NRC Image of fresh fuel pellets ready for assembly.
While fuels used in commercial reactors in the United States today are already extremely robust, there is always an interest in creating even more accident tolerant nuclear fuel.
DOE’s Savannah River National Laboratory (SRNL) is a partner with several other research institutions on a project to identify, develop and begin testing new fuel concepts. The program is under the auspices of DOE’s Office of Nuclear Energy. The overall goal represents a materials science challenge—to identify advanced materials and / or fuel cladding that would improve performance and safety, both during reactor service and during long-term fuel storage.
The shorthand name for the program is accident tolerant nuclear fuels.
Fuels with enhanced accident tolerance are those that, in comparison with the standard UO2—Zircaloy system, can tolerate loss of active cooling in the core for a considerably longer time period (depending on the system and accident scenario) while maintaining or improving the fuel performance during normal operations.
“There are a couple of different avenues,” says SRNL researcher Thad Adams. “One would be to increase the thermal conductivity of the fuel that’s inside the rod and drive the overall temperature down. The other is to put some kind of coating on the outside of the zircaloy tube so that the water and the zirc would not react.”
The latter way—involving advancements in coating—is the focus of the SRNL activity. The team partners will develop and test MAX phase materials for suitability as a coating system for fuel for the present generation of nuclear reactors or for next generation reactor designs.
MAX phase materials are a relatively new class of ternary metal carbides or nitrides that are fully machinable. These ceramic compounds have a unique set of properties, including good thermal conductivity, elevated temperature ductility and fracture toughness and are “weldable.” In addition, they have the typical ceramic characteristic of high temperature mechanical and chemical stability, and high resistance to chemical attack.
The MAX phases as a class are generally stiff, lightweight, and plastic at high temperatures. Some, like Ti3SiC2 and Ti2AlC, are also creep and fatigue resistant, and maintain their strengths to high temperatures. They exhibit unique deformation characterized by basal slip, a combination of kink and shear band deformation, and delaminations of individual grains. During mechanical testing, it has been found that polycrystalline Ti3SiC2 cylinders can be repeatedly compressed at room temperature, up to stresses of 1 GPa, and fully recover upon the removal of the load while dissipating 25% of the energy.The plan is for compounds from the MAX phase class of carbides to be selected and spray-applied to zircaloy substrates / claddings to yield an improved cladding compatible with the existing fleet of Light Water Reactor fuel, and highly resistant to high water temperature and steam conditions.
The program goal is to have a lead test assembly by 2020.
