The transporting of heat is an important part of our daily life, from boiling a pot of water to creating complex carbon-free energy technologies that power our cellphones, laptops, and home appliances.
Nuclear energy is an example of heat transfer. It has a proven track record spanning over seventy years. Nuclear power provides about twenty percent of the total electric energy generation in the U.S. A nuclear reactor is similar in principle to a fossil energy plant in that thermal energy (heat) is used to generate steam. The steam is then converted to electricity using a steam turbine. As the nuclear industry moves to advanced fuel cycle technologies, efficiently transporting heat energy becomes increasingly important. Meeting this challenge requires an understanding of microscopic mechanisms that control the transport of that heat.
In typical nuclear fuel, a uranium atom absorbs a neutron, becomes unstable and splits which results in the creations of two new lighter atoms. This process imparts sufficient kinetic energy to these new atoms to displace thousands of adjacent atoms from their equilibrium positions. This causes the creation of microscopic defects in the crystalline structure. Furthermore, this process produces heat that must flow through the fuel element. Then the heat must be transferred out of the coolant. Finally, the heat must produce steam for making electricity. The crystalline defects degrade the ability to transfer heat. This is a quality known as its thermal conductivity.
David Hurley is an Idaho National Laboratory Fellow and director of the Center for Thermal Energy Transport under Irradiation (TETI) which is an INL-led Energy Frontier Research Center supported by the Department of Energy’s Office of Science. He said, “Over the lifetime of the fuel in a reactor, its thermal conductivity decreases by as much as 70%.”
The degradation of a nuclear fuel’s thermal conductivity is a challenge for efficient nuclear power operations. However, researchers are learning that this degradation can be mitigated by improving the design of the chemistry and structure of the fuel. Fully achieving these mitigation strategies will require going beyond trial-and-error investigative approaches.
This is where TETI comes into play. The mission of the center is to accurately predict and ultimately improve the transport of thermal energy in nuclear fuels in extreme radiation and temperature environments. Hurley said, “Because our materials of interest contain uranium, quantum mechanical approximations such as density function approximations all fail at some level. What sets TETI apart is its ability to tackle this problem while staying as close to the first principles of quantum mechanics as possible.”
In its first four years, TETI scientists utilized inelastic neutron scattering to gain critical new insights into resolving the atomic scale mechanisms governing thermal transport in defect free, ceramic nuclear fuels. TETI scientists also applied beyond density functional approximation in order to discover a new energy state of uranium oxide, lower than previously reported. Linu Malakkal is one of the principal investigators on the TETI team. He said, “The lowest energy state is the correct state, and we are getting very close.”
Hurley said, “As we look to the future, it is important to realize that the tools developed by TETI researchers will not only impact advanced nuclear energy concepts but will offer opportunities for other energy-related technologies beyond nuclear energy, such as thermoelectric or photoelectric energy conversion, and/or new quantum materials.”