Uranium Isotopic Abundances Research Articles

The ubiquity of nuclear fission reactors throughout time and space

The evidence for a natural uranium fission reactor in Oklo, Gabon (Africa) begs the question as to whether this was the only one to have ever occurred on earth or elsewhere. Modern nuclear terminology classifies uranium as highly enriched uranium when the U235 content exceeds 20% which is shown comparable to projecting back ca 4.5e9 yr for terrestrial uranium isotopic abundances. At that time, the natural uranium content of the earth would have been classified as highly enriched uranium. With one verified natural criticality event, more events are postulated both on earth and throughout our galaxy. The latter effect should result in some background contribution to gamma ray burst events placing nuclear reactors as an expected ubiquitous natural phenomenon throughout time and space.

Production of Gas-Phase Uranium Fluoroanions Via Solubilization of Uranium Oxides in the [1-Ethyl-3-Methylimidazolium]+[F(HF)2.3]- Ionic Liquid.

A new methodology for gas-phase uranium ion formation is described in which UO2 is dissolved in neat N-ethyl,N'-methylimidazolium fluorohydrogenate ionic liquid [EMIm+][F(HF)2.3-], yielding a blue-green solution. The solution was diluted with acetonitrile and then analyzed by electrospray ionization mass spectrometry. UF6- (a U(V) species) was observed at m/z = 352, and other than cluster ions derived from the ionic liquid, nothing else was observed. When the sample was analyzed using infusion desorption chemical ionization, UF6- was the base peak, and it was accompanied by a less intense UF5- that most likely was formed by elimination of a fluorine radical from UF6-. Formation of UF6- required dissolution of UO2 followed by or concurrent with oxidation of uranium from the + 4 to the + 5 state and finally formation of the fluorouranate. Dissolution of UO3 produced a bright yellow solution indicative of a U(VI) species; however, electrospray ionization did not produce abundant U-containing ions. The abundant UF6- provides a vehicle for accurate measurement of uranium isotopic abundances free from interference from minor isotopes of other elements and a convenient ion synthesis route that is needed gas-phase structure and reactivity studies like infrared multiphoton dissociation and ion-molecule dissociation and condensation reactions. The reactive fluorohydrogenate ionic liquid may also enable conversion of uranium in oxidic matrices into uranium fluorides that slowly oxidize to uranyl fluoride under ambient conditions, liberating the metal for facile measurement of isotope ratios without extensive chemical separations. Graphical abstract ᅟ.

Uranium isotope-amount ratios in certified reference material 116-A—Uranium (enriched) metal assay and isotopic standard

Certified reference material (CRM) 116-A, uranium (enriched) metal assay and isotopic standard, was analyzed using TRITON and MAT261 thermal ionization mass spectrometer (TIMS) instruments to characterize the uranium isotope-amount ratios. The certified n(238U)/n(235U) “major” ratio in CRM 116-A was determined using a combination of two analytical techniques: total evaporation (TE) and modified total evaporation (MTE). The “minor” isotope-amount ratios n(234U)/n(235U) and n(236U)/n(235U) in CRM 116-A were characterized using a combination of MTE and conventional analysis techniques. For the n(234U)/n(235U) and n(236U)/n(235U) ratios, both the MTE and conventional analysis routines incorporate an internal mass bias correction using the measured n(238U)/n(235U) ratio as well as corrections for peak tailing from 235U to 238U. The abundance of 233U, present in CRM 116-A at trace levels, was characterized using a conventional analysis technique that incorporates a secondary electron multiplier (SEM) equipped with an energy filter. CRM 116-A isotope-amount ratios are traceable to the national measurement base and to the International System of Units (S.I.). The measurements leading to the certification of the uranium isotopic abundances in CRM 116-A are discussed.

