The future of plutonium science

Plenary speakers discuss the many avenues of research

Condensed Matter Physics

A parallel institution to Los Alamos in the sense of its broad diversity of nuclear-materials research, the European UnionÕs Institute of Transuranium Elements (ITU) was ably represented by Gerry Lander at the conference's first plenary presentation.

Lander was enthusiastic in elaborating some novel ITU approaches to the actinide electronic structure conundrum. He described a body of work that employed high pressure to alter the atomic volume in actinides. Displaying compressibility curves for uranium, americium, and an americium-curium alloy, Lander presented evidence for a transition from localized to bonding behavior in americium's 5f electrons when the element was compressed to 50 percent of its atomic volume.

He subsequently described a clever way of performing the reverse experiment, applying negative pressure to a thin deposited film of plutonium to illustrate what he described as a "pseudo phase change" in delta-phase plutonium. While he admitted that the exact interpretation was still controversial, he also enthused, "these experiments are very beautiful, and they give us a nice handle on the electronic structure of plutonium."

Continuing on this theme of understanding electronic structure, Lander described the discovery of superconductivity in plutonium compounds as "the most exciting thing in actinide science for the last fifty years." He was animated about the ITU-Los Alamos partnership, describing it as a collaboration in which the two organizations contributed complementary skills and were continuing to work very closely together.

Actinide Compounds and Complexes

Los Alamos staff member Wolfgang Runde the Isotope and Nuclear Chemistry Group (C-INC) offered a comprehensive primer in the selected chemistry of actinide compounds, particularly those relevant to modern methods of plutonium purification related to pit manufacturing.

Despite what he characterized as "intensive study of transuranic minerals," Runde emphasized that single-crystal structures of plutonium compoundsÑparticularly those of plutonium(V) and (VI)- remain rare. Because of their importance in plutonium processing and purifications, and in long-term nuclear waste management, his presentation focused on the oxalates, hydroxides, and silicates of plutonium, which despite their importance, are poorly characterized on a structural level. His talk covered aspects of both the preparation of such compounds and the insights acquired into their molecular structure.

In addition to the wealth of specific information presented in this talk, it also conveyed an overall sense of the interesting and complicated chemistry that keeps actinide researchers engaged in trying to comprehend the myriad of bonding behaviors that actinides display in both laboratory and natural environments.

The Nuclear Fuel Cycle On Dec. 8, 1953, President Dwight Eisenhower delivered his "Atoms for Peace" speech before the United Nations. Eisenhower's goal was to apply "the miraculous inventiveness of man" to change "the fearful atomic dilemma" into something that could benefit humanity.

Fifty years later, Vic Reis postulated that perhaps it was time to initiate what he called an "Atoms for Peace II." Before he outlined the possible vision for such an endeavor, he noted that this new vision would also be for fifty years, which is equivalent to the lifetime of one nuclear power plant. Reis, of Science Applications International Corp., is a former DOE assistant secretary for defense programs.

Reis' vision emphasized nuclear power. With the possible proliferation of next-generation nuclear power plants, electricity could become safe, plentiful, affordable, and environmentally friendly. A key component of making this happen would mean that countries would have to settle regional conflicts and work together. International terrorism and rogue states would have to be dramatically curtailed and wars would have to be minimized if not eliminated.

"Key tasks for the United States include extending licenses, such as opening Yucca Mountain. The United States government also must help industry with costs associated with reconstruction, continue the development of the advanced fuel cycle initiative, and design a fourth-generation nuclear reactor." (See ARQ 1st/2nd quarter 2003.)

Materials Science and Plutonium Properties

One of the more interesting characteristics of plutonium is how it ages. In essence, plutonium ages from "inside out" and "inside in," which means that it not only changes its own properties but also has the potential to change the properties of surrounding materials.

"The three most important aging effects in plutonium are the radiogenic decay of the various plutonium isotopes, the possible thermodynamic instability of the plutonium alloy itself, and the corrosion of plutonium's surface during both storage and function," said Joe Martz of Los Alamos' Materials Science and Technology (MST) Division. "These aging effects accumulate slowly over decades. Furthermore, such effects may not take place in a linear fashion."

To ensure the performance, safety, and reliability of the U.S. nuclear weapons stockpile, researchers must be able to predict when aging effects negatively influence a weapon's performance. To accomplish this goal, scientists must understand the properties of plutonium throughout its "lifetime," from the time it is manufactured to the time that it is "retired from service."

"Virtually all conditions in plutonium are ripe to age-related damage," Martz concluded. "Yet, we have found no first-order effects after several decades. Hence, we feel reasonably comfortable that the lifetimes of components will last at least several tens of years."

Actinides in the Environment

Teresa Fryberger, director of the DOE's Division of Environment Remediation Sciences, Office of Biological and Environmental Research, Office of Science, told her audience: "It is clear that we wonÕt be able to remove all contamination at all sites. We must understand what contaminants are mobile and under what conditions."

Such understanding, she said, will make it possible to develop science-based risk evaluations to produce better decision-making; more-effective remediation and containment strategies; and a solid understanding of what, when, and where to monitor at sites in long-term stewardship.

Current actinide research in the DOE Weapons Complex includes study of uranium migration under the tank farms at Hanford Site in Washington; plutonium transport at Rocky Flats Closure Project in Colorado; plutonium sources and mobility at Savannah River Site in South Carolina; and microbial effects on actinide speciation and mobility.

"We have learned a great deal about the molecular interaction of actinides in the subsurface," Fryberger said. "We have been able to provide scientific data that has impacted remediation decisions for specific problems at specific DOE sites; (but) we have far to go before we can describe the complex interactions and processes that are important in transport."

In the future, she said, scientists must use the available DOE computing capabilities and insert mechanistic and chemical information into models; use interdisciplinary teams to do comprehensive, long-term field studies that include modeling in design and interpretation; and develop improved characterization tools.

Detection and Analysis

Christopher Puxley of the Atomic Weapons Establishment at Aldermaston in the United Kingdom brought scientists up to date on the application of vibrational spectroscopy to actinide analysis.

Vibrational spectroscopy can be used for in situ nondestructive analysis of both bulk and trace actinide compoundsÑnotably for the detection and analysis of typical species and unexpected contaminants that occur on the surface of actinides.

Puxley made three points about using fiber-optic analysis on actinides. First, fiber-optic midinfrared analysis is applicable to organic materials and their degradation products. Its applicability to inorganic compounds is limited to oxyanion identification, low-molecular-weight atoms in materials (e.g., hydrides), and low-molecular-weight, multiply-bonded anions.

Second, fiber-optic Raman analysis is applicable to both inorganic and organic materials. It is extremely useful for the identification of inorganic compounds but is difficult to apply to compounds that are black.

And third, because of its high sensitivity and small spot size, fiber-optic Raman spectroscopy is a valuable technique to detect contamination and surface species on bulk materials.

-Reported by Vin LoPresti, Octavio Ramos Jr., and Charmian Schaller

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