An international collaboration to protect the world from nuclear threats got a boost in 2023 when a visiting researcher brought an understudied plutonium processing chemistry method to Pacific Northwest National Laboratory (PNNL) for hands-on research. 

“It’s not trivial to bring an outside researcher to PNNL and get them working in a glove box,” said PNNL nuclear forensics scientist and technical group leader Dave Meier. “I was given the opportunity to work with technical staff from the UK for six weeks before the pandemic, and we were committed to deepening our partnership with the opportunity to bring someone back to PNNL.”

The years-long preparation and training led to the first time an outside researcher performed extended radiological experiments in PNNL’s non-reactor nuclear facility’s glove boxes. It matters to the national security of both nations to be able to have advanced nuclear forensics capabilities. Two decades ago, nuclear forensics was regarded as a capability to explore where radioactive material had come from after-the-fact, in the event it had been found outside of regulatory control. Now, nuclear forensics is considered a primary deterrent to anyone who would try to use or facilitate the use of plutonium to harm others. 

Multiple paths of production

Plutonium is a radioactive, man-made material that is complicated to create, maintain, and dispose of. A highly fissile form of plutonium is one of the primary fuels used in nuclear weapons, but other forms can be found in pacemaker batteries, heat sources to power NASA space missions, fuel for some nuclear power plants, and as a byproduct in the spent fuel of uranium power plants.

“Plutonium processing requires sensitive techniques where many small changes can affect the outcome,” said PNNL chemist Richard Clark. Understanding these subtle differences in outcomes can help in the further development of nuclear forensics science. 

“Some just take practice,” said Clark. “For example, there’s a talent to how slow you need to go. You get more familiarity with the nuances when you’re watching someone who has done it before as opposed to reading about the procedure in a book.” 

Preparing for work in person

Because it is radioactive and highly toxic, even small amounts of plutonium require concerted handling. To protect themselves, researchers employ thickly walled, sealed containers called glove boxes that are equipped with windows and flexible gloves to handle nuclear material without exposure. 

It can be an awkward, slow process, akin to learning to use chopsticks after a lifetime of eating with a fork. And although visiting Bangor University researcher Stuart Dunn was a glove box worker with a lot of existing knowledge, part of the visit was learning about the different laboratory procedures at PNNL. This required significant coordination to translate his years of training into PNNL’s workflows that included “two or three hours a week for the year leading up to the visit, and intensive training the first week,” according to Dunn. 

“This is a stellar example of our support staff helping to facilitate this type of research,” said PNNL chemist Luke Sweet. “They adopted a real ‘let’s figure out how’ attitude, to get him working safely with the hazardous materials so we could all learn from each other.”

Plans for experiments were discussed for about a year prior to Dunn's visit. This lengthy preparation period made sure the experiment plans were well aligned with available materials, capabilities, expertise, and staff support available at PNNL. Laying out the foundation of expectations was key to making the most of Dunn's visit.

After the extensive planning processes, Dunn was able to jump right into experiments and gain more clarity of PNNL operations by witnessing them firsthand. He was familiar with staff and their expertise prior to his visit, so he came with a lot of questions and a list of things he was hoping to learn. The PNNL staff had also gotten to know Dunn over the years of lead-up to his visit, so they likewise were thrilled to meet and have more thorough conversations with him in person.

The exchange shapes up

Using small samples of plutonium, the team analyzed nearly 50,000 particles using Scanning Electron Microscopy (SEM); the resulting data were fed into the Morphological Analysis of Materials (MAMA) software from Los Alamos National Laboratory (LANL), which aligns with current R&D efforts focused on using open-source machine learning algorithms that can sort thousands of images into different categories in minutes. Dunn and PNNL staff first established a process for performing a hydroxide precipitation that purifies plutonium out of waste streams using his expertise. Dunn leveraged a capability that was developed at PNNL in 2016 to perform statistically designed studies for nuclear forensics. From 2016-2019, PNNL ran 76 different experiments, changing multiple variables (e.g., temperature, acid concentration, strike direction, etc.) per experiment to understand how plutonium characteristics change under varying conditions. 

Next, they worked together on a novel forensic technique that seeks to link the size and shape of plutonium particles with their processing provenance. Each particle was then imaged using a SEM to obtain the “fingerprint” that is analyzed and cataloged by MAMA to build a database that can sort and match any other samples that would be collected in the event plutonium is found out of regulatory control and could potentially point to a source. “This is a time- and labor-intensive process,” said Meier; “it would not be feasible to do this work on the same scale without machine learning or other rapid modern particle scanning capabilities.”

Innovative handling techniques were developed at PNNL to prepare plutonium samples and then take the material out of containment for characterization. Basically, small particles of plutonium material are stuck on a piece of special “tape” and are then able to be removed from the glove box and safely taken to a SEM. 

“There can be an inclination to take a perfectly good instrument, disassemble it into its smallest parts, and reconstruct it in the glove box, but PNNL has a way to adhere the material, so you’re confident surface interactions can be stabilized,” said Dunn. “Instead of putting a million-dollar instrument in a glove box, you spend 30 dollars to safely stick the material on the adhesive.”

“What we were doing at PNNL was something that is a known processing route for a specific material,” said Dunn. “But not many have the capability to look at the signature process. By applying the unique capabilities, we were able to look at a still fairly novel forensic signature from a nuclear forensics perspective under a microscope for the first time.” 

Dunn's experimental plan consisted of studying a plutonium-scrap recovery process that’s a known plutonium processing route that is not studied at PNNL. Using PNNL capabilities, he was able to produce pedigree samples under controlled conditions and analyze them to collect X-ray diffraction data and SEM images. These data are now being used to better understand the material properties of the species formed during the scrap recovery process; this insight is crucial to the development of new nuclear forensics signatures. 

This important national security work was enhanced by Dunn's involvement as an external researcher. “We had worked on some samples and questions regarding plutonium characterization, and we were ready to tackle this larger project with the UK and other DOE national laboratories, given how the work could benefit all organizations and the broader national and international security communities,” said Meier. “The NF community benefitted from learning to use new technologies and philosophies to create and use samples and leverage these new capabilities to advance their work.”

Funding was provided by the U.S. Department of Energy’s National Nuclear Security Administration, Office of Defense Nuclear Nonproliferation R&D. The work was completed in PNNL’s Radiochemical Processing Laboratory (RPL), a hazard category II nonreactor nuclear facility, that has supported national security, energy, environmental cleanup, and isotope research since the 1950s.