Re-certification of the CRM 125-A UO2 fuel pellet standard for uranium isotopic composition

The U.S. Department of Energy New Brunswick Laboratory (NBL) has been tasked to develop certified reference materials (CRMs) tailored for the demands of modern analytical methods in the field of nuclear forensics. As part of this effort, several existing uranium CRMs are being characterized for additional attributes. One of these materials is the CRM 125-A uranium oxide (UO2) pellet assay and isotopic standard (4% enriched in 235U), which is being developed as a CRM for U–Th age determinations. Reliable U isotopic composition data with relatively small uncertainties are essential for use as a uranium age standard. Therefore, re-characterization of the uranium isotopic composition of CRM 125-A was deemed necessary due to poorly constrained minor U abundances (i.e., 234U and 236U) resulting from instrumental limitations of the mass spectrometer used at the time of original certification (1996–1997). The analytical work presented in this study was undertaken with the purpose of reducing the uncertainties of certificate values for uranium isotope-amount ratios.Six randomly selected CRM 125-A pellets were characterized for the uranium isotopic abundances by thermal ionization mass spectrometry (TIMS). The re-certified major ratio n(235U)/n(238U) of 0.042301(25) was determined using the total evaporation (TE) and modified total evaporation (MTE) methods. The re-certified minor isotope ratios n(234U)/n(238U) and n(236U)/n(238U) of 0.00039130(38) and 0.0000040754(47), respectively, were measured by MTE and a conventional Faraday cup analysis method using internal normalization. Additionally, the material was checked for the presence of 233U using a secondary electron multiplier (SEM) equipped with a retarding potential quadrupole (RPQ). No 233U was observed within the instrumental detection limit. The homogeneity of CRM 125-A was confirmed by the absence of any statistically significant unit-to-unit variation in the uranium isotope amount ratios. The new values measured in this study represent a considerable refinement of the older data, in particular for the minor ratios, with uncertainties that are significantly smaller than those cited in the original certificate. The results of the characterization analyses are presented along with an explanation of the uncertainty estimates, which are compliant with the Guide to the Expression of Uncertainty in Measurement (GUM).

The use of solution nebulization ICP-MS combined with particle screening and micro-sampling for analysis of individual uranium-bearing particles

Solution nebulization inductively coupled plasma mass spectrometry (ICP-MS) was applied to the analysis of uranium isotope abundance ratios in individual particles. Particle screening was carried out before ICP-MS analysis using two different techniques: irradiation with thermal neutrons and fission track observation (so-called “fission track technique”) on the one hand and automated particle measurement (APM) in secondary ion mass spectrometry (SIMS) on the other hand. Uranium-bearing particles localized by the screening techniques were sampled individually by micro-sampling devices inside a scanning electron microscope and deposited on clean silicon wafers for subsequent chemical dissolution and ICP-MS measurement. The micro-sampling enables us to avoid polyatomic ion interferences and measure some uranium-bearing particles at the same time in isotopic analysis. Using both methods, particles from certified reference materials and a real-life sample taken at a nuclear facility were measured to evaluate the performance of the methods. Using ICP-MS combined with the fission track technique, the uranium isotope abundance ratios were successfully determined for individual particles with diameters ranging from 1.2 to 2.4 μm in a certified reference material (NBL CRM U100). The analysis of the real-life sample using ICP-MS combined with the fission track technique gave 235U/238U isotope abundance ratios between 0.0246 and 0.0376. When APM was used as the particle screening method for the real-life sample, 235U/238U isotope abundance ratios ranging from 0.0259 to 0.0432 were obtained. These values were higher than those obtained by a well-established method for particle analysis (SIMS). From these results, it can be concluded that both techniques allow selection and analysis of the uranium particles with the highest 235U abundance, which is the most important for nuclear safeguards.

Uranium isotope abundance ratios in natural uranium metal certified reference material 112-A

Certified reference material (CRM) 112-A, uranium metal assay standard, with “natural” U isotopic composition and CRM 145, uranium assay standard solution made by dissolving CRM 112-A metal were analyzed using TRITON Thermal Ionization Mass Spectrometer (TIMS) to characterize the uranium isotopic abundances. The certified 235U/238U “major” ratio of 0.0072543 (40) in CRM C112-A and CRM 145 is determined using the total evaporation (TE) and modified total evaporation (MTE) methods. In the MTE method, the total evaporation process is interrupted on a regular basis to allow correction of background from peak tailing, internal calibration of the secondary electron multiplier (SEM) detector versus the Faraday cups, peak-centering, and ion source re-focusing. For the “minor”234U/238U and 236U/238U ratio measurements using MTE, better precision and accuracy are achieved compared to the TE analyses. The certified 234U/238U ratio of 0.000052841 (82) in CRM C112-A and CRM 145 is determined using a conventional analysis technique that incorporates an internal mass bias correction utilizing the measured 235U/238U ratio and correction for peak tailing from 235U and 238U. The 236U/238U isotope abundance ratio in CRM C112-A and CRM 145 is estimated to be <5×10−9. The CRM 112-A and CRM 145 materials show no evidence for any statistically significant unit-to-unit variations in uranium isotope abundance ratios. The homogeneity of both CRMs is established. The measurements leading to the certification of uranium isotopic abundances are discussed.

