INL News Release
FOR IMMEDIATE RELEASE
June 11, 2013
NEWS MEDIA CONTACT:
Teri Ehresman, 208-526-7785, teri.ehresman@inl.gov
Misty Benjamin, 208-526-5940, misty.benjamin@inl.gov
NOTE
TO EDITORS: Images associated with this project are available on INL's
Flickr page: http://www.flickr.com/photos/inl/sets/.
Silver lining advances understanding of next-generation nuclear fuel
The
long search for the location of a rare element within nuclear fuel
particles has ended. Researchers have finally pinpointed where silver
congregates inside irradiated particles of a new type of nuclear fuel.
The
finding will help scientists better understand how select fission
products such as silver can escape from tristructural isotropic (TRISO)
fuel. This specialized fuel could run high-temperature gas-cooled
reactors (HTGRs) that have numerous enhanced safety features.
"This
is a major achievement for our research in TRISO particle fuel," says
David Petti, Idaho National Laboratory's director of the Very High
Temperature Reactor Technology Development Office. "Understanding the
behavior of fission products in our fuel is critical because of the
TRISO coating's containment function in the overall safety strategy for
HTGRs."
Next-generation HTGR designs incorporate safety systems
that rely on the natural laws of physics more than mechanical systems or
human intervention. These safety systems extend all the way down to the
design of the fuel itself.
For nearly 10 years, INL researchers
have been studying TRISO fuel, a spherical particle with uranium dioxide
or uranium oxycarbide at its core. The core is coated with layers of
carbon and silicon carbide — the TRISO coating — which acts as "the
primary containment" for fission products. The coated particles are
about the size of a poppy seed.
Researchers had known silver
fission products were amassing somewhere inside the coated particles.
Silver is one of the few fission products that can migrate outside the
particles, and scientists want to better understand such movement. But
they had not been able to adequately "see" inside the particles until
now. The research team reached the new milestone by spotting a tiny
sliver of silver using an extremely powerful microscope at the Center
for Advanced Energy Studies (CAES).
Scanning Transmission
Electron Microscope (STEM) examination enabled 1-nanometer
magnification, where the silver was identified and confirmed in the
particle's silicon carbide layer. Specifically, the silver resided along
the "grain boundaries," the place in a material where one crystal
lattice ends and another begins. The STEM microscope helped confirm both
the presence and location of the silver.
"The technique and
scale of these measurements on TRISO fuel, as well as the identification
of silver, are first-of-a-kind and are helping obtain a better
understanding of the transport mechanisms associated with these fission
products,” says Isabella van Rooyen, INL nuclear materials scientist and
principal investigator for the research effort. For the past nearly 40
years, researchers have studied silver in the particles without finding a
satisfactory explanation for its movement during and after irradiation.
This achievement was possible due to critical capabilities at
INL to prepare very small irradiated specimens using the focused ion
beam (FIB) at the Materials and Fuels Complex, the STEM at CAES, and the
team's supportive and innovative research approach. Only earlier this
year, modifications to the CAES licensing agreement were approved by the
Nuclear Regulatory Commission, allowing very small specimens with a low
radiation dose to be examined at CAES, which was previously impossible.
CAES is a research and education partnership between INL, Boise State
University, Idaho State University and University of Idaho.
Now
that the silver has been found, "the real fun begins," Van Rooyen
explains. It has been her dream to work on irradiated TRISO fuel and she
says this discovery "opens the door for many future applications of
TRISO fuel for both very high temperature reactors and small modular
reactors." Van Rooyen started the search for silver and the relevant
transport mechanism in 2006 on nonirradiated fuel while working on
pebble bed modular reactors in South Africa. That research brought her
to INL in 2011, and she is pleased that in two short years the research
is paying off.
Yaqiao Wu, a Boise State University research
associate professor and instrument lead of Materials and
Characterization Suite at CAES, echoes her excitement. "The expertise of
the staff, researchers and the equipment at CAES played critical roles
in making this discovery possible," he said. "The smallest electron beam
size with the STEM is around 1 nanometer, which is crucial for
revealing the nanosized silver-rich phase in this particle."
Fellow
INL researcher Tom Lillo agreed, saying, "We are very fortunate to have
state-of-the-art equipment and facilities to look at things that are
happening on an extremely small scale in order to understand the
behavior in large, advanced energy systems."
The next step for
the team is to continue to test and study the results. Team members say
that more research is needed to identify the silver transport mechanism
and to continue monitoring how the silver behaves.
For more information on the research, see the conference proceedings here.
INL
is one of the DOE’s 10 multiprogram national laboratories. The
laboratory performs work in each of the strategic goal areas of DOE:
energy, national security, science and environment. INL is the nation’s
leading center for nuclear energy research and development. Day-to-day
management and operation of the laboratory is the responsibility of
Battelle Energy Alliance.
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