Determining the origin and history of interdicted nuclear materials is a central challenge in nuclear forensics. The oxygen stable isotope composition of uranium oxide compounds has emerged as a promising forensic signature, attracting increasing attention since the early 2000s. This review examines analytical techniques for measuring oxygen isotope compositions in uranium oxides and evaluates how the nuclear fuel production cycle introduces or modifies these isotopic signatures. The potential for forensic geolocation is explored through workflows that calibrate the relationship between environmental water oxygen isotopes and those found in uranium oxides. Key strengths and limitations of this approach are assessed, including gaps in knowledge related to isotope fractionation during specific stages of the fuel cycle, and processing facility water inputs. The importance of proper sample handling and storage under inert atmospheres, as well as a deeper understanding of both intra-sample oxygen isotope heterogeneity, and hydrous uranium oxide phase formation, is highlighted for improving the reliability of forensic interpretations. The development of uranium oxide standards with well-characterized delta 18O values and international collaboration toward consensus on their use are identified as essential steps for advancing the field.

We have developed the capability to measure 17O/16O in H2O and OH- of hydrous minerals by a technique using thermogravimetric analysis coupled with isotope ratio infrared spectroscopy (TGA-IRIS) on a suite of liquid water standards, terrestrial mineral standards, multi-mineral synthetic mixtures of terrestrial samples, and Murchison carbonaceous chondrite. This 17O/16O analytical capability is combined with the previously developed TGA-IRIS capability of 18O/16O, which enables us to calculate Delta`17O values. Additionally, we have made 2H/1H measurements by TGA-IRIS on the suite of materials. Analyses of 328 standard water samples sealed into silver tubes (0.25 mu L, 174 VSMOW and 154 GISP) analyzed over the course of three years yielded delta 18O values by TGA-IRIS with precision of +/- 0.30 %o, delta 17O of +/- 0.24 %o, Delta`17O of +/- 0.126 %o, and delta 2H +/- 7.5 %o (+/- 1 Std. Dev.). TGA-IRIS analyses of a suite of hydrous minerals (oxy-hydroxide, mica, serpentines, sulfide, phyllosillicates) yields similar precision. TGA-IRIS analyses of the CM2 carbonaceous chondrite Murchison resulted in delta 18O, delta 17O, Delta`17O, and delta 2H values of the H2O and/or OH- from its hydrous minerals. Delta`17O values of Murchison H2O range from 1.034 %o at 350 degrees C, to - 0.368 %o at 650 degrees C, to - 1.75 %o at 850 degrees C. TGA-IRIS can generate Delta`17O values of H2O and/or OH- in hydrous materials that are well within the precision needed for the analysis of meteoritic samples, as well as samples returned from asteroids, such as Ryugu and Bennu.

Uranium isotopic composition can provide valuable information about the history and provenance of a nuclear material; therefore, uranium isotopic analyses are frequently made in the nuclear forensics, safeguards, and environmental monitoring communities. These measurements have always presented challenges due to the extreme variability in the relative abundance between the major (U-235, U-238) and minor (U-233, U-234, U-236) isotopes of uranium. The recently developed ATONA (Atto- to Nano-Amp) amplification system paired with Faraday cup detectors has a large dynamic range and low noise floor making it ideal for measuring uranium isotopic ratios in materials of both natural and anthropogenic origin. A wide variety of certified reference materials were analyzed to investigate the utility of the ATONA amplification system for determining uranium isotopic composition in samples ranging from depleted to highly enriched. The ATONA amplifiers provide nearly an order of magnitude improvement in external reproducibility over 10(11) Omega amplifiers when measuring the minor U-234/U-238 ratio in isotopically natural and depleted samples and when paired with a secondary electron multiplier can measure very low relative abundance uranium isotopes (i.e., U-236).

