Predicting the behavior of stored excess plutonium

Using x-ray photoelectron spectroscopy to probe the surface chemistry of plutonium oxides exposed to water

Excess plutonium metal and residues from decades of production must be processed, stabilized, and packaged for safe, long-term storage. Current DOE guidelines require that excess plutonium be converted to the dioxide form, which is unusable in nuclear weapons, and packaged in sealed steel containers.

Reactions at the interface between solid plutonium oxide particles and adsorbed gasses (which are either present initially or form during the vapor phase) play a dominant role in determining the pressure and atmosphere in these containers. Understanding the chemistry at the solid vapor interface, particularly with respect to reactions with water vapor, is crucial for predicting the storage behavior of these materials. Safe storage of plutonium oxides requires minimal exposure to water vapor to limit chemically or radiolytically driven gas-generation reactions. Ultimately, the presence of water vapor may also affect container corrosion rates and limit container storage lifetimes.

Surface chemical reactions and their products also play a role in environmental plutonium behavior. For example, higher-valent plutonium species (V or VI) present at the surface of particulate matter may exhibit increased solubility. Determining the reactions at the solid water interface is important to understanding plutonium adsorption and desorption from minerals and ceramics. X-ray photoelectron spectroscopy (XPS) is one of the main techniques used to characterize the surfaces of plutonium dioxide, plutonium hydroxide, and hyperstoichiometric plutonium oxide powders. XPS and the data obtained by it are only applicable to the outermost 2 to 5 nanometers (nm) of the surface. Because of its surface sensitivity, XPS complements the data obtained by x-ray absorption fine structure spectroscopy (XAFS), which are able to characterize the bulk of a sample.

The binding-energy values, distinctive line shapes, and peak widths obtained from well-characterized materials are used to interpret measured spectra of more-complex oxides, mineral-adsorbed species, and unknown forensic samples. In our study, we used reference spectra for the plutonium 4f and oxygen 1s levels of pure materials.

Previous articles in this issue of ARQ have either invoked the existence of a plutonium hydroxyl species or inferred its existence from EXAFS data. The data obtained from our experiments demonstrate its existence, provides quantitative estimates of amount, and further identify the valence of plutonium atoms in the surface region of the samples.

X-ray photoelectron spectroscopy (XPS) spectra from the oxygen 1s region of plutonium dioxide (PuO₂) following the treatments indicated.

Our studies

Hyperstoichiometric oxides are plutonium oxide powders with the formula PuO_2+x. In our study, we analyzed samples formed by the reaction of high-fired plutonium dioxide with water vapor at elevated temperatures. The calculated composition for the material is PuO_2.26, based on the pressure of hydrogen (H₂) generated during the reaction at 300 degrees Celsius. The slightly expanded cubic lattice parameter for this material was 5.4022 angstroms (�), compared to 5.3975 � for stoichiometric plutonium dioxide. While the structure of PuO_2+x is unknown, possible structural models have been suggested in other articles in this issue.

The fraction of plutonium existing at higher oxidation states in the PuO_2+x material depends on how charges are balanced within various proposed model structures. One such structure, mentioned in the article on Page 9, places a hydroxyl ion at the face-centered cubic (fcc) body center and requires the oxidation of plutonium(IV) to plutonium(V). A more correct formula for this model is Pu₄O₈OH, consisting of three plutonium(IV) atoms and one plutonium(V) atom.

Other proposed models place an extra oxygen atom in various positions in the fcc lattice, including in the central octahedral site. Some charge transfer from the existing plutonium atoms must occur to balance the additional oxygen atom, but the details are still unknown. For most of the related materials that we have analyzed, plutonium was in the (IV) oxidation state, with the exception of PuO₂CO₃, which was in the (VI) oxidation state. The value for x in PuO_2+x could be as high as 0.27, depending on preparation conditions, implying the presence of higher-valent plutonium in the material.

XPS is a convenient means of determining the valence and coordination chemistry at the surface of materials. One of the key technical advantages is that XPS can observe discrete differences in valence distributions in mixed-valent compounds in extended oxide structures. The time scale of the technique is applicable to processes occurring at a femtosecond (a quadrillionth of a second) and unless the valence is truly shared among the actinide atoms in the solid structure, determining the oxidation states is possible. For example, XPS has unequivocally demonstrated that the unit cell of the uranium oxide U₃O₈ contains two uranium(VI) atoms and one uranium(IV) atom.

X-ray photoelectron spectroscopy (XPS) spectra for hydrated plutonium dioxide (PuO₂), PuO_2.26, and Pu(OH)₄ centered about the 4f_7/2 transition.

Equally important are the distinctions contained in the light elements such as carbon and oxygen. For oxygen-containing species, a very clear and significant distinction in measured binding energy enables both identification and quantification in the chemical bonding states. This distinction is shown in data obtained from a variety of plutonium dioxide solids following reactions with water vapor. We began by examining XPS data recorded over the core level oxygen 1s state, as shown in the figure on the previous page. Spectra obtained from these solids demonstrate a complex multiple-component line shape indicative of several chemical-bonding states of oxygen-containing entities.

