There are perhaps no more controversial materials in our society than the actinides—the elements above actinium in the periodic table that include uranium, plutonium, and many other radioactive elements. They are at the same time vilified by some for their potentially catastrophic impact on the environment and global health and praised by others for their role in ending the Cold War. And yet, at the heart of both these issues is a tremendous amount of fundamental research and technology that is essential for the actinides' rational use, safe disposition, reliable long-term storage, and the remediation of their impact on the environment.

Plutonium symbolizes everything we associate with the nuclear age. It evokes the entire gamut of emotions from good to evil, from hope to despair, and from the salvation of humanity to its utter destruction. No other element bears such a burden. Its discovery in 1941, following the discovery of fission in 1938, unlocked the potential and fear of the nuclear age. During the Cold War, the primary interest in plutonium was to provide triggers for thermonuclear weapons that formed the basis of nuclear deterrence. Beginning in the 1950s, plutonium also became an integral part of the quest for nearly limitless electrical power. The end of the Cold War has dramatically altered the military postures of the United States and Russia, allowing each to reverse the engines fueling the nuclearweapons buildup. Now, both countries face the challenge of keeping the remaining stockpile of nuclear weapons safe and reliable without nuclear testing, as well as cleaning up nuclear contamination and preventing the spread of nuclear weapons and terrorism. Moreover, current concerns about energy availability and global warming have rekindled interest in nuclear power.

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.

Studies of the actinide elements and compounds were (and are) motivated by the need to characterize their structural and thermodynamic properties for the development of nuclear fuels and the treatment of waste, whether it be for long-term storage or ideas involving transmutation in high-powered accelerators. For the most part, tables giving these data exist, although the data for transuranium compounds are rather sparse. A much more difficult task is to understand the data and develop theories that have predictive power in this part of the periodic table. In doing this, however, we are confronted with the extremely complicated electronic structure of the 5f shell and the great paucity of high-quality data on materials containing transuranium isotopes.

Elemental plutonium is considered to be one of the most complex elements in the periodic table and, by some, one of the more toxic, given its moderate radioactivity and that it is a heavy metal. Essentially a manuf actured element (only traces are found in nature), it was first discovered and produced some six decades ago. Amazingly, hundreds of tons of plutonium are pre sently in existence. Crystalline solids formed by plutonium and nonmetallic elements (e.g., hydrogen, oxygen, halides, etc.) were some of the first compounds investigated, and oxides and oxide-related materials have always been important compounds.

The investigation of plutonium in glasses (amorphous ceramics lacking long-range order), in crystalline ceramics, and in composite materials composed of multiple crystalline or glass and crystalline phases, relieson multidisciplinary studies of physics, chemistry, and materials science. It involves the study of the plutonium atoms in materials with only short-range periodicity, as in glasses, to materials with long-range periodicity, as in crystals. The materials studied over the past 30 years include simple binary crystals, used to investigate the electronic structure of plutonium, to complex glasses and ceramics selected not only for the safety and durability that they provide for the immobilization of nuclear waste and plutonium, but also for the high flexibility they offer in composition. The lack of long-range order at the atomic level in glasses permits the inclusion of abroad range of waste elements, but it renders more difficult the interpretation of data from many commonly used experimental techniques. Regardless of the challenge, much of the research conducted in this field over the past few decades has been motivated by the use of plutonium as a surrogate for all nuclear-waste actinides or on its own in immobilization studies, in order to develop a durable glass or ceramic matrix that can resist leaching and mobilization of the plutonium on a geologic time scale.

Considerable progress has been made in the study of environmental plutonium science in the last 30-plus years, driven to a large extent by concerns about the release and migration of large amounts of plutonium into the accessible geosphere. Plutonium has been introduced into the environment through several pathways. Environmental contamination has been caused by nuclear-weapons production and testing, nuclear-reactor accidents, and accidents during the transport of nuclear weapons. Above-ground testing of more than 420 nuclear weapons has produced large amounts of radionuclides through fission and neutron activation products. More than three metric tons of plutonium have been distributed on the earth's surface by global fallout. For example, the MAYAK plutonium production complex in the former Soviet Union is located in the southern Urals, about 70 km north of Chelyabinsk and 15 km east of Kyshtym. Between 1949 and 1951, about 76 million m3 of liquid radioactive waste with a total activity of 100 PBq (2.7 MCi) were discharged into the Techa River.