Fission track–secondary ion mass spectrometry as a tool for detecting the isotopic signature of individual uranium containing particles

A fission track technique was used as a sample preparation method for subsequent isotope abundance ratio analysis of individual uranium containing particles with secondary ion mass spectrometry (SIMS) to measure the particles with higher enriched uranium efficiently. A polycarbonate film containing particles was irradiated with thermal neutrons and etched with 6M NaOH solution. Each uranium containing particle was then identified by observing fission tracks created and a portion of the film having a uranium containing particle was cut out and put onto a glassy carbon planchet. The polycarbonate film, which gave the increases of background signals on the uranium mass region in SIMS analysis, was removed by plasma ashing with 200W for 20min. In the analysis of swipe samples having particles containing natural (NBL CRM 950a) or low enriched uranium (NBL CRM U100) with the fission track–SIMS method, uranium isotope abundance ratios were successfully determined. This method was then applied to the analysis of a real inspection swipe sample taken at a nuclear facility. As a consequence, the range of 235U/238U isotope abundance ratio between 0.0276 and 0.0438 was obtained, which was higher than that measured by SIMS without using a fission track technique (0.0225 and 0.0341). This indicates that the fission track–SIMS method is a powerful tool to identify the particle with higher enriched uranium in environmental samples efficiently.

Establishing reactor operations from uranium targets used for the production of plutonium

This paper presents results from the examination of a number of archived neutron-irradiated uranium targets used for past plutonium production testing. Three of these targets were destructively characterized using Los Alamos National Laboratory actinide analytical chemistry capabilities. A validated conduct-of-operations protocol was followed for this characterization effort. Chemical analyses included measurements for radionuclides, uranium assay, uranium isotopic abundances, trace actinides, trace metals, and non-metals. Material scientists also examined materials for morphological and microstructural properties and individual particles were examined for trace impurities. After characterization of the targets was completed, a reactor modeling effort was undertaken to corroborate target details in historical records. Time since irradiation calculations utilized both activation and fission products. The described examination of uranium targets has a tremendous impact from a safeguards verification and nuclear forensics perspective.

The applicability of MGA method for depleted and natural uranium isotopic analysis in the presence of actinides ( 232Th, 237Np, 233Pa and 241Am)

The multi-group analysis (MGA) method for the determination of uranium isotopic abundances in depleted uranium (DU) and natural uranium (NU) samples is applied in this study. A set of non-destructive γ-ray measurements of DU and NU samples were performed using a planar Ge detector. The relative abundances of 235U and 238U isotopes were compared with the declared values of the standards. The relative abundance for 235U obtained by MGA for a “clean” DU or NU sample with a content of uranium >1 wt% is determined with an accuracy of about ±5%. However, when several actinides such as 232Th, 237Np, 233Pa and 241Am are present along with uranium isotopes simulating “dirty” DU or NU, it has been observed that MGA method gives erroneous results. The 235U abundance results for the samples were 6–25 times higher than the declared values in the presence of above-mentioned actinides, since MGA is utilized the X-ray and γ-ray peaks in the 80–130 keV energy region, covering XK α and XK β regions. After the least-squares fitting of the spectra, it is found that the increases in the intensities of the X-ray and γ-ray peaks of uranium are remarkably larger in the complex 80–130 keV region. On the other hand, it is observed that the interferences of the actinide peaks are relatively less dominant in the higher γ-ray region of 130–300 keV. The results imply the need for dirty DU and NU samples that the MGA method should utilize the higher energy γ-rays (up to 1001 keV of 234mPa) combined with lower energies of the spectra, which may be collected in a two detector mode (a planar Ge and a high efficient coaxial Ge).

