Oxygen stable isotopes in uranium oxides processed through the nuclear fuel cycle may have the potential to provide information about a material's origin and processing history. However, a more thorough understanding of the fractionating processes governing the formation of signatures in real-world samples is still needed. In this study, laboratory synthesis of uranium oxides modeled after industrial nuclear fuel fabrication was performed to follow the isotope fractionation during thermal decomposition and reduction of ammonium diuranate (ADU). Synthesis of ADU occurred using a gaseous NH3 route, followed by thermal decomposition in a dry nitrogen atmosphere at 400, 600, and 800 degrees C. The kinetic impact of heating ramp rates on isotope effects was explored by ramping to each decomposition temperature at 2, 20, and 200 degrees C min(-1). In addition, ADU was reduced using direct (ramped to 600 degrees C in a hydrogen atmosphere) and indirect (thermally decomposed to U3O8 at 600 degrees C, then exposed to a hydrogen atmosphere) routes. The bulk oxygen isotope composition of ADU (delta O-18 = -16 +/- 1%) was very closely related to precipitation water (delta O-18 = -15.6%). The solid products of thermal decomposition using ramp rates of 2 and 20 degrees C min(-1) had statistically indistinguishable oxygen isotope compositions at each decomposition temperature, with increasing delta O-18 values in the transition from ADU to UO3 at 400 degrees C (delta(UO3)-U-18 - delta(18)OADU = 12.3%) and the transition from UO3 to U3O8 at 600 degrees C (delta(OU3O8)-O-18 - delta(OUO3)-O-18 = 2.8%). An enrichment of O-18 attributable to water volatilization was observed in the low temperature (400 degrees C) product of thermal decomposition using a 200 degrees C min(-1) ramp rate (delta(OUO3)-O-18 - delta(18)OADU = 9.2%). Above 400 degrees C, no additional fractionation was observed as UO3 decomposed to U3O8 with the rapid heating rate. Indirect reduction of ADU produced UO2 with a delta O-18 value 19.1% greater than the precipitate and 4.0% greater than the intermediate U3O8. Direct reduction of ADU at 600 degrees C in a hydrogen atmosphere resulted in the production of U4O9 with a delta O-18 value 17.1% greater than the precipitate. Except when a 200 degrees C min(-1) ramp rate is employed, the results of both thermal decomposition and reduction show a consistent preferential enrichment of O-18 as oxygen is removed from the original precipitate. Hence, the calcination and reduction reactions leading to the production of UO2 will yield unique oxygen isotope fractionations based on process parameters including heating rate and decomposition temperature.
