A New Understanding of Nuclear Fuel

Unexpected anisotropy in uranium dioxide may inhibit efficiency

Technology can be employed without fully understanding it. You might not expect that to happen in the nuclear industry, but it appears that is the case. New research on uranium dioxide is revealing things are not always as they seem.

According to the Nuclear Energy Institute, “America’s 100 nuclear power plants generate almost 20 percent of the nation’s electricity…” With that sort of market share, you’d think the materials responsible for creating the energy would be thoroughly understood. Materials tend to keep an ace or two up their sleeve, however, and uranium dioxide (UO2) is a perfect example.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.

As described in a Los Alamos National Lab (LANL) press release, the property under investigation was thermal conductivity. The heat generated from the fission of the radioactive material heats water which is converted to steam either directly or indirectly. That steam then drives a turbine which generates electricity. Heat is important, so the thermal conductivity of the fuel rods is important too.

Researchers at LANL discovered that UO2 behaves anisotropically in heat conduction although it has long been assumed isotropic. It is thought that interplay of phonon and magnetic spin factors are responsible. According to David Andersson, a Los Alamos National Laboratory scientist on the project, “…in single crystals the measured thermal conductivity is different along the side or edge of the cubic unit cell than along the diagonal…”

UO2 has a cubic structure which is typically associated with isotropic properties. The researchers concluded that phonon-spin scattering with the applied temperature gradient was breaking the symmetry associated with the structure. This “errant” behavior may be responsible for historical scatter in data for UO2. Odd behavior may have been mistakenly attributed to other causes.

What is ultimately hoped from better understanding the material is a better utilization of it. Higher efficiency and greater safety are just a couple of benefits. Probing the fundamental science of material can reveal new information, and that information can pay dividends in the applications.


Photo courtesy of LANL