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Are Uranium Clusters the Key to Cleaner Nuclear Energy?

Written by: Marijke Wijnen

“Those ones are obviously part of our Christmas celebration. They also participate in our Thanksgiving display and our Easter display. They get reworked into every display.”

Professor Peter Burns, who is wearing a yellow vest and has the demeanor of a warm-hearted jokester, points to a collection of colorful polyhedrons surrounding a miniature Christmas tree on display outside his lab in Stinson-Remick Hall at the University of Notre Dame.

“Doesn’t it entice you that this formed?” He plucks a yellow polyhedron model of the U60 cluster from the display and rotates it in his hands, admiringly. “We call it U60 because it has 60 vertices. It’s kind of outrageous but this molecule has exactly the same topology as a soccer ball.”

Professor Burns and his lab have discovered a totally new group of uranium-based nano-clusters that have the potential to transform how nuclear energy is created and how nuclear waste is cleaned up. Professor Burns did the initial synthesis in 2003 over winter break when his students were away. He had family in town and on a whim he snuck to his lab for a short period of respite between holiday festivities and ran a series of experiments and grew the first uranium cluster. After that his lab went on a self-described synthesis 

binge, discovering over 120 uranium clusters, which Professor Burns describes as, “unrivaled in their beauty.”

Mateusz Dembowski, one of Professor Burns’ graduate students, explains that forming these uranium clusters is like baking a batch of chocolate chip cookies -- it’s really easy if you know the recipe. All you have to do if put the correct solution together and allow it to evaporate, eventually the incredibly stable uranium clusters crystallize.

Uranium in nano-clusters has completely different properties than uranium dissolved in water and oxygen, which could lead to a greener nuclear fuel cycle. Professor Burns and his team are working to better understand the properties of these clusters and the stepwise mechanism by which they form.

Professor Burns’ lab is not the only lab trying to elucidate the science behind these clusters. In 2008 the Department of Energy put out a call for researchers to put together proposals for Energy Frontier Research Centers (EFRCs), major collaborative efforts focused on topics that could transform our energy future. Professor Burns assembled a team of top researchers from across the country to study actinides, the bottom row of elements on the periodic table, including uranium. All actinides are radioactive and are thus the basis for nuclear energy. Professor Burns’ proposal for a Materials Science of Actinides Energy Frontier Research Center was one of 36 projects selected by the Department of Energy to receive approximately 20 million over 5 years. In 2014 the Department of Energy renewed the project’s funding.

About a third of the researchers who are a part of the EFRC are studying uranium nano-clusters and their potential applications. One promising property of these nano-clusters is that they can be filtered out of solution. This has potential applications in both reprocessing fuel and dealing with nuclear waste. If uranium is separated from more benign fission products, it reduces the amount of hazardous waste that needs to be stored securely for many years to come.

Professor Burns’ main goal is to continue to uncover the fundamental science of actinides, given their societal importance. He explains, “They are the fuel of nuclear energy, are important for national security and nuclear non-proliferation, are essential for medical isotope production, are major components of nuclear waste, and are serious environmental contaminants at many sites related to the nuclear fuel cycle (i.e. uranium mining) and weapons production.”

But aside from their societal significance, Professor Burns is deeply interested in the puzzle of these nano-clusters themselves. Still holding the 3D printed model of the U60 cluster, Professor Burns continued, “If I posed the question, how many ways are there to put 60 uranium polyhedra together to form a cluster that’s closed that contains twelve pentagons. It’s a difficult calculation but after you do it. It’s 1,812. Wouldn’t you wonder why this particular one forms and none of the other eighteen hundred and eleven?”

It’s this deep curiosity that keeps Professor Burns excited about his research.