Illustration of anisotropic thermal conductivity in uranium dioxide (UO2). Scientists are studying the thermal conductivity related to the material’s different crystallographic directions, hoping to improve our understanding of UO2’s efficiency as reactor fuel. Courtesy/LANL
Nearly 20 percent of the electricity in the United States is generated by nuclear energy from uranium dioxide fuel, but mysteries still surround exactly how the material controls the electrical production: Poor thermal conductivity can limit the conversion rate of heat produced by fission, however we don’t know the physics underlying this behavior or, as it turns out, some of the properties to which it gives rise.
“A deeper understanding of the physics that governs the performance of important engineering materials, such as uranium dioxide, should lead to improvements in efficiency and safety,” said David Andersson, a Los Alamos National Laboratory scientist on the project, “which are ultimate goals of the Department of Energy’s program to develop advanced predictive computer models of nuclear reactor performance.”
New research at Los Alamos is showing that the thermal conductivity of cubic uranium dioxide is strongly affected by interactions between phonons carrying heat and magnetic spins. “This leads to unexpected behavior of the thermal conductivity: For example, in single crystals the measured thermal conductivity is different along the side or edge of the cubic unit cell than along the diagonal, even at and above room temperature,” said Andersson.
A solid is said to be isotropic if the properties are equal in all directions: UO2 has always been assumed to be isotropic, Andersson said, but “after our discovery of anisotropic thermal conductivity we realized that there were hints of this behavior as the material wasn’t always behaving as expected.”
This matters for the nuclear power industry because some of the uncertainty in historical experimental data may have been due to hidden anisotropy rather than measurement uncertainties or sample impurities.
“UO2 has always been assumed to be isotropic due to its cubic crystal structure. In contrast to this, the measured thermal conductivity anisotropy shows us that UO2 really does not behave like we would expect for a simple cubic material”, Andersson noted. This result is surprising because there is no clear evidence of non-cubic distortions for either the paramagnetic or the antiferromagnetic phases of UO2 in diffraction experiments.
The anisotropy is explained by the phonon-spin scattering mechanism coupling to the applied temperature gradient and thus breaking cubic symmetry. “This insight made us realize that the same phonon-spin scattering is responsible for the poor UO2 thermal conductivity in general, which is one of the most important and limiting factors in determining nuclear fuel performance and safety.”
The paper was published today in Nature Communications (http://www.nature.com/ncomms/2014/140801/ncomms5551/full/ncomms5551.html).