Thermal conductivity measurement of non-stoichiometric UO2 pellet for advanced nuclear material accountancy

Abstract

Currently, the IAEA utilizes thermogravimetric analysis (TGA) to identify UO2 fuel stoichiometry or O/U ratio; however, the method is unfit for on-site inspection, since it requires a large heating device to achieve high temperature (900 °C) to fully oxidize UO2±x to U3O8. In order to enable the development of a hand-carry device for this matter, we suggest the utilization of UO2 thermal conductivity which sensitively varies with the fuel stoichiometry at low temperature (< 300 °C). In this study, various hyper-stoichiometric UO2+x pellets (0 < x < 0.025) were sintered under mixed gas of CO/CO2 and H2/Ar, referring to the equilibrium oxygen potential of constitutional U-O phase diagram. The stoichiometry of fabricated pellets was reaffirmed following the standard TGA method, which appears to be matched with intended stoichiometry. The thermal conductivity of fabricated UO2+x pellets was measured from 25 °C to 300 °C using laser flash analyzer (LFA) and compensated to 96 % theoretical density (TD) considering various sintered pellet density from 89 to 92 %TD. LFA-measured fuel thermal conductivity show 8% decrease at room temperature over 0.004 stoichiometry deviation from 2.007 to 2.011, which indicates clear potential of LFA method for advanced material accountancy of commercial nuclear fuel.