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Summer 2017

Happy First Birthday to Four EFRCs

These centers, new in 2016, focus on understanding how to manage existing nuclear waste

Rebecca Palmer

Researchers from IDREAM. Image courtesy of Aurora Clark

Researchers from CAST. Image courtesy of Thomas Albrecht-Schmitt

Researchers from CHWM. Image courtesy of Gregory Morrison

Researchers from WastePD. Image courtesy of Gerald Frankel
 

The United States has tons of legacy waste from nuclear weapons development and used nuclear fuel from energy production that needs to be recycled, processed and stored. The Department of Energy (DOE) is charged with processing and storing the majority of nuclear waste in the United States. To store it effectively, we need to fundamentally understand the science that governs this complicated and toxic waste. In August 2016, four new Energy Frontier Research Centers (EFRCs) were funded to tackle the thorniest problems in legacy nuclear waste.

What is going on in that waste? Understanding how nuclear waste changes

The scientific legacy of the nuclear era is complicated, and nowhere is this better illustrated than the Hanford Site, the world’s first plutonium production facility, in Washington State. Operating for more than 50 years, Hanford has an immense nuclear legacy. It includes 200 million liters of a complex mixture of aluminum dissolved in concentrated sodium hydroxide solution. The solution contains many radioactive elements. Aluminum is the dominant element, which comes from the casings used to hold the nuclear fuel and materials used within the reactors themselves.

Scientists with Interfacial Dynamics in Radioactive Environments and Materials (IDREAM) work to understand how the composition of the nuclear waste changes over time, with a particular emphasis on the role of aluminum and its transformations in a radiation environment. Understanding which species are present and how they react with other species in the waste as well as how those species will precipitate into aluminum oxyhydroxide species will help develop better ways for processing waste. The effects of radiation are especially important, as the ionizing radiation drives chemical reactions far from equilibrium and challenges traditional kinetic models and reaction pathways.

At IDREAM, researchers are using a combination of experimental methods, including neutron and X-ray scattering, to learn how radiation is changing the fundamental chemistry of nuclear waste. This experimental approach is integrated with a tailorable computational approach that ranges from molecular processes up to particle-particle interactions. Together, experiments and modeling will help scientists understand these interactions and their influence on aggregation and the ability of the waste to flow as it is prepared to be vitrified into glass for long-term storage.

“The integrated computational and experimental effort, and knowledge of how radiation influences complex interactions in solution over a long time period will have significant impact outside of the nuclear energy arena,” says Aurora Clark, deputy director of IDREAM.

Sifting through the sludge: Separating radionuclides in nuclear waste

In nuclear waste processing, separating radioactive elements is difficult because nuclear waste contains many different radionuclides such as technetium, plutonium and americium along with other elements such as aluminum, chromium or phosphorus. At the Center for Actinide Science and Technology (CAST), researchers investigate how to selectively isolate these radionuclides from nuclear waste. Scientists design molecules that will selectively bind to a specific radioactive element. The designer molecules separate different elements, allowing for better processing and storage.

Scientists at CAST are studying the structures of compounds with a radioactive element and specially designed binding molecules. Specialized monitoring of mass, using a technique called mass spectrometry, informs on how the elements are changing over time. Researchers model the compounds and materials to understand how to optimize them. From the separated radioactive elements, researchers also develop and study new materials to potentially store the radionuclides.

The jelly in the doughnut: Studying hierarchical materials for nuclear waste storage

Knowledge must be gained to develop new materials that are able to store higher concentrations of nuclear waste, and that don’t degrade or leach nuclear waste into the environment. One way to do this is by using hierarchical materials. Like a jelly-filled doughnut, a hierarchical material has one type of material for the outside and another type of material filling in the middle. The Center for Hierarchical Waste Form Materials (CHWM) studies four types of hierarchical materials as models of highly concentrated nuclear waste materials. The four materials are salt inclusion materials, metal-organic frameworks, hierarchical nanomaterials and porous silica materials. Three groups at CHWM work together to synthesize, characterize and model these systems. By studying these materials, scientists at CHWM are learning how to immobilize waste in persistent structures, lasting tens of thousands of years.

Their work involves using non-radioactive model systems, as well as samples that contain radioactive elements. Researchers characterize structures through X-ray crystallography initially, and over time to see how the materials change. They also gain an understanding of how crystals of these materials grow, monitoring their growth at synchrotron and neutron facilities. Using a technique called solution calorimetry, researchers can measure the energy given off when dissolving these materials in a liquid, which helps them determine the material’s stability. Then, scientists in the modeling group can use this information to learn how these materials change over time and offer suggestions regarding what could improve them.

Contain it: Studying materials for holding nuclear waste

Researchers at the Center for Performance and Design of Nuclear Waste Forms and Containers (WastePD) are investigating the materials that will contain different radioactive species present in nuclear waste. Gerald Frankel, director of WastePD, says “important improvements are possible” for the waste forms and containers that will be used for nuclear waste. Not much is understood about the corrosion process of these structures made from glass, metal and ceramics. Even glass corrodes, and a better understanding of this process would help in assessing how nuclear waste will be isolated over long periods of time.

Nuclear waste could be stored underground in deep repositories, so it will be exposed to water. Understanding layers that form at the interface where the environment meets the waste is key to slowing the release of radioactive materials into the environment. “The degradation rate of glass is slow, but it can take off, depending on the change of this protective layer,” says Frankel. We lack understanding of these interfaces because they are complicated to study, with some of these waste glasses containing 30 components.

At WastePD, three teams work together to understand these interfaces for metals, ceramics and glass. Frankel says that before WastePD, these three materials had never been studied in one center. Interactions between people who are experts in each category of material will help the different groups learn from one another. The teams use different advanced surface techniques, including focused ion beam milling and atom probe tomography, which enable them to actually observe and understand these complex surfaces. Scientists at WastePD also use computer models to understand how these interfaces between the container and the environment change over many years.

The road ahead

From learning about the complex nature of the waste, to understanding the containers that hold nuclear waste, these four EFRCs are on their way to providing a road map to safer nuclear waste storage.

Acknowledgments

CAST, CHWM, IDREAM, and WastePD are Energy Frontier Research Centers funded by the Department of Energy, Office of Science, Basic Energy Sciences.

About the author(s):

  • Rebecca Palmer is a graduate student at Northwestern University in Evanston, Illinois. She is part of the Argonne-Northwestern Solar Energy Research Center (ANSER). She studies mixed metal oxides deposited on a porous support and investigates these materials’ ability to oxidize water.

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.