By Scott Miller
Photography by Ashley Jones & Josh Wilson

Clemson researchers are pioneering ways to use medical imaging technology to study the movement of radionuclides through soil

Four years ago, scientists with Clemson University and Savannah River National Laboratory buried the radioisotope neptunium-237 in a 2-foot-long, soil-filled PVC column to analyze how it would react in the environment.

Last year, Kathryn Peruski dug it up. Using Clemson’s Electron Microscopy Facility, the Clemson Ph.D. candidate captured the first image of the miniscule fragmenting of particles off of the neptunium, a radioactive byproduct of nuclear power generation and nuclear weapons production that is stored underground.

“She really got extraordinarily interesting results. It has really changed our view of what is going on,” says Dan Kaplan, senior research fellow with the Savannah River National Laboratory (SRNL). “In particular she found that neptunium was moving in a form that had never been observed before.”

Her discovery is part of a long effort to better understand how disposed nuclear waste could move through soil and potentially enter water and food supplies. And now, the experiments go on. Peruski replaced the buried radioisotope with another that a future Clemson student will dig up years later, possibly in a decade or more.

OUTDOOR TEST BEDS

Peruski is working under the guidance of Brian A. Powell, Fjeld Professor in Nuclear Environmental Engineering and Science at Clemson. Powell holds a joint appointment with SRNL to advance research on environmental remediation and radioactive waste disposal. SRNL’s Joint Appointment program provides an opportunity for university faculty to explore collaborative research opportunities with SRNL researchers and strengthen the relationship between SRNL and the joint appointee’s home university.

Powell began working closely with Kaplan in 2001. At the time, Powell was a graduate student — he earned both his master’s degree and Ph.D. in environmental engineering and science from Clemson under the mentorship of Professor Robert Fjeld. Powell, Fjeld and Kaplan were testing a theory that the radioactive element
plutonium was more mobile than believed, that it could unexpectedly change its oxidation state and move rapidly underground. They found that it was indeed mobile but not at the alarming rate some scientists feared. In fact, plutonium, regardless of the chemical state it was buried in, would convert to a specific form that was largely immobile, they discovered.

“Those results may not be sexy, but having the field data to prove our theoretical understanding of plutonium geochemistry is critical for the Department of Energy to confirm that when they bury waste, they know it is going to be safe. The data is there to support that,” Powell says. “These lysimeter facilities are really supporting the risk analysis that the DOE has to do when disposing of waste.”

A lysimeter is a large bed of concrete with holes where scientists can insert the 2-foot-long, 4- to 6-inch-thick PVC columns filled with soil and some form of radioisotope. The lysimeters are placed outdoors, so the isotopes are then exposed to natural elements like rain and temperature. The testbeds are engineered with plumbing to capture the rainwater.

Powell and Kaplan have designed and built lysimeter test beds both at Savannah River National Lab and at Clemson’s innovation campus in Anderson County. Having multiple locations gives them opportunity to instrument the facilities differently and study different aspects of radionuclide disposal risk.

LONG-TERM EXPERIMENTS

The columns are left in the lysimeters for 10 or 11 years before being pulled out and analyzed using advanced electron microscopy, medical imaging techniques and three-dimensional X-rays to view changes. Computer modeling will help predict how those 10-year changes might happen over tens of thousands of years and even longer.

Long-term modeling is critical. The neptunium Peruski is studying has a half-life — the time it takes for half of the material to undergo radioactive decay — exceeding 2 million years. While environmental contamination from neptunium is highly unlikely now, scientists must have plans to safely handle nuclear waste well into the future, Peruski says.

“You don’t want to wait until this is a problem to solve it,” she adds.

Through her research, Peruski hopes to better understand what causes neptunium to move in the environment so engineers can design effective storage methods. She will analyze several neptunium samples exposed to environmental variables and document their changes over time.

These long-term experiments coupled with laboratory experiments and computer modeling will lead to a better understanding of the physical, chemical and biological processes that govern the mobility of radionuclides in natural and engineered systems. The work is important to evaluate the risk posed by subsurface contamination, to design remediation strategies for contaminated sites and to facilitate the use of safe disposal practices.

