Oxygen Kinetic Isotope Effects in the Thermal Decomposition and Reduction of Ammonium Diuranate

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 °C. The kinetic impact of heating ramp rates on isotope effects was explored by ramping to each decomposition temperature at 2, 20, and 200 °C min^-1. In addition, ADU was reduced using direct (ramped to 600 °C in a hydrogen atmosphere) and indirect (thermally decomposed to U₃O₈ at 600 °C, then exposed to a hydrogen atmosphere) routes. The bulk oxygen isotope composition of ADU (δ¹⁸O = -16 ± 1‰) was very closely related to precipitation water (δ¹⁸O = -15.6‰). The solid products of thermal decomposition using ramp rates of 2 and 20 °C min^-1 had statistically indistinguishable oxygen isotope compositions at each decomposition temperature, with increasing δ¹⁸O values in the transition from ADU to UO₃ at 400 °C (δ¹⁸O_UO3 - δ¹⁸O_ADU = 12.3‰) and the transition from UO₃ to U₃O₈ at 600 °C (δ¹⁸O_U3O8 - δ¹⁸O_UO3 = 2.8‰). An enrichment of ¹⁸O attributable to water volatilization was observed in the low temperature (400 °C) product of thermal decomposition using a 200 °C min^-1 ramp rate (δ¹⁸O_UO3 - δ¹⁸O_ADU = 9.2‰). Above 400 °C, no additional fractionation was observed as UO₃ decomposed to U₃O₈ with the rapid heating rate. Indirect reduction of ADU produced UO₂ with a δ¹⁸O value 19.1‰ greater than the precipitate and 4.0‰ greater than the intermediate U₃O₈. Direct reduction of ADU at 600 °C in a hydrogen atmosphere resulted in the production of U₄O₉ with a δ¹⁸O value 17.1‰ greater than the precipitate. Except when a 200 °C min^-1 ramp rate is employed, the results of both thermal decomposition and reduction show a consistent preferential enrichment of ¹⁸O 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.