Researchers illuminate nuclear material origin stories

Everyone has an origin story, and when it comes to nuclear materials, the tales of where they began and what they've been through can have blockbuster-level implications for national security. At the Department of Energy's Oak Ridge National Laboratory, nuclear security scientist Jordan Roach is developing new ways to quickly uncover clues about one of the most common forms of uranium in the nuclear fuel cycle. To get these clues, he's using a tool movie detectives might employ to draw out answers in interrogation rooms: light.

Roach's latest work, published in the Journal of Nuclear Materials, uses Raman spectroscopy, a popular analytical tool, in a new way to identify previously unobservable characteristics related to each nuclear material's processing history.

Uranium ore concentrates (UOC) are the bulk material produced after uranium mining and leaching to remove other minerals. They are relatively stable uranium compounds, making them easier and safer to transport and store compared to enriched uranium or spent fuel. That stability can also make UOCs attractive targets for diversion or misuse.

In the study, Roach and collaborators examined eight uranium oxide samples derived from different UOC source materials and processing pathways. Each had a different origin story. The team sintered the samples - a process using heat to bind particles - at 800 degrees Celsius. This process produces U3O8, an intermediate form of uranium on its way to conversion to a fuel source. This is the precursor product of uranium that can be used as a fuel source.

"[The samples] were all tossed in the oven overnight at 800 degrees Celsius, and then out came oxides," Roach said. "What specifically? We didn't know just by looking at them, and I didn't want that to taint the results, or at least my analysis of them."

This is where the plot thickens.

Roach and the team then used Raman spectroscopy on the eight samples. To do this, they used light from a laser to interact with atoms and bonds in the sample. As the light energy interacts with the molecular vibrations in the sample, a tiny fraction of the laser light scatters. The resulting light spectrum produces a substance's unique spectroscopic signature.

Roach said the team's goal was to test whether the molecular-level signature of the material would preserve any telltale signs of its origin, even after high-temperature processing.

"Is there some sort of spectroscopic signature that survives that process, or is unique to where it came from?" Roach said.

"[Raman spectroscopy] gives us a window into the material on the molecular scale."

When UOCs undergo sintering, non-uranium, oxygen-based molecules should turn to gas and release from the compound. Theoretically, all that should remain is pure U₃0₈. In fact, Roach said, that's what their paper co-authors from the University of Utah initially observed when they used the more traditional powder X-ray diffraction (PXRD) method to analyze the samples. PXRD measures X-ray diffraction within crystalline structures, with each compound and element producing a unique diffraction pattern. However, Raman spectroscopy is more sensitive to smaller particles and can analyze both crystalline and non-crystalline materials.

"We looked at the spectra and started to observe some very clear differences," Roach said. "Crystallographically these are essentially the same material but using spectroscopy we can actually see differences between them, suggesting there's some component that is surviving the sintering process."

Raman spectroscopy can help researchers and nonproliferation experts determine a material's entire history, not just its current chemical composition. Like most interesting characters, there's more than meets the eye.

The findings open the door to more effective nuclear forensics. While Raman spectroscopy is already used to identify uranium oxide phases, this study suggests the technique can also reveal processing-related differences

"From a nonproliferation standpoint, it's another tool for identifying materials that might be outside of regulatory means," Roach said. "It's an extra indicator we could look at to determine the source of these materials."

The technique is relatively fast, requires minimal sample preparation and may provide new insights about a uranium compound thought to be well studied and documented.

"There's still some mystery… even from a computational standpoint, we don't have an ideal model of U₃O₈," Roach said. "When you add certain elements to the structure, what does that do to the Spectra? Where do they go in the material itself? Do they substitute uranium? Do they fit in between the actual interstitial spaces?"

Roach said the team plans to further study how trace elements influence Raman spectra in uranium materials.

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.