A new method for quantifying 64Cu in nuclear debris samples.

Quantifying 64Cu in post-detonation nuclear debris samples can provide important diagnostic information regarding the structural materials used within a nuclear device. However, this task is challenging due to the weak gamma emissions associated with the decay of 64Cu, its short half-life (12.701 h), and the presence of interfering fission product radioisotopes. Large quantities of debris sample are generally needed to accurately quantify 64Cu, which can be problematic in sample-limited scenarios where other radiometric analyses are required. Herein, we present a new method for the separation of 64Cu from solutions of mixed fission products and demonstrate the quantification of its activity through use of gas-flow proportional beta counting. The new method was validated through a series of rigorous tests and was shown to improve the detection limit of 64Cu by over two orders of magnitude, from 2.5 × 106 to 1.3 × 104 atoms/sample for 100 min measurements.

Oxygen Isotope Fractionation in U3O8 during Thermal Processing in Humid Atmospheres.

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.

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 (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.

Impact of Controlled Storage Conditions on the Hydrolysis and Surface Morphology of Amorphous-UO3.

The hydration and morphological effects of amorphous (A)-UO3 following storage under varying temperature and relative humidity have been investigated. This study provides valuable insight into U-oxide speciation following aging, the U-oxide quantitative morphological data set, and, overall, the characterization of nuclear material provenance. A-UO3 was synthesized via the washed uranyl peroxide synthetic route and aged based on a 3-factor circumscribed central composite design of experiment. Target aging times include 2.57, 7.00, 14.0, 21.0, and 25.4 days, temperatures of 5.51, 15.0, 30.0, 45.0, and 54.5 °C, and relative humidities of 14.2, 30.0, 55.0, 80.0, and 95.8% were examined. Following aging, crystallographic changes were quantified via powder X-ray diffraction and an internal standard Rietveld refinement method was used to confirm the hydration of A-UO3 to crystalline schoepite phases. The particle morphology from scanning electron microscopy images was quantified using both the Morphological Analysis of MAterials software and machine learning. Results from the machine learning were processed via agglomerative hierarchical clustering analysis to distinguish trends in morphological attributes from the aging study. Significantly hydrated samples were found to have a much larger, plate-like morphology in comparison to the unaged controls. Predictive modeling via a response surface methodology determined that while aging time, temperature, and relative humidity all have a quantifiable effect on A-UO3 crystallographic and morphological changes, relative humidity has the most significant impact.

Nuclear proliferomics: A new field of study to identify signatures of nuclear materials as demonstrated on alpha-UO3.

The use of a limited set of signatures in nuclear forensics and nuclear safeguards may reduce the discriminating power for identifying unknown nuclear materials, or for verifying processing at existing facilities. Nuclear proliferomics is a proposed new field of study that advocates for the acquisition of large databases of nuclear material properties from a variety of analytical techniques. As demonstrated on a common uranium trioxide polymorph, α-UO3, in this paper, nuclear proliferomics increases the ability to improve confidence in identifying the processing history of nuclear materials. Specifically, α-UO3 was investigated from the calcination of unwashed uranyl peroxide at 350, 400, 450, 500, and 550 °C in air. Scanning electron microscopy (SEM) images were acquired of the surface morphology, and distinct qualitative differences are presented between unwashed and washed uranyl peroxide, as well as the calcination products from the unwashed uranyl peroxide at the investigated temperatures. Differential scanning calorimetry (DSC), UV-Vis spectrophotometry, powder X-ray diffraction (p-XRD), and thermogravimetric analysis-mass spectrometry (TGA-MS) were used to understand the source of these morphological differences as a function of calcination temperature. Additionally, the SEM images were manually segmented using Morphological Analysis for MAterials (MAMA) software to identify quantifiable differences in morphology for three different surface features present on the unwashed uranyl peroxide calcination products. No single quantifiable signature was sufficient to discern all calcination temperatures with a high degree of confidence; therefore, advanced statistical analysis was performed to allow the combination of a number of quantitative signatures, with their associated uncertainties, to allow for complete discernment by calcination history. Furthermore, machine learning was applied to the acquired SEM images to demonstrate automated discernment with at least 89% accuracy.