Fundamental Studies of Plutonium Aging

Plutonium metallurgy lies at the heart of science-based stockpile stewardship. One aspect is concerned with developing predictive capabilities to describe the properties of stockpile materials, including an assessment of microstructural changes with age. Yet, the complex behavior of plutonium, which results from the competition of its 5f electrons between a localized (atomic-like or bound) state and an itinerant (delocalized bonding) state, has been challenging materials scientists and physicists for the better part of five decades. Although far from quantitatively absolute, electronic-structure theory provides a description of plutonium that helps explain the unusual properties of plutonium, as recently reviewed by Hecker. (See also the article by Hecker in this issue.) The electronic structure of plutonium includes five 5f electrons with a very narrow energy width of the 5f conduction band, which results in a delicate balance between itinerant electrons (in the conduction band) or localized electrons and multiple lowenergy electronic configurations with nearly equivalent energies. These complex electronic characteristics give rise to unique macroscopic properties of plutonium that include six allotropes (at ambient pressure) with very close free energies but large (∼25%) density differences, a lowsymmetry monoclinic ground state rather than a high-symmetry close-packed cubic phase, compression upon melting (like water), low melting temperature, anomalous temperature-dependence of electrical resistance, and radioactive decay. Additionally, plutonium readily oxidizes and is toxic; therefore, the handling and fundamental research of this element is very challenging due to environmental, safety, and health concerns.