Within the front end of the nuclear fuel cycle, many processes impart forensic signatures. Oxygen-stable isotopes (delta O-18 values) of uranium-bearing materials have been theorized to provide the processing and geolocational signatures of interdicted materials. However, this signature has been minimally utilized due to a limited understanding of how oxygen isotopes are influenced during uranium processing. This study explores oxygen isotope exchange and fractionation between magnesium diuranate (MDU), ammonium diuranate (ADU), and uranyl fluoride (UO2F2) with steam (water vapor) during their reduction to UOx. The MDU was precipitated from two water sources, one enriched and one depleted in O-18. The UO2F2 was precipitated from a single water source and either directly reduced or converted to ADU prior to reduction. All MDU, ADU, and UO2F2 were reduced to UOx in a 10% hydrogen/90% nitrogen atmosphere that was dry or included steam. Powder X-ray diffraction (p-XRD) was used to verify the composition of materials after reduction as mixtures of primarily U3O8, U4O9, and UO2 with trace magnesium and fluorine phases in UOx from MDU and UO2F2, respectively. The bulk oxygen isotope composition of UOx from MDU was analyzed using fluorination to remove the lattice-bound oxygen, and then O-2 was subsequently analyzed with isotope ratio mass spectrometry (IRMS). The oxygen isotope compositions of the ADU, UO2F2, and the resulting UOx were analyzed by large geometry secondary ion mass spectrometry (LG-SIMS). When reduced with steam, the MDU, ADU, and UO2F2 experienced significant oxygen isotope exchange, and the resulting delta O-18 values of UOx approached the values of the steam. When reduced without steam, the delta O-18 values of converted ADU, U3O8, and UOx products remained similar to those of the UO2F2 starting material. LG-SIMS isotope mapping of F impurity abundances and distributions showed that direct steam-assisted reduction from UO2F2 significantly removed F impurities while dry reduction from UO2F2 led to the formation of UOx that was enhanced in F impurities. In addition, when UO2F2 was processed via precipitation to ADU and calcination to U3O8, F impurities were largely removed, and reductions to UOx with and without steam each had low F impurities. Overall, these findings show promise for combining multiple signatures to predict the process history during the conversion of uranium ore concentrates to nuclear fuel.

Exchange of oxygen stable isotopes (delta O-18 values) between precipitation waters and uranium oxides is governed by thermodynamics or kinetics. It has been assumed that meteoric waters can be related to precipitation waters in uranium ore concentrates and their calcined and reduced uranium oxide products. With this assumption, the delta O-18 values of uranium materials could provide forensic signatures that identify the production history and geolocation of nuclear materials. To further exploit the potential of delta O-18 values in nuclear material analysis, this study examines the oxygen stable isotope exchange in two UOCs, magnesium diuranate (MDU) and sodium diuranate (SDU). MDU and SDU were synthesized from solutions of uranyl nitrate hexahydrate using precipitation waters with unique oxygen isotope compositions. The structures of the MDU and SDU were analyzed using powder X-ray diffraction (p-XRD) and thermal mass loss curves, while the delta O-18 values of waters generated during thermal decomposition were analyzed using a thermogravimetric analyzer coupled to an isotope ratio infrared spectrometer (TGA-IRIS). By p-XRD, theMDUwas uniformand amorphous across all syntheses with residual crystalline material incorporated as aminor component. Combinedwith the TGA results, all of the MDU is likely amorphous MgU(2)O7 center dot 3H(2)O with MgO impurities present throughout. In contrast, the SDU synthesis resulted in multiple phases with many samples exhibiting crystalline phases including a combination of Na(UO2)(4)O-2(OH)(5)center dot 5H(2)O and Na-2(UO2)(6)O-4(OH)(6)center dot 8H(2)O with a Na2U2O7 minor phase. A small fraction of the SDU samples were amorphous with no crystalline XRD peaks observed. Mass loss curves of the SDU samples revealed that the amorphous samples contained inclusions of similar crystalline phases compared to the crystalline materials. The uniformity of the MDU samples enabled highly reproducible measurements of delta O-18 values of the water vapor yielded from two dehydration events at 170 degrees C and 500 degrees C. In contrast, the multiphase composition of the SDU samples resulted in poor reproducibility in delta O-18 values. Neither system revealed any correlation between the delta O-18 values of precipitation water and the waters released during dehydration of the UOCs.

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.

In year 2 of this project (2017), our group focused on determining the causes of isotopic signatures in fentanyl. In order to better predict isotopic signatures in fentanyl, we analyzed the nitrogen isotope compositions of two key intermediates of fentanyl, NPP and ANPP. In addition, we analyzed additional fentanyl products produced using the Valdez route, along with five other alternative fentanyl routes. A new, more sensitive GC-C-IRMS method has been developed, which allows for compound specific isotope analysis of Chemical Attribution Signatures (CAS) in fentanyl.

While there is a clear need for standardized reference materials for analytical calibrations and for inter -laboratory comparisons, there are not currently any for the oxygen stable isotopic composition of uranium ox-ides. In this paper we summarize the results from four laboratories by seven different methods of oxygen stable isotope analyses using fluorination techniques of CRM 125-A UO2 Standard Reference Material. We synthesize these data and methods to arrive at a consensus oxygen stable isotope composition for CRM 125-A 818O = -9.63%o (+/- 0.29%o) VSMOW. We discuss methodological differences between analytical approaches, including furnace vs laser heating, fluorination using BrF5 or ClF3, as well as calibration strategies. We highlight the po-tential effects of calibration scale compression from single-point calibrations using reference material with 818O values having a large relative difference from the sample being analyzed. We demonstrate how calibration scale compression can yield differences in calibrated 818O values up to-2%o for samples with-20%o difference from a single reference material, if the calibration slope of different analytical systems differs by 0.1. We suggest the use of liquid water calibration standards sealed in silver capillary tubes for multi-point calibrations of fluorination analysis systems.

