Silicon Carbide Clad Thoria Plutonia Fuel for Light Water Reactors

Statement of Problem: Light water reactor fuel used in current reactors uses slightly enriched uranium oxide (UO2) fuel pellets with zirconium alloy (zircaloy) clad. Zircaloy clad loses all of its tensile strength at temperatures of 500 oC, and balloons outward during the early phases of a loss of coolant accident, thereby blocking the flow of emergency coolant. Also, it reacts exothermically when exposed to emergency coolant after an accident. This behavior was a principal cause of the severe fuel damage during the TMI reactor accident and was the most likely cause of fuel damage and radioactivity release at Fukushima, Japan. Silicon Carbide (SiC) cladding, on the other hand, retains its strength to 1600 oC, would not balloon and block flow during an accident, and would not react exothermically with emergency coolant. Even though SiC cladding has been under development for over seven years, it still has several major technology development challenges that must be addressed before licensing and deployment. Examples of remaining challenges are development of a robust end joint to seal the fuel within the cladding, improving the composite structure for greater impact resistance, and creating new processes with the capability to produce 14 foot long clad tubes. And because the cladding is inelastic, the fuel will run hotter, requiring a change in fuel form such as annular UO2 fuel at higher enrichment, or use of a high thermal conductivity high melting temperature fuel such as thoria-plutonia. This work is intended to (1) demonstrate the superior behavior of SiC triplex cladding during severe accidents, and including a spent fuel pool loss of coolant situation, (2) to identify and develop creative solutions to the remaining technical challenges, and (3) to examine solutions to the higher fuel temperature issue such as use of thoria-plutonia fuel and higher enriched UO2. It is intended that the resulting SiC triplex clad product would be compatible with a variety of fuel forms that evolve based on ongoing industry and DOE fuel development programs. In Phase I we demonstrated three key characteristics of SiC fuel cladding. (1) unlike the zircaloy cladding used in current reactors throughout the world, the SiC does not react exothermically with hot water, or air, at temperatures of 1400 oC and higher; (2) it is chemically compatible with alternate fuel forms such as thoria based fuels that have higher thermal conductivity and melting temperatures than UO2; and (3) when coupled with a thoria-plutonia fuel form, it has the capability of very long fuel cycles (e.g. over 7 years), very high burnup (over 120 mwd/kg) and very high plutonium destruction rates (over 60% of initial Pu loading). Commercial Applications and Other Benefits: Because of its robustness and durability during severe accidents such as at Fukushima, SiC triplex cladding has the potential to avoid core damage during severe accidents, and thus enable the continuation of safe nuclear operations in existing reactors throughout the world. When coupled with new fuel forms such as thoria-plutonia fuel, it can operate to higher burnups, thus reducing the amount of spent fuel and the burden on the US waste repository, and can also enable the disposition of plutonium from excess weapons disposition, and recycled spent fuel without the penalty of producing new plutonium, as in uranium based mixed oxide fuel.