Plutonium Science Challenges Future Researchers

The International "Plutonium Futures: The Science" Conference in Santa Fe in early July was a convincing testament to the resurgence of interest in plutonium science. More than 400 attendees from 15 countries gathered to discuss advances and challenges in plutonium chemistry and materials science. The technological challenges stem from the applications of plutonium‹principally stockpile stewardship and nuclear power, and from dealing with the problems left behind by those applications‹environmental concerns, waste disposal issues, and potential proliferation concerns. The scientific challenges stem from the fact that plutonium is the most complex element in the periodic table.

Plutonium is of interest because of the extraordinary nuclear properties of the isotope plutonium-239. Extracting the energy of its nucleus provides a "factor of millions" advantage in energy production or explosive power. However, electrons determine its chemical, physical, and mechanical properties and its engineering behavior. In the metal, it takes little provocation to change its density by as much as 25 percent. It can be as brittle as glass or as malleable as aluminum; it expands when it solidifies‹much like water freezing to ice; its shiny, silvery, freshly machined surface will tarnish in minutes, producing nearly every color in the rainbow. It is highly reactive and a very strong reducing agent when in solution, readily forming compounds and complexes during chemical processes or in the environment. It transmutes itself by radioactive decay, causing damage to its crystalline lattice and leaving behind helium, americium, uranium, and other impurities. It also damages all materials or solutions in contact, making it difficult to store and difficult to predict its transport. No wonder its principal applications are limited to those for which the "factor of millions" is crucial.

Figure 1. Connected binary-phase diagram (temperature vs. composition) of the actinides. Such diagrams demonstrate the transition from typical metallic behavior at thorium to the enormous complexity at plutonium and back to typical metallic behavior past americium. With little provocation plutonium will change its density by as much as 25 percent. It can be as brittle as glass or as malleable as aluminum; it expands when it solidifies. Its unusual behavior is just one of the challenges of understanding plutonium.

We have never fully understood the fundamental reasons for the unusual properties of plutonium. We have learned just enough about its chemical behavior to allow us to separate it from the reactor products and purify and recover it from scrap, and enough about its metallurgical behavior to shape it and engineer it for its applications. The early quest for fundamental understanding slowed down considerably in the 1980s and early 1990s, but now interest has been revived for two principal reasons. First, the era of no nuclear testing ushered in by the end of the cold war places a premium on understanding plutonium better because we can no longer do the proof tests that allowed us to bridge the gap between our understanding of physics and actual weapon function. Second, we must now find more efficient and cost-effective methods to deal with the environmental problems created during the cold war and to make certain that we do not create additional problems in the future. In addition we must provide protection for fissile materials and cost-effective disposition of plutonium declared excess to the nuclear weapons programs. Thus, we must resolve fundamental questions about the physics, chemistry, and metallurgy of plutonium.

Figure 2. The color of plutonium oxidation states. Each oxidation state, ranging from Pu(III) to Pu(VII), has a characteristic color in solution. Plutonium will often change oxidation states in solution, making its interaction with the natural environment inordinately complex.

Much of the current work in the actinides was reviewed in Santa Fe. The single most unusual characteristic of plutonium is its instability. In the metal, it is extremely sensitive to the slightest changes in temperature, pressure, or chemistry. Small changes, in turn, produce very large changes in physical and mechanical properties. In solution, changes in oxidation state, which occur readily through small changes in solution chemistry or even through radiolysis, lead to a variety of molecular complexes, each with a characteristic solubility and chemical reactivity. Recent electronic structure calculations provide valuable insight to demonstrate how the unusual properties of plutonium result from the systematic variation in the bonding of the 5f electrons across the actinide series. By examining the trends across the series, we find that the peculiarities of plutonium are not a single anomaly but the culmination of a systematic trend of bonding behavior of the 5f electrons. Plutonium sits right at the knife-edge in the transition between bonding or chemically active 5f electrons and localized or chemically inert 5f electrons.

While the science of plutonium is thus complex and exciting to study of itself, we must redouble our efforts to bring fundamental knowledge to bear on the demanding set of applications we face this century. For example, to certify nuclear weapons without testing we must develop a better understanding of aging effects, particularly the effect of self-irradiation damage on plutonium¹s already notorious instability. Furthermore, although plutonium is a man-made element created an atom at a time in reactors, the world now has more than 200 tonnes of plutonium in its military stockpiles and more than 1000 tonnes (800 tonnes are contained in spent-fuel elements) in civilian inventories. Its protection from unauthorized use, its potential utilization as reactor fuel, or its geologic disposition represents not only a great scientific challenge, but also a national security imperative.

We also recognize that during the cold war significant quantities of plutonium and other radioactive materials were released into the environment. In addition, civilian nuclear power programs continue to create nuclear waste waiting for an acceptable method of disposal. These problems represent one of the most challenging applications of modern chemistry because of the inherent complexity of plutonium and the corresponding complexity of the natural environment.

Figure 3. Pourbaix diagram for plutonium: another demonstration of the complexity of plutonium in the environment. This Eh vs. pH diagram is calculated for plutonium in water containing hydroxide, carbonate, and fluoride ions. The red dots are triple points, where plutonium can exist in three different oxidation states. The range of Eh/pH values found in natural waters is bounded by the solid black outline. Dashed lines define the area of water stability.

To enhance our understanding of the fundamental behavior of plutonium and to apply this knowledge to solving these challenging problems, we must attract the next generation of scientists and engage the international scientific community. The Plutonium Futures Conference took an important step in these directions. Also, as Dave Clark pointed out in a previous editorial, we must rekindle the educational programs in transactinium science. To attract the next generation of researchers and the academic community, we must continue to instill a spirit of scientific excitement that was so evident in Santa Fe.

The greatest challenge, however, will be to turn around the crisis of confidence that has developed in the operation of the nation's plutonium facilities during the past decade. The inordinate difficulty in accomplishing experimental plutonium work today, which underlies this crisis, is having a chilling effect on the ability to recruit and retain the best and the brightest. Our ability to operate nuclear facilities safely and successfully is of utmost importance in regaining public confidence and attracting the next generation of plutonium scientists.

This editorial was provided by S. S. Hecker, senior fellow at the Laboratory. Illustrations courtesy of Los Alamos National Laboratory.

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