From comparison to known reference compounds, the spectral assignments were ascribed to photoemission from oxygen present as lattice oxide ions, coordinated hydroxyl ions, and adsorbed water molecules. Following various treatment conditions, the distribution of different chemical states varied at the exposed plutonium oxide surface. However, in every example we studied the ubiquitous presence of surface hydroxyls was manifested.

Returning to the question of valence about the plutonium center in these various plutonium dioxide-treated samples, shown in the figure at left, shows XPS data centered about the dominant core-level line for plutonium, namely the 4f_7/2 transition. An extensive discussion concerning XPS line shapes is beyond the scope of this article, but suffice it to say that the PuO_2.26 and PuO₂ samples treated with moisture both exhibited core-level shifts consistent with several chemical-bonding states present at values higher than nominally seen for plutonium(IV). In fact, the existence of plutonium(V) can be invoked based on comparison to reference data.

These spectra show the plutonium 4f_7/2 (left) and oxygen 1s (right) core levels for a plutonium dioxide powder in the as-received condition, through a series of thermal treatments, then followed by exposure to 10¹⁰ L H₂O. The plutonium 4f peaks narrow and become more bulk dioxide-like as the oxygen 1s dioxide peak grows with respect to the hydroxide intensity. Exposure to water vapor restores some surface hydroxides and eliminates the reduced plutonium oxide.

The thermal stability and reversibility of reactions involving water and plutonium oxides can be examined by monitoring the surface using XPS following various thermal excursions. Our samples were maintained in a rigorous vacuum following heating and were subject to an intrinsically reducing environment.

The figures above show stack plots of the plutonium 4f_7/2 and oxygen 1s regions for a hydrated plutonium dioxide sample that was successively dehydrated by systematically heating the sample to progressively higher temperatures. Note the progressive shift and narrowing of the plutonium line back to a line shape close to that observed for plutonium dioxide at 400 degrees Celsius. For thermal treatments above 400 degrees Celsius, the appearance of a slightly lower binding-energy state in the plutonium spectra is observed and is consistent with that seen for plutonium(III) as in the plutonium oxide form Pu₂O₃.

The data show significant loss in the oxygen 1s features attributed to surface water and hydroxyls as well as the reappearance of the lattice oxygen chemical bonding states upon heating. The upper-most spectra were recorded following re-exposure of the plutonium sample to water; note the redevelopment of the feature hydroxyl along with the commensurate loss of the plutonium(III) entity.

Our research showed that surface hydroxylation occurred rapidly at very low water-vapor exposure, indicating that it is important to minimize the exposure to water vapor of samples intended for long-term storage. At room temperature, the hydroxylated layer is typically 3 nanometers (nm) thick.

The surface hydroxides are tenacious and resist thermal decomposition up to 590 degrees Celsius. Important factors influencing surface chemistry include processing conditions (thermal treatment, in particular) and specific activity.

Active sites for the reaction of water and other small molecules can be renewed by thermal energy or by effects of radiation. Although not specifically addressed in our study, we expect that other properties such as surface area, particle size, and particle morphology be parameters in describing reactivity in the study of the surface chemistry of plutonium oxides.

PuO_2+x shows evidence of extensive hydroxylation and spectroscopic features that are consistent with higher plutonium oxidation states, suggestive of plutonium(V). Curve fitting of the plutonium 4f levels indicates that about 10 percent of the plutonium is in this higher oxidation state. Plutonium oxide that has been exposed to air will display the complex surface chemistry demonstrated by the hyperstoichiometric plutonium oxide, including a small fraction of oxidation states greater than (IV).

Implications

High-fired (heated to 900 degrees Celsius or higher for hours or longer) plutonium dioxide has been shown to be more highly ordered and more stoichiometric-that is, behaving more like plutonium(IV)-and so is more difficult to dissolve than low-fired material. The higher degree of surface hydroxylation observed for PuO_2+x powders may effectively increase apparent plutonium solubility because hydrated plutonium dioxide (PuO₂H₂O or Pu(OH)₄) has a higher free energy than stoichiometric high-fired plutonium dioxide and is therefore more soluble and prone to surface corrosion. Solubility studies of tetravalent actinides found greater solubility for amorphous plutonium dioxide than for crystalline plutonium dioxide in acidic solutions.

Our research has demonstrated several advantages inherent in surface analysis by XPS, including determining surface valency, quantifying stoichiometry, and in the case of oxygen species (or other light elements such as carbon), indirectly monitoring hydrogen- containing chemical states. Plutonium ions adsorbed from water on mineral surfaces is often observed by XPS in the (IV) state with peak positions and line shapes corresponding to those observed for the humid-air-exposed plutonium dioxide and Pu(OH)₄ samples. The XPS spectra observed on air-exposed plutonium metal also share many of the same features-shake-up satellites characteristic of plutonium(IV) and broad peaks that indicate multiple chemical states for both plutonium and oxygen. Plutonium 4f binding energies and curve fitting of the oxygen 1s region for these samples have provided a more solid basis for understanding plutonium oxide surface chemistry.

This article was contributed by Doug Farr of the Nuclear Materials Technology Division, Roland Schulze of the Materials Science and Technology Division, and Mary P. Neu of the Chemistry Division.

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