Researchers at the Department of Energy's Oak Ridge National Laboratory are breathing new life into the scientific understanding of neptunium, a unique, radioactive, metallic element - and a key precursor for production of the plutonium-238, or Pu-238, that fuels exploratory spacecraft.
The ORNL team's research arrives during a period of increased national interest in the use of Pu-238 in radioisotope thermoelectric generators, or RTGs. Often used in space missions such as NASA's Perseverance Rover for long-term power, RTGs convert heat from radioactive decay into electricity. Advancing RTG knowledge and application possibilities also requires the same high-level evaluation of both chemical reactions and structural characterization, two key aspects of the materials science for which ORNL is known.
"When people want to do scientific experiments in space, they need something to power their instruments, and plutonium is typically the power source because things like solar and lithium ion batteries don't withstand deep space," said Kathryn Lawson, radiochemist in ORNL's Fuel Cycle Chemical Technology Group and lead author of the new study published in the Royal Society of Chemistry .
"Plutonium is the solution, so the more scientific inquiry there is to go on a spacecraft, you need to have these types of batteries," said Lawson. "The demand always exceeds our supply. Hence, we need to have a great understanding of neptunium chemistry to support that production of Pu-238."
The team's findings, which illuminate important chemical and structural aspects of neptunium, were discovered through thermal decomposition - or by breaking down neptunium samples with heat - helping scientists to identify critical intermediate phases and guide more precise thermal treatments to discover additional neptunium insights.
At ORNL, this and other modern characterization techniques provide useful data on neptunium's chemistry, helping advance the lab's Pu-238 production process. A critical radioisotope for use in space exploration being produced at ORNL .
Neptunium is an important precursor of Pu-238, meaning it participates in a nuclear reaction that eventually results in Pu-238. Unlocking neptunium's complex secrets for a better understanding of its chemistry will enable more efficient, effective Pu-238 production. By improving how we understand and work with neptunium, ORNL is helping to ensure a secure, domestic supply chain for this mission-critical material, while fueling deep space exploration and advancing U.S. energy independence.
In the early 1980s, ORNL designed the original thermal decomposition process for uranium analysis. Today, Lawson and her colleagues across the lab are deploying the method to break down neptunium samples with steadily increasing heat, from 150 to 600 degrees Celsius - 302 to 1,112 degrees Fahrenheit - and using multiple measurement techniques to analyze the resulting chemical reactions.
By integrating the results from each categorization and measurement technique - including Raman spectroscopy, which bombards samples with lasers to examine molecular vibrations, and computational modeling, which offers a mathematic comparison point against the team's scaled experiments - the researchers discovered new mechanistic and materials chemistry information. Including the first detailed Raman fingerprint of a key neptunium oxide, these findings help advance a general understanding of neptunium while improving the lab's specific process for Pu-238 production.
"My group and I were looking at this from a structural point of view, to help understand the decomposition mechanism. Essentially what we do is use Raman spectroscopy to shoot lasers at the sample and excite different vibrations in the structure of the material," said nuclear security scientist Tyler Spano of ORNL's Materials and Chemistry Group, a contributing researcher to the recently published study.
"What comes out is a series of peaks, a spectrum. That tells us something about the current material structure, but what we did was follow the heating pathway that Kathryn [Lawson] uses for the material," said Spano. "We made these measurements as we were heating the material, so we could see how the chemical bonding environments were changing as we increased the temperature."
The study reunited a familiar cohort of early-career staff spanning three research directorates at ORNL, including National Security Sciences, Fusion and Fission Energy and Science, and Isotope Sciences and Enrichment. Now colleagues and collaborators at ORNL, Spano and Lawson met more than a decade ago at the University of Notre Dame, when Spano served as Lawson's graduate teaching assistant, and they worked side-by-side in the same university research lab. In 2020, they were reunited when Lawson joined Spano at ORNL.
"I had started a couple years before that and was so excited when Kathryn started," said Spano. "Our community is small, and the pool of people who have the expertise that you can work with in this field is so limited. It's exciting to know that someone else who thinks literally about science and the important questions is going to be here."
Lawson, Spano and their ORNL colleagues and collaborators on neptunium research are contributing to the history of understanding radioactive, metallic elements, or actinides, such as uranium, neptunium and plutonium. Meanwhile, they're adding to the resurgence of interest in neptunium's possibilities for advancing Pu-238 science.
"We're already working on our next project. It's again, very basic science-focused, and we're looking at the decomposition of some other neptunium compounds," said Lawson. "Essentially, we've figured out that we have these capabilities, and so we're looking at a number of other poorly understood neptunium compounds that are all going to be potentially of interest for Pu-238 production."
Together, Lawson, Spano and their fellow early-career researchers are helping to reinvigorate the study of neptunium chemistry and contributing to a shared understanding of the natural world, even though naturally occurring neptunium is exceedingly rare.
"It's so fun to get to research with your friends. It's all in the family," said Spano. "There are so few people who do this type of research. We all know each other, and it's just really beneficial to work together and pool our resources and capabilities."
At a place like ORNL, that unique collection of people and capabilities can yield important innovations in energy research and scientific discovery.
"This work is fun because it supports RTGs, and that side of things really captivates people when they talk about space," said Lawson. "That's what I get to tell my family: I make batteries for space! That's the nutshell. Making the best and the most batteries for space, so that we can continue to explore our solar system."
This research is supported by the Science Mission Directorate of the National Aeronautics and Space Administration, administered by DOE's Office of Nuclear Energy.
UT-Battelle manages ORNL for DOE's Office of Science, the single largest supporter of basic research in the physical sciences in the United States. The Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science . - Chris Driver