“For radioactive waste disposal, we’re putting the radioisotopes into very specific chemical and physical forms, and then we bury them in the ground,” Powell says. “So understanding what type of transformations happen to them while they’re buried is very important. Has it decomposed? Has it degraded in any way that would release radioisotopes?”

Powell and Kaplan also can test the effectiveness of burying waste in metal containers or embedding waste in glass, ceramic or concrete.

“Part of it is materials testing and part of it is risk analysis to understand how fast particular radioisotopes will move once they are released from an engineered waste disposal facility,” Powell says.

    ONE OF A KIND

    The lysimeters give Kaplan and Powell a method to put laboratory results from small-scale experiments through longer, more extensive experiments in natural environments.

    “They really are one of a kind,” Kaplan says. “There are no other facilities in the world that study radionuclides under controlled experimental conditions in the field.”

    Designing the lysimeters to be safe while exposing waste to environmental pressure and maintaining experimental control has been a challenge. Some scientists have been suspicious, Powell acknowledges, but in September, Powell and Kaplan were invited to speak at an international conference in Japan to share details of their engineered lysimeter test beds.

    “Other scientists are becoming increasingly interested as they see some of the data that Dan and I have published from these newer experiments,” Powell says. “Lysimeters allow us to bridge the gap from the laboratory to larger field-scale experiments and ultimately to a large, full-scale, engineered waste disposal facility.”

    Kaplan adds: “The plutonium that Brian studied as a graduate student, after 11 years in the field, its movement was barely detectable. These are very slow processes that are very hard to study. The more time you have, the better prediction you have about how [the material] will behave. Once we get this information, we can model and calculate how far it moves, how fast it may move and how much risk there may be that it moves into the human food chain.”

    A LONG-TERM PARTNERSHIP

    The Savannah River National Laboratory is a multiprogram national laboratory for the U.S. Department of Energy’s Office of Environmental Management. Located at the Savannah River Site near Jackson, S.C., the laboratory works to provide cost-effective solutions for the nation’s environmental, nuclear security, energy and manufacturing challenges.

    SRNL has partnered with Clemson scientists for three decades or more and offered numerous internships to Clemson graduate students studying environmental sciences. 

    “Part of our program at SRNL is to work with universities both for extending our technical capabilities and for workforce development. We went to Clemson because they had expertise in this area,” Kaplan says.

    That partnership was solidified further last year when Powell was awarded a dual appointment with Clemson and SRNL.

    Powell’s research has received more than $7 million since 2014 from the U.S. Department of Energy’s Experimental Program to Stimulate Competitive Research (EPSCoR). The project involves a multidisciplinary team of around 85 scientists with expertise in radiochemistry, radiation detection and measurement, nuclear engineering, environmental engineering, civil engineering, hydrology, geology, materials science, physics, plant physiology, soil science, and quantitative modeling.

    The long-term objective of the EPSCoR project is to support closure of DOE legacy weapons production sites, disposal of radioisotope-bearing wastes and disposal of spent nuclear fuel from commercial energy production.

    Eighteen master’s students and six Ph.D. students have graduated as a result of the work conducted thus far on the project, and an additional six Ph.D. students will graduate before the end of the project in 2020. Thirteen postdoctoral researchers have also participated in the project since 2014. Many of these researchers have continued their careers at DOE laboratories, such as Los Alamos National Laboratory, Pacific Northwest National Laboratory and the National Energy Technology Laboratory. Others have joined the private sector or other universities.

    Scientists working on EPSCoR have authored 35 articles published in peer-reviewed journals and pioneered new ways of using technology developed for medical imaging to study the movement of radionuclides through soil.

    “One of the big questions we are trying to answer is how well do we understand the way water moves through soil and how does that movement affect the chemistry of radioisotopes and how they move through the soil?” Powell says. “We haven’t connected that yet. Water moving through soil sounds simple, but it is incredibly complex, possibly unpredictable under some circumstances.

    “We are really getting beyond scratching the surface of these ideas. We are putting together some pretty sophisticated models to simulate the lysimeter data quite well. But there is a lot more work to go.”

    Scott Miller is director of communications for the Division of Research.

    0 replies

    Leave a Reply

    Want to join the discussion?
    Feel free to contribute!

    Leave a Reply

    Your email address will not be published. Required fields are marked *