Simultaneous detection of uranium and fluorine isotopes advances nuclear non-proliferation monitoring

By combining two techniques, analytical chemists at the Department of Energy’s Oak Ridge National Laboratory have become the first to simultaneously detect fluorine and different isotopes of uranium in a single particle. Because fluorine is essential for converting uranium into a form suitable for enrichment, locating the two elements together can help inspectors from the International Atomic Energy Agency, or IAEA, determine the intended use of nuclear material.

The results, published in the Journal of the American Chemical Societyare pushing the limits of how quickly individual particles can be characterized in terms of their chemical, elemental and isotopic compositions. Essential to understanding chemical processes and dating materials, isotopes are different forms of a chemical element with the same number of protons but a different number of neutrons.

“Determining isotope ratios on single particles is very time consuming,” said Benjamin Manard of ORNL, who led the study. “We have enabled rapid particle analysis for isotopic determination of fluorine and uranium.” His team combined two techniques to analyze 40 particles, each the size of a red blood cell, in less than five minutes.

The first technique is laser-induced breakdown spectroscopy, or LIBS. It quickly identifies the fluorine element with great sensitivity.

“LIBS vaporizes a sample, such as a particle of uranyl fluoride, breaking it down and forming a plasma, or cloud of excited ions. As the plasma cools, light is emitted,” said Hunter Andrews of the ‘ORNL, LIBS project manager of the study. Spectroscopy then measures the light to characterize the elements in the plasma.

“It’s like fireworks,” Manard said. “Different elements emit different colors or wavelengths.”

Simultaneously, helium gas scans atoms from the plasma into a mass spectrometer, where uranium isotopes are characterized via the second technique, called laser ablation multicollector inductively coupled plasma mass spectrometry, or ICP-MS. Inductive coupling means that the radio frequency energy heats the plasma. It reaches 8,000 kelvins, hotter than the surface of the sun.

“LIBS tells us if and how much fluorine is in the particle, while ICP-MS tells us all the uranium isotopes present,” Manard said. “This integrated equipment provides a one-stop shop for measuring fluorine and uranium isotopes simultaneously.”

Uranyl fluoride, a molecule containing uranium, oxygen and fluorine, indicates the occurrence of certain nuclear processes. “The detection of uranium and fluorine in the same particle is significant from a nuclear nonproliferation perspective,” said ORNL co-author Brian Ticknor. “Being able to determine the amount of fluorine versus uranium can give you additional information about where that particle came from, the processes that produced that particle, and how long ago those processes occurred.”

He added: “While these tools are developed for national security purposes, they could have applications in a variety of areas, including next-generation battery manufacturing, fuels for advanced nuclear reactor design, and environmental science. revealing the transport and fate of microplastics. »

Why hadn’t anyone put these two pieces of equipment together before?

“The ICP-MS needs a positive charge to make our measurements,” Ticknor said. “Uranium, plutonium, most metals, much of the periodic table, just happen to be positive ions. Part of the reason we have to use this simultaneous technique is that elements like fluorine don’t not amenable to the ICP-MS that we perform for uranium. This has to do with the electronegativity of fluorine. With ICP, the goal is to form positive ions to inject into the mass spectrometer. Fluorine wants very, very strongly to have a negative charge.

Manard explained further: “This is why we had to do the LIBS for the fluorine part, then the mass spectrometry for the isotopic determination of uranium. Many people have done LIBS and ICP-MS separately, but no one had done both in a multicollector. -fashion based on fashion.”

Manard conceptualized experiments for a multidisciplinary team at ORNL’s Ultra-trace Forensic Science Center. The advanced characterization of nuclear materials carried out by the center provides a better understanding of new processes in the fuel cycle.

Ticknor, Cole Hexel and Paula Cable-Dunlap, all of ORNL, served as advisors for the project. Andrews performed LIBS experiments. Daniel Dunlap and Alex Zirakparvar, both of ORNL, prepared the ICP-MS instrument to perform high-precision isotope ratio measurements. ORNL’s Tyler Spano prepared samples and compared them using Raman spectroscopy, a traditional method for measuring chemical composition that can take weeks.

Veronica Bradley, a former ORNL postdoctoral fellow, helped map out laser ablation. C. Derrick Quarles Jr. of Elemental Scientific Inc., which makes the LIBS system, helped set up methods and set up equipment.

Manard and Ticknor envision applications for their technique beyond nuclear nonproliferation. In fact, when ORNL purchased the LIBS instrument to target difficult elements such as fluorine, the research literally gained teeth. Manard conducted experiments with researchers from Savannah State University, the University of Maine and ORNL using the new equipment to map the distribution of fluoride in shark teeth obtained at the Aquarium from Georgia. Shark tooth enamel is rich in fluorine and has revealed environmental conditions, both current and prehistoric.

The innovative integration of the two techniques can reveal the origin and evolution of isotopes in various fields, including geology, biology, chemistry and nuclear materials.

“My research has recently focused on high-throughput particle analysis,” Manard said. “With improvements in laser ablation, we have been able to push the limits of how quickly we can characterize single particles. We are trying to pass thousands of particles in 24 hours with this technology. The speed is still at the drags my mind.”

After using joint techniques to differentiate uranium oxide from uranyl fluoride, the researchers wish to deepen their research.

“What would be really cool and interesting is if we could distinguish other types of uranium compounds,” Manard said.

The team also wants to extend these techniques to other complex compounds related to nuclear processes, such as those containing chlorine, which is electronegative like fluorine.

“Chlorine is just below fluorine on the periodic table and has many of the same properties,” Ticknor said. “Uranium chloride has been difficult to measure in the past, and it would potentially be amenable to this type of measurement.”