Copyright © Amy E. Hixon. All rights reserved.

The interactions between uranyl peroxide cage clusters (shown to the left) and solid phases are important for developing nanoscale control of actinides in an advanced fuel cycle. In order to determine the factors controlling cluster-mineral interactions, we conduct experiments in which the solid phase and cluster concentration, pH, ionic strength, time, and temperature are varied. Current work in the Hixon group is focused on examining U60 interactions with high-purity quartz.

This work is supported as part of the Materials Science of Actinides, and Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

Actinides are present in the environment as a result of nuclear weapons product and nuclear fuel disposition and pose a long-term environmental concern due to their toxicity and long half-lives. Understanding and predicting their mobility is important for risk management. By using a combination of wet chemistry, instrumental, and modeling techniques, members of the Hixon group are able to understand actinide aqueous speciation, the properties of mineral surfaces, and how the two react. When our knowledge is combined with microbial and hydrogeologic influences, we are able to predict actinide behavior in both natural and engineered systems.

​​Actinide Science at Notre Dame

Development of mechanistically accurate surface-complexation models.

There are several mechanisms by which actinides may interact with a mineral surface: inner-sphere sorption, outer-sphere sorption, ion exchange, surface (co)precipitation, and structural incorporation. The aqueous actinide concentration, specific mineral phase, and solution matrix all influence which process will dominate. Many studies rely on empirical approaches, such as Kd measurements, to describe these interactions. These approaches are insufficient because they lack specificity and are only valid for the particular conditions of the experiment. For example, the Kd approach is unable to predict actinide sorption under changing conditions of solution concentration, ionic strength, and pH. Conversely, mechanistic approaches can differentiate between interaction mechanisms and predict actinide sorption as a function of solution chemistry. Thus, mechanistic approaches provide a structured and consistent method of examining experimental data. When they are coupled with spectroscopic data describing the bonding environment of actinides associated with a solid phase, the resulting surface-complexation model is based on both micro- and macro-scale observations.

Current research in the Hixon group is focused on examining actinide sorption to several aluminum oxide and hydroxide minerals in an effort to connect surface acidity to sorption behavior.

The behavior of uranyl peroxide cage clusters at interfaces.

Environmental aging of nuclear materials.

Accurate nuclear materials identification is critical to the assessment of potential proliferation activities at declared and potentially undeclared nuclear facilities. The heightened concern about nuclear terrorism following the 9/11 attacks in the USA has given rise to a rapid expansion of techniques and applications of nuclear forensics. In recent years, element analysis and characterization of surface morphologies and microstructures of nuclear materials have been applied to complement isotopic analysis for both nuclear proliferation and nuclear smuggling studies. However, none of the collected samples are in their pristine condition-- they will have aged due to interactions with the surrounding environment, self-irradiation, and the interplay between these two effects. Deciphering the aging processes of complex nuclear materials in various environments is crucial for understanding the changes these materials have experienced from the time of their production and use to the time of sample collection.

Research in the Hixon group is targeted at examining the oxidation and oxidative dissolution of U and Pu oxides and metals, as well as of more complex nuclear materials, with the objective of determining molecular-scale reaction mechanisms and relating those mechanisms to aging scenarios.

This work is supported by the Department of Homeland Security through the Advanced Research Initiative (ARI).

Glass matrix synthesis and rapid dissolution for selective actinide recovery.

Following a nuclear detonation, radiochemists and post-detonation diagnosticians are tasked with performing high-quality analysis of fallout debris samples and accurate device interpretation. Debris formed following a nuclear detonation commonly consists of a heterogeneous glassy matrix containing uranium, plutonium, neptunium, americium, and fission/activation products. Homogeneous and well-characterized reference materials are critical for accurate post-detonation debris analysis. Such reference materials can include aged nuclear explosion debris from the Nevada Test Site (e.g., Trinitite) as well as "fresh" doped-glass material. Recent work has highlighted the often heterogeneous nature of historical debris, which presents a problem from the aspect of safeguards accountancy and verification as well as micro-scale characterization (i.e., microbeam analysis).

In this regard, research in the Hixon group is focused on the creation of homogeneous, "fresh" doped-glassy standards containing uranium, plutonium, surrogate fission products (e.g., Sr, Cs, Pm, Sm, Eu), and urban materials (e.g., Fe and Ca from construction materials, stainless steel, aluminosilicate phases as a surrogate for dirt).

This work is supported through the Nuclear Forensics Junior Faculty Award Program sponsored by the U.S. Department of Homeland Security, Domestic Nuclear Detection Office.