Actinide oxides: exploring their structure, bonding, and reactivity

Understanding the structure, properties, and reactivity of actinide oxides is central to our ability to properly use actinide materials, whether in fabricating nuclear fuel elements, processing materials for long-term storage, or ensuring the integrity of nuclear weapons in the stockpile. This issue of Actinide Research Quarterly (and the upcoming 3rd quarter issue) addresses these topics and represents a distillation of a series of talks given at a January 2004 Seaborg Institute workshop titled "Actinide Oxides in the Environment, as Stored Material for Nuclear Fuel Fabrication, and in Practical Weapons Components."

This issue of ARQ focuses on the structure, bonding, and reactivity of plutonium dioxide (PuO₂) and oxides derived from reactions of this material. The role of lattice oxygen and structural implications in regard to reactivity of the hypervalent PuO_2+x (PuO₂ with additional oxygen atoms in the host lattice) have been of considerable debate in recent scientific literature.

The issue begins with an account by Luis Morales of the inherent thermal reactivity of plutonium dioxide with water. This unique-in fact at times perverse-behavior has frequently been attributed to the intrinsic radioactivity that this heavy element possesses. This thermal reactivity, when coupled with radiation-driven chemistry, is an emerging important contributor to actinide-oxide material performance and behavior. From investigations initially begun at Rocky Flats in Colorado and conducted later here at Los Alamos, preliminary suggestions regarding the nature and bonding of the hypervalent PuO_2+x were put forth. More recently, other views have emerged based on new data and theoretical considerations.

In the second article, Los Alamos actinide pioneer Robert Penneman provides an alternative view of the structure, bonding, and valence of PuO_2+x. The article describes the enormous contributions that Los Alamos workers and collaborators made in unraveling the mysteries of bulk and surface actinide oxides. More importantly, the Zachariasen rules for structural bonding relationships in the actinide oxides and fluorides are discussed and used to describe the local order and bonding in the PuO_2+x entity. The predominant motivation for many early studies was simply to understand the structure and bonding in actinide oxides and fluorides. The structure-property relationships of PuO_2+x and other actinide oxides of similar stoichiometry are topical, controversial, and significant in the context of understanding the behavior of this material and its congeners when processed and stored.

Additional data have been gathered using modern techniques that complement structural determination data of hypervalent PuO_2+x. The new technologies include extended x-0ray absorption fine structure (EXAFS) analysis and x-ray photoelectron spectroscopy (XPS).

EXAFS provides information on the local order and structure regularity and is discussed in a third article by a frequent contributor to ARQ, Steven Conradson. This information, coupled with other available experimental evidence concerning the nature of the chemical bonding in PuO_2+x, presents a picture that is significantly more complex than originally imagined-and at considerable odds with previously postulated structures and stoichiometries.

In the final article, Doug Farr, Roland Schulze, and Mary Neu address the role of the actinide-oxide surface in directing the thermal and radiation-driven reactivity using XPS. Their contribution paints a picture that contrasts with previously postulated structural elements and stoichiometry of PuO_2+x. Their article also highlights the importance of other coordination stoichiometries in addressing plutonium-dioxide reactivity with water.

This issue of ARQ presents results of studies that tackle the technical detail that has been obscured by lore and an incomplete understanding of the unique complex structure and properties of PuO_2+x. The next issue of ARQ will provide pragmatic examples of where these factors come into play and justify why and how a strong technical basis ensures the safety and proper stewardship of actinide oxide materials.

Will the real PuO₂ please step forward?

These photos show a wide variability in color and general appearance for samples of plutonium dioxide (PuO₂). This variability in the appearance of PuO₂ samples is well known, and while PuO₂ is normally olive green, samples of yellow, buff, khaki, tan, slate, and black are also common. It is generally believed that the color is a function of chemical purity, stoichiometry, particle size, and method of preparation-although the color of PuO₂ resulting from a given method of preparation is not always reproducible.