Study on effect of process parameters and mixing on morphology of ammonium diuranate

Ammonium Polyuranates: Old Dog, New Structural Tricks

The crystal structure of ammonium polyuranates xUO3·yNH3·zH2O has been investigated. Powder X-ray diffraction (PXRD) has been employed to define single-phase samples within a series of synthesized compounds, which are further characterized by elemental analysis to ascertain the stoichiometry, revealing compositions of 3UO3·NH3·5H2O and 2UO3·NH3·3H2O. Analysis using extended X-ray absorption fine structure and vibrational spectroscopy has elucidated that both 3UO3·NH3·5H2O and 2UO3·NH3·3H2O possess a local structure similar to the metaschoepite─layered U(VI) oxohydroxide UO3·2H2O, but with H2O and NH4+ groups in the interlayers. The structures of ammonium polyuranates are solved from PXRD data, revealing their relationship to the U(VI) oxohydroxide with the established composition of NH4[(UO2)3O2(OH)3]·3H2O and NH4[(UO2)2O2(OH)]·2H2O for 3UO3·NH3·5H2O and 2UO3·NH3·H2O, respectively. These structures maintain the arrangement of U-O polyhedra as pentagonal bipyramids. However, disparities in lattice parameters, space group, and layer topology from UO3·2H2O emphasize significant structural modifications resulting from the substitution of water by ammonium. Moreover, the anion topology of the NH4[(UO2)2O2(OH)]·2H2O has no analogues among uranium oxohydroxide minerals. Notably, ammonium polyuranates, when compared, have minimal alterations in lattice parameters regardless of the presence of ammonia within the structure. The revealed results contribute valuable insights into the UO3-NH3-H2O system and hold potential applications in the field of nuclear power as ammonium polyuranates form during actinide precipitation in back-end of the nuclear fuel cycle and also serve as precursors in the fabrication of nuclear fuel.

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.

A response surface model of morphological changes in UO₂ and U₃O₈ following high temperature aging

The morphological changes that take place during the processing and storage of uranium oxides can provide valuable information on the processing history and storage conditions of an interdicted sample. In this study microstructural changes in two uranium oxides (UO2 and U3O8) due to changes in the aging conditions at elevated temperatures were quantified and modeled using a response surface methodology approach. This allowed the morphological changes to be used as a signature for the aging conditions for nuclear forensic analysis. A Box-Behnken design of experiment was developed using the independent variables: temperature from 100 to 400 °C, aging times from 2 to 48 h, and partial pressure of O 2 ( P O 2 ) ${{\rm{O}}_2}({{\rm{P}}_{{{\rm{O}}_{\rm{2}}}}})$ between ~0.0 kPa and 21.3 kPa. The design of experiment consisted of 54 samples per uranium oxide. Each aged sample was characterized using scanning electron microscopy (SEM) for image analysis. Utilizing the Morphological Analysis for Materials (MAMA) software package, particle size and shape were quantified using the acquired SEM images. Analysis of the particle attributes was completed using the Kolmogorov–Smirnov two sample test (K–S test) to determine if the particle size and shape distributions were statistically distinct. This data was then used to create response surfaces of the quantitative morphological changes based on the developed design of experiment. The U3O8 samples showed no statistically quantifiable differences due to the aging conditions. However, the UO2 samples had distinct morphological changes due to the experimental aging conditions. Specifically, the temperature factor had an increasing effect on the particle area, and a decreasing effect on particle circularity.