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 NH ₃ 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 U ₃ O ₈ at 600 °C, then exposed to a hydrogen atmosphere) routes. The bulk oxygen isotope composition of ADU (δ ¹⁸ O = -16 ± 1‰) was very closely related to precipitation water (δ ¹⁸ O = -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 δ ¹⁸ O values in the transition from ADU to UO ₃ at 400 °C (δ ¹⁸ O _UO3 - δ ¹⁸ O _ADU = 12.3‰) and the transition from UO ₃ to U ₃ O ₈ at 600 °C (δ ¹⁸ O _U3O8 - δ ¹⁸ O _UO3 = 2.8‰). An enrichment of ¹⁸ O attributable to water volatilization was observed in the low temperature (400 °C) product of thermal decomposition using a 200 °C min ^-1 ramp rate (δ ¹⁸ O _UO3 - δ ¹⁸ O _ADU = 9.2‰). Above 400 °C, no additional fractionation was observed as UO ₃ decomposed to U ₃ O ₈ with the rapid heating rate. Indirect reduction of ADU produced UO ₂ with a δ ¹⁸ O value 19.1‰ greater than the precipitate and 4.0‰ greater than the intermediate U ₃ O ₈ . Direct reduction of ADU at 600 °C in a hydrogen atmosphere resulted in the production of U ₄ O ₉ with a δ ¹⁸ O 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 ¹⁸ O as oxygen is removed from the original precipitate. Hence, the calcination and reduction reactions leading to the production of UO ₂ will yield unique oxygen isotope fractionations based on process parameters including heating rate and decomposition temperature.
Competing Interests: The authors declare no competing financial interest.
(© 2021 The Authors. Published by American Chemical Society.)