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

Abstract

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 (δ18OUO3 – δ18OADU = 12.3‰) and the transition from UO3 to U3O8 at 600 °C (δ18OU3O8 – δ18OUO3 = 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 (δ18OUO3 – δ18OADU = 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.

Oxygen Isotope Fractionation in U3O8 during Thermal Processing in Humid Atmospheres

“…This is supported by previous work showing the calcination of the same ADU to result in a preferential loss of 16 O under an inert atmosphere using the same heating rate and ultimate temperaturein which case the δ 18 O value of U 3 O 8 was −0.7‰. 2 Interestingly, fractionation factors between each of the resulting U 3 O 8 compounds and the water vapor sources were different, with U 3 O 8 _d having Δ 18 O = −17.1‰ and U 3 O 8 _e having Δ 18 O = −7.9‰. A re-examination of the calcination procedures revealed a difference in methods; namely, termination of the calcination of U 3 O 8 _d was performed with water vapor still present using a slow cooling rate (see Figure S2) while the calcination of U 3 O 8 _e was ended by switching the atmosphere to dry N 2 prior to a fast cool-down.…”

“…The precipitation and structural characterization of ADU have been previously reported. 2 Briefly, ammonia was introduced into a 0.1 mol•L −1 solution of uranyl nitrate (δ 18 O water = −15.6‰) at 65 °C until a pH of 7.5 was attained. After a 30-min settling period, the precipitate was filtered and washed with deionized water.…”

“…The ubiquitous presence of oxygen bound to uranium during processing on the front end of the nuclear fuel and weapons cycles establishes a forensic significance for the stable isotopes of oxygen. Indeed, oxygen isotopes in uranium oxides have been identified in recent studies to provide information relevant to geolocation and processing history. , In the precipitation of uranyl peroxide from water of a known oxygen isotope composition, the precipitate was shown to incorporate oxygen isotopes from the aqueous solution during mineralization with a consistent fractionation . When this compound was then calcined in an atmosphere of dry air, the original signature from process water was lost, and oxygen isotopes from atmospheric oxygen, O 2 , were rapidly incorporated.…”

“…When this compound was then calcined in an atmosphere of dry air, the original signature from process water was lost, and oxygen isotopes from atmospheric oxygen, O 2 , were rapidly incorporated. Another study showed that in the absence of an exchanging atmosphere (i.e., inert), the thermal decomposition of ammonium diuranate (ADU) between 400 and 800 °C was associated with a consistent loss of 16 O . While these studies highlight the potential for the use of oxygen isotopes as a forensic signature of industrially processed uranium oxides, one important atmospheric component was absent during the calcining and thermal decomposition stages: water vapor (i.e., humidity).…”

“…15 days total use per reservoir), and similar reservoir size (20 L), the change in δ18 O of the vapor in these experiments was assumed to be negligible.ADU Calcination. ADU used to make U 3 O 8 samples for aging experiments was previously determined to have a bulk δ 18 O value of −16 ± 1‰ 2. Calcination of this precipitate to make starting compounds for subsequent aging experiments was performed at 800 °C for 72 h in atmospheres containing either H 2 O_d or H 2 O_e diluted in N 2 .…”