FY2010 - Oak Ridge National Laboratory

Seed Money Fund—
Reactor and Nuclear Systems Division
enrichments, burn-up limits, and heat transfer rates. DOE NE has either provided or promised additional
funding for this project as follows. The Advanced Fuels Campaign of the DOE NE Fuel Cycle Research
and Development program has provided $100,000 to prepare a 3 year, $5 million proposal to continue this
line of research. The LWR Sustainability Program has promised a minimum of $200,000 next fiscal year
to continue this work. ORNL has also applied for a patent on this new type of fuel.
Results and Accomplishments
Significant progress has been made on the project tasks. After a slow start, which brought to light the
safety and administrative limitations of working with UO 2 , it was decided to do the initial work using
CeO 2 as a surrogate material. CeO 2 is widely used in research as a surrogate for UO 2 because of its similar
chemical and thermal conductivity properties. The initial work was divided into two parallel tasks:
(1) developing a method for interacting a continuous SiC layer on graphite fibers and (2) developing a
method of mixing CeO 2 particles and fibers in a homogeneous mixture without damaging the fibers.
Graphite interacts with both CeO 2 and UO 2 above 1000°C. To prevent this interaction both during
sintering and reactor operations, a SiC layer is created on the outer surface of the graphite fibers. The SiC
layer needs to be firmly bonded to the graphite fibers. Simply applying a coating of SiC particles will not
provide a uniform continuous SiC layer or a tight bond with the graphite. A process was developed that
interacts silicon into the outer layer of the graphite fiber by passing Ar–4% silane gas (SiH 4 ) over the
graphite fibers in a reaction chamber at elevated temperatures. The method forms a fixed integral bond
between the graphite and SiC of uniform depth that is continuous over the entire outer surface of the fiber.
To provide efficient heat conduction paths through the pellet, the fibers need to be homogeneously
dispersed throughout the pellet with minimal clumping. Several UO 2 fiber mixing methods were
investigated. Initial methods involved mixing the CeO 2 particles and fibers using a high shear blender.
The fibers shattered using this method, exposing the graphite, which interacted with the CeO2 during
sintering. The method was attempted again using 10-µm-diameter, 5-mm-long tungsten wires in place of
fibers. The wires did not disperse but instead twisted into a ball. Adding liquids of different densities and
viscosities or reducing the mixing speed did not improve the results. A liquid mixing technique, which
combined the oxide and fibers in a solution with dispersants, was developed. Interatomic forces prevented
the fibers from clumping and allowed them to homogeneously mix with the oxide. The mixture was then
dried, pressed, and sintered to form a pellet. Pellets of ~40% theoretical density have been formed.
Forming a denser green pellet and sintering at higher temperatures should allow pellets to reach densities
in excess of 90% theoretical density.
Information Shared
Hollenbach, D. F., and L. J. Ott. 2010. “Improving the Thermal Conductivity of UO 2 Fuel with the
Addition of Graphite Fibers.” Trans. Am. Nucl. Soc. 102, 485–487.
Hollenbach, D. F., L. J. Ott, T. M. Bessman, J. W. Klett, and B. L. Armstrong. 2009. “Composite Nuclear
Fuel Pellet.” U.S. Patent Application 13/489,118, filed July 30.
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