Bucky-Balls Go Nuclear

Controlling uranium chemistry at nanoscales

Laurent Karim Beland

A representation of U₁₂₄P₃₂ showing the tetrahedral symmetry and multiple uranyl cages. Reproduced by permission of The Royal Society of Chemistry

Dressed in her protective attire, including a lab coat and dosimeters, Jie Qiu carefully manipulates small uranium-based compounds, creating new shapes and functions. It looks impressive, but she has a trick: the uranyl peroxide phosphate clusters she studies can auto-assemble and take form into structures reminiscent of Antoni Gaudi's street art. Recently, she and her co-authors, members of the Materials Science of Actinides Center (MSA), based at the University of Notre Dame, managed to concoct what may well be a blockbuster chemical: a tiny water-soluble uranyl phosphate cluster.

After being mined, enriched, and manufactured, nuclear fuel is used in power plants. While the United States stores the spent nuclear fuel, other countries, such as France and Japan, reprocess the fuel so that it can be partly reused in power plants. During the reprocessing and disposal phases, uranium is typically dissolved in acid for further treatment. Full recovery of uranium in solution is a technological challenge that, if met, could significantly improve these phases of the nuclear fuel cycle. Developing such an ability to manipulate actinides—the family of heavy elements at the bottom of the periodic table that includes uranium and plutonium—is one of the main objectives of the MSA.

Writing in Chemical Science, the MSA researchers report that by adding hydrogen peroxide, lithium hydroxide, and potassium chloride, they can synthesize uranyl nano-clusters that are probably large enough to be filtered out using commercial membranes.

Exactly how large are the clusters? X-ray scattering indicates that these tetrahedral structures measure 4 nanometers in diameter. Chemical compositional analysis indicates that they are formed by 124 uranium atoms, held to 32 phosphate atoms using 152 bridging peroxide groups (oxygen-oxygen bonds) that hold the arrangement together. Qiu and her team, led by Peter Burns, identified five sub-structures that together are the novel multi-caged uranyl cluster: four symmetrical cages distributed around a larger central cage. It is the only uranyl-based cluster that consists of multiple cages, according to the team.

Furthermore, mass spectroscopy measurements showed that upon dissolution in water, the clusters, U₁₂₄P₃₂, remain intact. Another remarkable feature of these clusters is that they are formed starting directly with uranyl nitrate, while most metals that form similar tetrahedral superstructures are synthesized starting with smaller clusters.

Compounds used in current-day reactors are typically not very soluble in water. The insolubility retards the mobility of uranium during the reprocessing phase of the nuclear fuel cycle. The high solubility of the novel material discovered by the MSA could accelerate and increase the effectiveness of reprocessing, perhaps bringing leftover uranium concentrations in solution below drinking water standards. This could be a game-changer, as this recycled uranium would not need to be disposed of and its use in American nuclear power plants could curtail mining and other dangerous activities. "The world-wide supply of uranium-235 is rather limited, and reprocessing will be necessary to make nuclear energy viable on the time-scale of centuries, instead of only decades," said Peter Burns, MSA Director. The MSA group has completed some experiments that simulate uranium recovery from nuclear waste. Results appear in ACS Applied Materials and Interfaces. While the United States, at the moment, opts not to reprocess spent fuel, the MSA's progress may give nuclear energy authorities improved options.

At the moment, Jie Qiu and other members of the MSA are improving the synthesis efficiency of the uranyl cluster and testing long-term water stability. They are also concocting other novel uranyl peroxide clusters.

No doubt, the MSA has a lot of interesting work ahead of it, which might lead to significant improvements in the nuclear fuel cycle. While one might associate nanotechnology with microelectronics and emerging renewable energies, the team's research has proven that it can also augment more traditional industries, such as nuclear energy production.