Oxygen Isotope Fractionation in U3O8 during Thermal Processing in Humid Atmospheres

Abstract

The incorporation of oxygen isotopes from water into uranium oxides during industrial processing presents a pathway for determining a material's geographical origin. This study is founded on the hypothesis that oxygen isotopes from atmospheric water vapor will exchange with isotopes of oxygen in solid uranium oxides during thermal processing or calcination. Using a commonly encountered oxide, U₃O₈, the exchange kinetics and equilibrium fractionation with water vapor (in a concentration range of 50-55% relative humidity) were investigated using processing temperatures of 400, 600, and 800 °C. In an atmosphere containing only water vapor diluted in N₂, oxygen isotope equilibration in U₃O₈ occurred within 12 h at 400 °C and within 2 h at 600 and 800 °C. Fractionation factors (1000lnα, U₃O₈-H₂O) between the water and oxide were -12.1, -11.0, and -8.0 at 400, 600, and 800 °C, respectively. With both humidity and O₂ present in the calcining atmosphere, isotopic equilibration is attained within 2 h at and above 400 °C. In this mixed atmosphere, which was designed to emulate Earth's troposphere, isotopes are incorporated preferentially from water vapor at 400 °C and from O₂ at 600 and 800 °C. Rapid and temperature/species-dependent isotope exchange also elucidated the impact of retrograde exchange in humid air, showing a shift from O₂-dependent to H₂O-dependent fractionation as U₃O₈ cooled from 800 °C. These results confirm that uranium oxides inherit oxygen isotopes from humidity during thermal processing, illuminating an important mechanism in the formation of this forensic signature.

Figure 1

Setup for calcination experiments. Water vapor diffuses through membrane tubing submerged in the reservoir and is subsequently diluted with a second stream of dry gas. The RH is controlled by flowmeters for each gas stream.

Figure 2

U₃O₈_d (δ¹⁸O = −43.3‰) was calcined in an atmosphere of vapor from H₂O_e (δ¹⁸O_v = −8.8‰) diluted to 50–55% RH in N₂. Samples heated at 600 and 800 °C appear to achieve equilibrium with water vapor within 2 h, while U₃O₈ calcined at 400 °C reaches equilibrium within 2–12 h.

Figure 3

Oxygen isotope fractionation factors between U₃O₈ and water vapor compared to calculated fractionation for other U-oxides from equations provided by Hattori and Halas and Zheng.^,

Figure 4

U₃O₈_e (δ¹⁸O = −16.7‰) was calcined in an atmosphere of vapor from H₂O_d (δ¹⁸O_v = −26.2‰) diluted to 50–55% RH in dry air (δ¹⁸O_O2 = +23.2‰). Samples calcined at 400, 600, and 800 °C all appear to equilibrate within 2 h.

Figure 5

Fractionation between U₃O₈ and water vapor [top] and fractionation between U-oxides and atmospheric O₂ [bottom] during 72 hr calcining experiments. Data for fractionation between UOx and dry air (N₂/O₂, gray squares) are reproduced from Klosterman et al., with amorphous UO₃ at 400 °C and U₃O₈ at 600 and 800 °C and calcination times of 20 h.

Figure 6

δ¹⁸O values for U₃O₈ (initial δ¹⁸O = −16.7‰) calcined for 1 h in a mixed atmosphere (¹⁸O-depleted H₂O vapor + air with ¹⁸O-enriched O₂). U₃O₈ was cooled from 800 °C in either an inert atmosphere or the mixed atmosphere at different rates.