U3O8 and UO2. While aliquoting NSR-F-270409-2 for analysis, a metal staple of unknown origin was discovered in the sample. The presence of 236U and 232U in both samples indicates that the uranium feed stocks for these samples experienced a neutron flux at some point in their history. The reactor burn-up calculated from the isotopic composition of the uranium is consistent with that of spent fuel from natural uranium (NU) fueled Pu production. These nuclear forensic conclusions allow us to categorically exclude Australia as the origin of the material and greatly reduce the number of candidate sources.

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.

Hydrated secondary mineralization readily forms on the surface of UO2 particles exposed to humidity in an oxidizing environment. The oxygen stable isotope composition of the secondary uranium oxide may reflect that of the water vapor, as well as the hydrogen and oxygen stable isotopic composition of the mineral hydration water. The geospatial organization of δ2H and δ18O values of atmospheric humidity and precipitation is increasingly well understood, which suggests that the hydrogen and oxygen stable isotopes in secondary mineral hydration water may yield information on the environment in which the mineralization formed. UO2 powders were exposed to air with constant 30%, 61%, and 91% relative humidity, and constant H and O stable isotope composition. Aliquots were sampled from the UO2 materials at intervals of 1–10 days through the total humidity exposure duration of 180 days. Scanning electron microscopy, transmission electron microscopy, and x-ray diffraction analysis of the humidity-exposed UO2 indicates that schoepite/metaschoepite [(UO3)•2H2O] secondary phases had formed on the underlying UO2. The δ2H and δ18O values of mineral hydration waters were determined by thermogravimetry-enabled isotope ratio infrared spectroscopy (TGA-IRIS). Results indicate that hydrogen in the surface sorbed and mineral hydration waters is exchangeable and thus their δ2H values are difficult to interpret. However, oxygen in these waters is less exchangeable, and thus the oxygen stable isotope composition of the schoepite/metaschoepite hydration water is likely to be related to that of the exposure water vapor. After formation of schoepite/metaschoepite, the δ18O values of the hydration water in schoepite/metaschoepite does not change in response to changes in exposure vapor δ18O values, which suggests that the δ18O values of the hydration water is relatively durable. These findings suggest that information about the origin and storage history of a UO2 sample may be discernable from δ18O values of schoepite/metaschoepite hydration water.

The interaction between uranium and oxygen-containing substrates is nearly ubiquitous within the nuclear fuel cycle. Given the well-known and predictable oxygen stable isotope compositions of atmospheric oxygen and meteoric waters around the world, the use of these isotopes as a potential geolocation or processing signature of uranium compounds has been of interest within the nuclear safeguards community. This study focuses on measuring the oxygen-stable isotope composition of synthetically produced uranium oxides to determine the mechanism and extent of fractionation during materials processing relevant to the nuclear fuel cycle. Metastudtite [UO2O2(H2O)(2)] samples were first produced using water and hydrogen peroxide of a known oxygen isotope composition. This starting material was calcined in dry air at temperatures ranging from 300 degrees C to 1000 degrees C for 20 h producing oxides ranging in composition from UO3+x (0 <= x <= 0.5) to U3O8. In addition, calcinations between 500 and 800 degrees C were performed at different time intervals to evaluate the rate of isotopic equilibration. The delta O-18 values of water bound to metastudtite were measured by thermogravimetric analysis coupled to isotope ratio infrared spectroscopy (TGA-IRIS). A redesigned fluorination manifold was constructed at the University of Utah and was used to extract the bulk oxygen from precipitated and calcined products using bromine pentafluoride (BrF5). Mineralization water on metastudtite (measured by TGA-IRIS) was closely associated with the aqueous environment of precipitation with a fractionation of +3.5 +/- 0.6 parts per thousand. In contrast, bulk oxygen from metastudtite (measured by fluorination and gas chromatography combined with isotope ratio mass spectrometry) was found to have an inconsistent fractionation with precipitation water. Samples calcined in dry air exhibited a wide range of oxygen isotope compositions which increased from delta O-18 = +1.4 +/- 0.9 parts per thousand at 300 degrees C to +17.3 +/- 0.9 parts per thousand at 1000 degrees C. This variation in delta O-18 values was a result of oxygen exchange with atmospheric O-2 and was found to be rapid at calcination temperatures greater than 600 degrees C-with equilibration occurring in less than 2 h. These results provide insights into the incorporation and exchange of oxygen isotopes during precipitation and calcination processes comparable to those employed within the nuclear fuel cycle and establish a foundation for future investigations to build upon.

