Silicon Carbide Clad Thoria Plutonia Fuel for Light Water Reactors

Light water reactor (LWR) fuel used in current reactors use slightly enriched uranium oxide fuel pellets with zirconium alloy (zircaloy) clad. Because the clad oxidizes in the reactor coolant, the fuel burnup is limited to a maximum of five years, or a batch average of 55 gigawatt days per ton (gwd/t). An advanced silicon carbide ceramic cladding has been developed which is more corrosion resistant than zircaloy and can achieve twice the burnup, thus reducing the volume of spent fuel requiring storage and disposal. Because it has v ery slow reaction during Loss of Coolant Accident (LOCA) conditions, this cladding is much safer during severe accidents. However, this cladding is rigid, and does not creep during operation; hence the pellet to clad gap remains open for a longer period of time, increasing the temperature of the uranium oxide fuel, leading to higher fission gas release, and less margin to centerline melt during transients. Another fuel form, thorium oxide, has been proposed for light water reactors because it has superior thermal properties, and when enriched with excess reactor grade plutonium has the potential to destroy current plutonium stockpiles, and thus reduce the amount of plutonium that must be disposed of in a repository. Traditional mixed oxide fuel generates more plutonium during operation. However, the thoria-plutonia fuel, when clad with zircaloy, cannot achieve the high burnup required for economic operation and plutonium destruction. We propose to marry the advanced silicon carbide cladding with the thoria-plutonia fuel form, and thereby, solve both problems at once. Since the silicon carbide clad has better nuclear properties than zirconium alloys (about 30% less parasitic absorption), it would further enhance the benefits of a thoria plutonia fuel, achieving even higher burnups and reactor economics. As a first step in Phase 1, we propose to conduct experiments to determine the chemical compatibility of thoria with silicon carbide at normal operating temperatures and at accident temperatures (up to 1600 oC). We will also run tests on the silicon carbide cladding to confirm its relative passivity in steam during LOCA accidents up to 1600 C. If Phase 1 tests are acceptable, Phase 2 will involve the development of a fuel design and fabrication of test specimens for an ATR loop test. Commercial Applications and Other Benefits: Commercial LWR fuel enabled by this new ceramic cladding material and thoria-plutonia fuel system will enhance passive safety at the heart of the reactor, will operate to higher burnups, thus reducing the amount of spent fuel and the burden on the US waste repository, and will enable the disposition of plutonium from recycle PWR spent fuel without the penalty of producing new plutonium as in uranium based MOX fuel.