AVS2014 Session AC+AS+MI+SA+SS-TuM: Synchrotron Radiation and Laboratory Based Investigations of Actinides and Rare Earths

Advanced x-ray synchrotron radiation techniques, addressing spatial and temporal fluctuations of structural and electronic degrees of freedom, hold outstanding scientific promises for the future of actinide research [1]. Indeed, by probing hidden order parameters and elementary electronic excitations with high sensitivity and resolution, element- and edge-specific (resonant and non-resonant) x-ray scattering experiments provide the bricks for building the novel conceptual frameworks necessary to unravel the complexity of actinides. Here, I will present selected results from these experiments and discuss what they tell us.

Whereas resonant x-ray diffraction (RXD) with photon energies tuned to the M_4,5 absorption edges of actinide ions are used to study the order of electric quadrupole moments in oxides and intermetallics [2-4], resonant x-ray emission spectroscopy (RXES) and non-resonant inelastic X-ray scattering (NIXS) are applied to study the bulk electronic configuration in solids, liquids and gases [5,6]. In particular, the high penetration depth of hard X-rays employed in NIXS enables flexible containment concepts, facilitating investigations of radioactive materials in the liquid phase or under extreme conditions. Finally, inelastic x-ray scattering (IXS) can be used to map phonon dispersion branches with an energy resolution comparable to the one afforded by inelastic neutron scattering, but using crystal samples with sizes orders of magnitude smaller than those required by neutrons.

In the first example, I will show how a combination of diffraction, RXES, and absorption near-edge spectroscopy experiments at high-pressure can be used to study the correlation between polymorphism, mixing of different electronic configurations, and hybridization effects in elemental americium [7]. The potential of RXD in elucidating the nature of "hidden order" will be illustrated by the examples of the low-temperature phases in NpO₂ [8] and URu₂Si₂ [9]. Then, I will present NIXS results interrogating the O_4,5 absorption edges of uranium and plutonium materials, and results of IXS studies of the vibrational dynamics in PuCoGa₅ and NpO₂.

[1] R. Caciuffo, E. C. Buck, D. L. Clark, G. van der Laan, MRS Bulletin 35, 889 (2010)

[2] P. Santini et al., Rev. Mod. Phys. 81, 807 (2009)

[3] H. C. Walker et al., Phys. Rev. Lett. 97, 137203 (2011)

[4] Z. Bao et al., Phys. Rev. B 88, 134426 (2013)

[5] T. Vitova et al., Phys. Rev. B 82, 235118 (2010)

[6] R. Caciuffo et al., Phys. Rev. B 81, 195104 (2010)

[7] S. Heathman et al., Phys. Rev. B 82, 201103(R) (2010)

[8] N. Magnani et al., Phys. Rev. B 78, 104425 (2008)

How do you look inside a nuclear waste package without breaking it open? This question is important when the contained corrosion products are potentially flammable and radioactive. Synchrotron x-rays have been used to perform micro-scale in situ observation and characterisation of uranium entrapped in grout; a simulation for some intermediate level waste. Using specially designed analysis cells X-ray tomography and x-ray diffraction have been used to generate both qualitative and quantitative data from a grout encapsulated uranium sample before, and after, deliberately constrained H₂ corrosion. Tomographic reconstructions determined the extent, rates and mechanisms of the oxidation reaction by assessing the relative densities between the materials and the volume of corrosion products. The oxidation of uranium in grout was shown to follow the anoxic U + H₂O oxidation regime, and the pore network within the grout was observed to influence the induction period for the initiation of uranium hydride formation across the surface of the metal. Powder diffraction analysis identified the corrosion products UO₂ and UH_3, and permitted measurement of corrosion induced stress. Together, x-ray tomography and diffraction provide a means of accurately determining the types and degree of uranium corrosion occurring, thereby offering a future means for studying the reactions occurring in real full-scale waste package systems.