When exposed to humidity in an oxidizing atmosphere hydrated uranium oxide grows as a secondary mineral on aged U3O8 and may incorporate the oxygen stable isotope signature of the water vapor into the secondary uranium oxide, as well as hydrogen and oxygen stable isotopes into any mineral hydration water. Because geospatial variation in delta H-2 and delta O-18 values of atmospheric humidity and precipitation is well understood, the H and O stable isotope composition of mineral hydration waters can give information on the environment of mineral formation. We exposed powdered U3O8 to humidity with known H and O stable isotope composition at constant 30%, 61% and 91% relative humidity. We sampled and analyzed the U3O8 powders along with any secondary hydrated minerals that had formed on the particle surfaces at various intervals from 1 to 10 days throughout the 180-day humidity exposures. We present stable H and O isotope results of mineral hydration waters in uranium oxide materials analyzed by thermogravimetry-enabled isotope ratio infrared spectroscopy (TGA-IRIS), which uses precise heating by thermogravimetric analyzer to liberate water vapor for subsequent online isotope analysis via a laser-based isotope ratio infrared spectroscopy instrument (Picarro L-2130i). Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and x-ray diffraction (XRD) analysis of humidity-exposed U3O8 heated to 350 degrees C indicates that the U3O8 had a measurable metaschoepite ((UO3)center dot 2H(2)O) phase along with alpha-U3O8. After heating to 350 degrees C, the metaschoepite is eliminated but the alpha-U3O8 is retained, meaning that the heating successfully extracted the water in the metaschoepite but did not disturb the crystalline structure of the U3O8. Analysis of the bulk oxygen isotope composition by fluorination of these samples before and after heating, indicates that the oxygen isotope value of the U3O8 is also not affected by heating to 350 degrees C. Precision for delta H-2 and delta O-18 values of metaschoepite mineral hydration water stable isotope values yielded by the TGA-IRIS method on metaschoepite are 5.86%0 for delta H-2, and 0.34%0 for delta O-18. Hydrogen in the various water reservoirs is highly exchangeable and thus the delta H-2 values of these waters is not easily interpreted. However, oxygen in the separate water reservoirs is much less exchangeable, and thus delta O-18 values of the metaschoepite mineral hydration water is likely to reflect that of the water vapor the sample was exposed to. Once metaschoepite is formed on the surface of U3O8 particles, the oxygen isotope signature of the mineral hydration water in metaschoepite does not respond to changes in exposure vapor isotopic composition, and thus appears to be fairly durable in this regard. These results suggest that it may be possible to discern information about the provenance and history of an oxidized U(3)O(8 )sample from oxygen stable isotope measurements of metaschoepite mineral hydration water.

Nitrate is a significant water-quality issue in California, the United States as a whole, and the world. Critical to addressing nitrate contamination is understanding the presence and extent of denitrification, and further refining the techniques used to identify nitrate sources. The use and understanding of nitrate isotopic signatures to identify nitrate sources have advanced tremendously; however, knowledge gaps remain concerning specific fractionation pathways and the role of denitrification in altering source values. Using a large unique database of California groundwater nitrate isotopic compositions, we explored the utility of nitrate–oxygen isotope ratios in determining specific nitrate origins. Lawrence Livermore National Lab (LLNL) samples were supplemented by United States Geological Society (USGS) data to create a dataset of over 1200 dual-isotope results. Methods used at LLNL allowed for the determination of δ15N-NO3−, δ18O-NO3−, δ18O-H2O, δ2H-H2O, excess air, major dissolved gases, and excess N2. Results were examined for the degree to which δ18O-NO3− conforms to the model of nitrification in which two atoms of oxygen are sourced from ambient water and one from the atmosphere. Almost 80% of the results fall within one standard deviation of predicted values. However, 19% of samples had significantly higher values, suggesting the preservation of a synthetic nitrate source signature, mixing of sources, or widespread denitrification. Results were examined with respect to general land-use classifications and, while nitrate concentrations followed the expected pattern of being higher in agricultural settings, δ18O-NO3−patterns are complicated by application of N-fertilizer in various forms, and subsequent N cycling in the soil zone. We found that the current understanding of oxygen isotope-fractionation mechanisms cannot yet explain the prevalence of oxygen-isotope compositions with higher than predicted δ18O values, but when paired with related data such as land use and indicators of denitrification, oxygen-isotope compositions of nitrate can help to assess nitrogen cycle dynamics.