On the other hand, wet routes employ a dissolution step, in which uranium is first dissolved in an aqueous solution and then precipitated, thermally processed, and reduced under a hydrogen atmosphere. In the dry route, 235U-enriched uranium hexafluoride (UF 6) is hydrolyzed to UO 2F 2 followed by thermal decomposition and reduction to UO 2. (11) Fabrication of UO 2 can occur following either dry or wet routes. (10) A heavy water-moderated reactor, for example, may employ a uranium fuel of natural isotopic composition (99.3% 238U, 0.7% 235U, trace 234U), while a light water-moderated reactor generally requires an enrichment of the fissile isotope, 235U. The fabrication route utilized to produce UO 2 fuel pellets for use in a reactor can depend upon the type of reactor the fuel is destined for. The wide variation in oxygen isotope compositions measured among real-world industrial uranium oxides suggests a more complex mechanism of oxygen fractionation, likely involving atmospheric humidity, heating and cooling rates, and isotope effects─i.e., inherent properties which impact the physicochemical behavior, and ultimately partitioning, of isotopes of an element as a result of mass differences. (7) In that work, the oxygen isotope composition of process water was preserved for a uranyl peroxide precipitate and measured by analysis of co-mineralized water however, subsequent calcination in dry air resulted in a loss of this original signature and rapid incorporation of isotopes from atmospheric oxygen (O 2). Recent work has provided insights into fractionation on the front end of the fuel cycle during yellowcake precipitation and dry air calcination. (8) Similar applications regarding uranium materials have been difficult to interpret, given the limited knowledge of how oxygen fractionates during processing steps within the nuclear fuel cycle. (1−7) Oxygen stable isotopes (as δ 18O values defined vs VSMOW: Vienna Standard Mean Ocean Water see below) have a well-established history in Earth and biological sciences as an origin tracer for rocks, fluids, organisms, and contaminants─primarily as a result of interaction with meteoric water. The use of oxygen stable isotopes as a signature of geolocation and processing history in uranium oxides has garnered interest in recent years. 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. Except when a 200 ☌ 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. Direct reduction of ADU at 600 ☌ in a hydrogen atmosphere resulted in the production of U 4O 9 with a δ 18O value 17.1‰ greater than the precipitate. Indirect reduction of ADU produced UO 2 with a δ 18O value 19.1‰ greater than the precipitate and 4.0‰ greater than the intermediate U 3O 8. Above 400 ☌, no additional fractionation was observed as UO 3 decomposed to U 3O 8 with the rapid heating rate.
An enrichment of 18O attributable to water volatilization was observed in the low temperature (400 ☌) product of thermal decomposition using a 200 ☌ min –1 ramp rate (δ 18O UO3 – δ 18O ADU = 9.2‰). The solid products of thermal decomposition using ramp rates of 2 and 20 ☌ min –1 had statistically indistinguishable oxygen isotope compositions at each decomposition temperature, with increasing δ 18O values in the transition from ADU to UO 3 at 400 ☌ (δ 18O UO3 – δ 18O ADU = 12.3‰) and the transition from UO 3 to U 3O 8 at 600 ☌ (δ 18O U3O8 – δ 18O UO3 = 2.8‰). The bulk oxygen isotope composition of ADU (δ 18O = −16 ± 1‰) was very closely related to precipitation water (δ 18O = −15.6‰). In addition, ADU was reduced using direct (ramped to 600 ☌ in a hydrogen atmosphere) and indirect (thermally decomposed to U 3O 8 at 600 ☌, then exposed to a hydrogen atmosphere) routes.
The kinetic impact of heating ramp rates on isotope effects was explored by ramping to each decomposition temperature at 2, 20, and 200 ☌ min –1. Synthesis of ADU occurred using a gaseous NH 3 route, followed by thermal decomposition in a dry nitrogen atmosphere at 400, 600, and 800 ☌. 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). However, a more thorough understanding of the fractionating processes governing the formation of signatures in real-world samples is still needed. 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.