Examination of the performance exhibited by a single detector double focusing magnetic sector ICP-MS instrument for uranium isotope abundance ratio measurements over almost three orders of magnitude and down to pg g−1 concentration levels

Calibration experiments were performed on a commercially available double focusing magnetic sector single detector ICP-MS instrument to evaluate its potential for accurate isotope abundance ratio measurements down to the 1 pg g−1 isotope concentration level (without involving any preconcentration step). Its performance, for isotope abundance ratios varying over almost three orders of magnitude from ≈1∶1 to ≈1∶500, was checked using the IRMM-072 series of Isotopic Reference Materials. Routine experimental settings were applied and remained unchanged for the entire series of experiments. It is shown that all the certified ratios (certified to within 0.03% expanded combined uncertainty, k = 2) could be matched experimentally within the expanded (k = 2) combined uncertainty of the measurements (from ±0.11 to ±0.33% for 233U/235U, and from ±0.16 to ±0.31% for 233U/238U). For 1∶1 ratios (235U/238U) a 0.05% precision could be reproducibly obtained of which only 33% could be accounted for by Poisson statistics, whereas for 1∶500 ratios a 0.16% precision was obtained which could be completely explained by the same Poisson theory. In addition to the experimental precision other possible sources of uncertainty were investigated. The way isotope abundance ratios are measured (acceleration voltage is varied while the magnetic field is kept constant) is discussed as a potential extra source of mass discrimination. The crucial importance of a proper correction for dead-time effects is demonstrated. Finally, the hypothesis of a contamination with natural uranium that would have biased the results could be ruled out for all samples.

U-Pb geochronology: Systematic development of mixing equations and application of Monte Carlo numerical simulation to the error propagation in the Concordia diagram

Geochronological U-Pb information is obtained in the Concordia diagram through two successive stages: 1. (1) the computation of the ratios of radiogenic Pb to uranium isotopic abundances, their uncertainties and correlations; 2. (2) the computation of the ages and their uncertainties on the basis of a linear model, by the determination of the intercepts of the best-fit line with the Concordia curve. The systematic development of equations related to binary and ternary mixtures describing the analysed Pb enables us easy calculation of the atomic abundances of radiogenic Pb isotopes and of common 206Pb. As many parameters are involved, a classical computation of error propagation is tedious. Here, we show how a Monte Carlo-type iterative computation, in which all the parameters vary randomly within their confidence intervals, advantageously resolves the problem. This method implicitly and globally takes into account the totality of the uncertainties and correlations of all the parameters. It enables us the determination and visualization of confidence ellipses around experimental points and allows the discussion of final errors and correlations according to the precision of each initial measurement. The same approach can be used for the determination of the ages and their uncertainties, by iterative computation of the principal axes related to sets of points randomly selected from the confidence ellipses as well as the calculation of ages corresponding to the intercepts of those principal axes with the Concordia curve. The results of this computation are discussed and compared with previous methods of simple and weighted linear regression. The main advantages of the use of this approach are the following: 1. (1) ages and uncertainties are obtained by basic statistics on the distribution of the sets of calculated ages; 2. (2) weighting, always arbitrarily chosen, is here implicitly taken into account. This implicit weighting is more robust than previous methods employed and it corresponds to a stricter application of the postulate of experimental points alignment in the Concordia diagram.

High accuracy determination of235U in nondestructive assay standards by gamma spectrometry

High precision gamma spectrometry measurements have been made on five sets of uranium isotope abundance reference materials for nondestructive assay (NDA). These sets are intended for international safeguards use as primary reference materials for the determination of the235U abundance in homogeneous uranium bulk material by gamma spectrometry. The measurements were made to determine the count rate uniformity of the235U 185.7 keV gamma-ray as well as the235U isotope abundance for each sample. Since the samples were packaged such that the U3O8 is infinitely thick for the 185.7 keV gamma-ray, the measured count rate was not dependent on the material density. In addition, the activity observed by the detector was collimated to simulate calibration conditions used to measure bulk material in the field. The sample-to-sample variations observed within the 5 sets of samples ranged between 0.005–0.11% (1s) with standard deviations of the mean ranging from 0.01–0.02%. This observed variation appears to be due predominantly to counting statistics and not to material inhomogeneity and/or packaging. The results of this study indicate that accuracy of235U determinations via gamma spectrometry, in the range of few hundredths of a percent (2σ), is achievable. The main requirement for achieving this level of accuracy is a set of standards whose235U isotope abundances are known to within 0.01% (2σ).