Plutonium is one of the most complex elements in the periodic table, displaying highly unusual behavior and a total of six allotropic phases between room temperature and its low melting point (640 °C) at ambient pressure. The high-temperature δ-phase (face-centered cubic) of plutonium is the most interesting phase for metallurgical applications as it is the most ductile compared to the other phases—in particular the α-phase (simple monoclinic), which is the stable phase at room temperature and relatively brittle. Notably, the δ-phase can be stabilized at room temperature and pressure by alloying with gallium in small amounts. Although δ-phase plutonium oxidizes at a lower rate than other phases, it still remains very sensitive to corrosion, which may be severe in long-term storage under an inadequately controlled atmosphere.
High temperature oxidation of δ-phase plutonium under an oxygen atmosphere leads to the formation of an oxide scale composed of an outer layer of face-centered cubic PuO2 and an inner layer of Pu2O3. This inner layer of Pu2O3 exhibits a polymorphic character as both cubic and hexagonal (α- and δ-phases) crystallographic structures may be present. The overall oxide scale growth kinetics consists of an initial parabolic growth stage mainly resulting from the thickening of the Pu2O3 layer, followed by linear growth of PuO2. Furthermore, oxidation leads to an unexpected destabilization of the δ phase with the appearance of a pure phase of plutonium during oxidation, corresponding to the emergence of a gallium-depleted layer below the oxide scale as well as an enrichment of this element on the outer surface of the scale. Although formation of Ga2O3 is suspected, the formation enthalpy of PuO2 is energetically more favorable than that of Ga2O3. Alternatively, gallium may be incorporated into plutonium oxide scale: ab-initio calculations indicate that it is energetically favorable to place gallium atoms in plutonium substitutional sites of PuO2 lattice.
The exact role played by the gallium solute element during oxidation is not yet understood. Several important questions need to be answered: Does the gallium diffuse through the oxide scale up to the scale/gas interface in order to be oxidized instead of plutonium atoms? This would have the effect of limiting the formation of the plutonium oxide. Also, does the gallium remain inside the oxide scale and where exactly? At present, there is limited experimental data for gallium behavior and consequently many questions remain unanswered.
Figure 1. Fourier Transforms (FTs) of the averaged EXAFS spectra, recorded in fluorescence mode at the Ga K-edge for an incidence angle of 3° with PuO2 lattice (blue line) and β-Ga2O3 as standard material (red line). (Note that FTs exhibit pseudo radial distribution functions from which real inter-atomic distances cannot be read directly).
Experimental characterization of the gallium local environment
To provide a better understanding of gallium behavior during oxidation, we investigated how gallium atoms are localized in the plutonium oxide crystalline lattice, focusing on the characterization of the atomic local environment around the gallium atoms within a PuO2 layer. Therefore, EXAFS (extended X-ray absorption fine structure) measurements were performed on an oxidized δ-PuGa 1 at.% alloy at the MARS (multi-analysis on radioactive samples) beamline located at the SOLEIL synchrotron facility near Paris, France. Measurements were performed in fluorescence mode at the Ga K-edge (10,367 eV) in grazing incidence geometry (to analyze the PuO2 layer) and at the Pu L3-edge (18,057 eV) in a normal incidence-grazing-exit geometry to reduce the so-called self-absorption effect.
In the Fourier transform (FT) of the Ga K-edge-averaged EXAFS data (which can be considered as a pseudo-radial distribution function, Fig. 1), only a well-ordered first coordination shell seems to be detected. Thus, the local order around the gallium atoms may be reduced to first-nearest neighbors (1NN). Next, Ga2O3 was analyzed as reference standard material to check if gallium was present in the oxide scale as Ga2O3. Comparing the EXAFS spectra shows that the 1NN shells of both compounds are similar, indicating that gallium in the oxide scale is likely surrounded by oxygen atoms with Ga–O distances close to those in Ga2O3. However, the 2NN contribution in the EXAFS spectrum of Ga2O3 corresponding to gallium cations is clearly missing in the EXAFS spectrum of the PuO2 layer. This demonstrates that gallium is not present as Ga2O3 (precipitates or layer). Therefore, gallium atoms are very likely incorporated in the PuO2 lattice and surrounded by oxygen atoms. However, since no 2NN shell appears in the EXAFS spectrum at the Ga K-edge, the local structure around gallium atoms may be significantly disordered and remains unresolved. Consequently, the best way to determine this local structure was to analyze EXAFS data using the reverse Monte Carlo (RMC) method, which allows exploration of numerous local atomic configurations.
Figure 2. Initial structure 2×2×2 supercells of PuO2 containing one gallium atom (green sphere) in the following positions: (i) plutonium substitutional site, (ii) octahedral site, (iii) oxygen substitutional site, and (iv) interstitial site (with coordination to two neighboring oxygen atoms). The RMC calculations were performed using periodic boundary conditions (PBC) with a 5×5×5 simulation box size containing 12,000 independent atoms including 125 Ga atoms.
