Fabrication Strategies and Thermal Conductivity Assessment of High Density UO 2 Pellet Incorporated with SiC
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Fabrication Strategies and Thermal Conductivity Assessment of High Density UO2 Pellet Incorporated with SiC Sunghwan Yeo1, Edward Mckenna1, Ronald Baney1, Ghatu Subhash2, James Tulenko1 1 Materials Science and Engineering Department, University of Florida, Gainesville FL 32611, U.S.A. 2 Mechanical Engineering Department, University of Florida, Gainesville FL 32611, U.S.A. ABSTRACT Enhanced thermal conductivity oxide fuels offer increases in both safety and efficiency of commercial light water reactors. Low-temperature oxidative sintering and Spark Plasma Sintering (SPS) techniques have been used to produce UO2-SiC composite pellets. Oxidative sintering performed for 4 hours at 1200~1600oC and SPS was employed only for 5 mins at the same temperature. While oxidative sintering failed to achieve enhanced thermal conductivity, the SPS sintered pellet obtained promising features such as higher density, better interfacial contact, and reduced chemical reaction. Thermal conductivity measurement at 100oC, 500oC, and 900oC revealed maximum 62% higher thermal conductivity value, when compared to UO2 pellets, in SPS sintered UO2-10vol% SiC composite pellet. The result shows that the SPS technique is required to sinter UO2-SiC nuclear fuel pellets with a high value of thermal conductivity. INTRODUCTION Although uranium dioxide (UO2) is the most common variety of nuclear fuel, its poor thermal conductivity causes both steep temperature gradients and high center-line temperatures fuel pellets during a reactor’s operation. Thermal stress caused by the large temperature gradient results in either cracking in a low temperature region or plastic deformation in the high temperature region. In a loss of coolant accident (LOCA), hydrogen gas, which is produced by the reaction of water and zircaloy cladding, is mainly due to residue heat of the fuel pellet and may cause a reactor explosion. The idea of incorporating high thermal conductivity material into a UO2 pellet has been suggested [1], and silicon carbide (SiC) is a possible material to form the heat release path in the fuel matrix. Silicon carbide has sixty times the thermal conductivity of uranium dioxide at room temperature [2]. Moreover, it has a low thermal neutron absorption cross section, a high melting point, and good irradiation and chemical stabilities. In this study both oxidative sintering [3] and Spark Plasma Sintering (SPS) are employed to fabricate UO2-SiC composite pellets. Oxidative sintering refers to the sintering technique which utilizes enhanced diffusivity of hyper-stoichiometric uranium dioxide powder. SPS or Field Assisted Sintering Technique (FAST) is an advanced sintering process using high pulsed DC current flowing through powder particles. Columbic joule heat or spark discharge between the particles induce uniform and localized heating process. Mechanical pressure is applied and absorbable gases on the particle surface are removed to assist the sintering process. Even though those features lead to advantages such as fast sintering rate and enhanced sinte
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