples.

a, Initial Pu-O material before heating. b, Pure f-PuO2 after heating under O2. The observed X-ray diffraction patterns (green curves) and calculated Rietveld refinement models (orange curves) are compared against the Bragg reflections for f-PuO2, α-Pu2O3, and β-Pu2O3 (blue, teal, and red vertical ticks). The black curves show the differences between the X-ray data and refinement models, divided by the estimated standard uncertainties.

Extended Data Fig. 6 Fitting of a Lorentzian function to the structure factor’s first principal peak, for molten PuO1.76.

The fit53 includes the Lorentzian contribution mirrored across Q = 0 Å−1 and is constrained15 to \(S\left(0\right)=1-\left\langle\; {f(0)}^{2}\right\rangle /{\left\langle\; f(0)\right\rangle }^{2}={-}1.113\). The X-ray diffraction data were extrapolated to Q = 0 using this Lorentzian fit, prior to the Fourier transform to obtain the PDFs (Eqs. 3, 4).

Extended Data Fig. 7 Effect of top hat convolution on X-ray data.

Comparison of the (a) X-ray structure factor and (b) total PDF for molten PuO1.76, processed with and without the top hat convolution54 in GudrunX. For this comparison, Qmax = 11.9 Å−1 was used for both structure factors to avoid large truncation oscillations in the PDF without top hat.

Extended Data Table 1 Faber-Ziman weighting factors
Full size table

Supplementary information

Supplementary Information

Supplementary Figs. 1–3, Tables 1–4 and discussion.

Supplementary Data 1

X-ray total structure factor for molten PuO1.76.

Supplementary Data 2

X-ray total structure factor for molten CeO1.75.

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Wilke, S.K., Benmore, C.J., Alderman, O.L.G. et al. Plutonium oxide melt structure and covalency. Nat. Mater. (2024). https://doi.org/10.1038/s41563-024-01883-3

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