Reverse Monte Carlo simulation
The RMC method, as implemented in the EvAX software package, is an atomistic simulation technique based on an evolutionary algorithm (EA) that allows the 3D atomic structure to be reconstructed by minimizing differences between experimental and simulated EXAFS spectra. Both Ga K-edge and Pu L3-edge EXAFS spectra were simultaneously analyzed using this RMC/EA method. For this simulation, a structural model consisting of a 2×2×2 supercell of PuO2 was built by incorporating one gallium atom inside the lattice in one of the four possible crystallographic sites: a plutonium substitutional site, an octahedral site, an oxygen substitutional site, or an interstitial site (with coordination to two neighboring oxygen atoms), as shown in Fig. 2. At each iteration, during the simulation, a new atomic configuration was generated by randomly displacing all of the atoms. After this, the corresponding Ga K-edge and Pu L3-edge configuration-averaged EXAFS spectra were calculated.
At the end of the simulation, the final atomic configuration that gave the best agreement with the experimental EXAFS data was saved and used to calculate the partial radial distribution functions (RDFs) g(R) around the gallium and plutonium atoms for further analysis. Good agreement between the experimental and theoretical EXAFS spectra was obtained with the Pu L3-edge (Fig. 3a) whatever the initial gallium position in the PuO2 lattice. This means that the long-range order of the PuO2 crystalline structure lattice is not significantly affected by the presence of gallium. The final RDF gPu–X(R) (Fig. 4a) exhibits enlarged peaks which do not perfectly match the positions of atoms in the different crystallographic sites of the ideal crystalline lattice. This broadening is however characteristic of the effects of vibrational motion of the atoms since EXAFS experiments were performed at room temperature.
Figure 3. Experimental and calculated Pu L3-edge (a) and Ga K-edge (b) EXAFS spectra FTs. RMC/EA simulations were performed from supercells of PuO2 containing initially one gallium atom within the lattice either in: plutonium substitutional site, octahedral site, oxygen substitutional site, or interstitial site (with coordination to two neighboring oxygen atoms).
With the Ga K-edge data, a good agreement between experimental and theoretical EXAFS spectra was obtained only for the PuO2 structures with the gallium atom in the plutonium substitutional site or the octahedral site (Fig. 3b). The RDF gGa–X(R) for gallium (Fig. 4b) in the plutonium substitutional site exhibits a large local disorder around the gallium atoms, with a splitting into two main contributions of the 1NN shell peak corresponding to eight oxygen atoms, initially located at 2.34 Å. The former consists of two thin peaks at a distance of 1.83 and 1.91 Å, each corresponding to one oxygen atom. The latter is shifted to longer distance by 0.4 Å and includes a series of four peaks at 2.35, 2.54, 2.77, and 2.96 Å, corresponding to the remaining six oxygen atoms of the initial 1NN shell. The loss of local order around the gallium atoms is experimentally responsible for the extinction of the second contribution of the experimental Ga K-edge EXAFS spectrum FT (Fig. 1). The RDF gGa–X(R) for gallium in the octahedral site after the RMC/EA simulation also exhibits changes of the same scale. In addition, the study of gallium, oxygen, and plutonium atom displacements reveals that gallium atoms are no longer located in their initial high symmetry site. The gallium atoms move to be closer to two oxygen atoms and further from the other six oxygen atoms.
In parallel, DFT calculations highlighted that placing the gallium atom in the plutonium substitutional site appears to be energetically favorable because of its atomic radius in addition to the positive electron interaction effects, as it maintains the electronic structure of PuO2.
We obtained a good agreement between structural analysis and DFT calculations when incorporating a gallium atom into the plutonium substitutional site of the PuO2 lattice. Thus, during the oxidation process, gallium atoms likely diffuse into the plutonium substitutional site in the PuO2 layer. There is a driving force corresponding to an energy gain when incorporating gallium atoms into plutonium vacancies, which supports this process. Therefore, plutonium vacancies must exist, possibly resulting from self-irradiation of plutonium and also from the oxygen gradient inducing point defects in the oxide scale.
Figure 4. Radial distribution functions (a) gPu–X(R) and (b) gGa–X(R) corresponding to the final atomic configuration obtained after RMC/EA simulation giving the best agreement with the experimental EXAFS. Calculation was performed from supercells of PuO2 containing initially one gallium atom in a plutonium substitutional site.
Summary
High temperature oxidation of PuGa alloys under oxygen atmospheres leads to the formation of an oxide scale composed of an outer layer of PuO2 and an inner layer of Pu2O3. This leads to a surprising destabilization of the δ phase, suggesting that it also modifies the structure of the alloy. Coupling of EXAFS experiments with reverse Monte Carlo (RMC) simulations permitted us to characterize the atomic local environment around the plutonium and gallium atoms in the outer PuO2 layer formed on δ-PuGa 1 at.% alloy. The results have highlighted the presence of gallium in the plutonium oxide scale. However, no formation of gallium aggregates or gallium oxides was detected. Instead, RMC analysis of experimental data indicates that gallium atoms are present within the PuO2 lattice. More precisely, the results showed that gallium atoms may occupy a plutonium substitutional site or octahedral site. This incorporation leads to a large local disorder with displacement of gallium atoms from their initial high symmetry crystallographic site. A noteworthy agreement is obtained by DFT calculations when incorporating gallium in a plutonium substitutional site in the PuO2 lattice. In conclusion, diffusion of gallium atoms into plutonium vacancies in the PuO2 lattice is likely to be the main process responsible for the observed δ-phase destabilization.
