Characterizing uranium oxide reference particles for isotopic abundances and uranium mass by single particle isotope dilution mass spectrometry

Uranium and plutonium particulate test materials are becoming increasingly important as the reliability of measurement results has to be demonstrated to regulatory bodies responsible for maintaining effective nuclear safeguards. In order to address this issue, the Institute for Reference Materials and Measurements (IRMM) in collaboration with the Institute for Transuranium Elements (ITU) has initiated a study to investigate the feasibility of preparing and characterizing a uranium particle reference material for nuclear safeguards, which is finally certified for isotopic abundances and for the uranium mass per particle. Such control particles are specifically required to evaluate responses of instruments based on mass spectrometric detection (e.g. SIMS, TIMS, LA-ICPMS) and to help ensuring the reliability and comparability of measurement results worldwide. In this paper, a methodology is described which allows quantifying the uranium mass in single micron particles by isotope dilution thermal ionization mass spectrometry (ID-TIMS). This methodology is characterized by substantial improvements recently achieved at IRMM in terms of sensitivity and measurement accuracy in the field of uranium particle analysis by TIMS. The use of monodisperse uranium oxide particles prepared using an aerosol generation technique developed at ITU, which is capable of producing particles of well-characterized size and isotopic composition was exploited. The evidence of a straightforward correlation between the particle volume and the mass of uranium was demonstrated in this study. Experimental results have shown that the uranium mass per particle can be measured via the ID-TIMS method to a relative expanded uncertainty of about 10% (coverage factor k = 2). The availability of reliable and validated methods for the characterization of uranium particles is considered to be essential for the establishment of SI-traceable measurement results. It is therefore expected that the method developed in this study is valuable for the certification of particulate materials in which the isotopic composition and the content of uranium must be accurately known.     

Uranium isotopic signatures measured in samples of dirt collected at two former uranium facilities

Nuclear forensics is a multidisciplinary science that uses a variety of analytical methods and tools to explore the physical, chemical, and isotopic characteristics of nuclear and radiological materials. These characteristics, when evaluated alone or in combination, become signatures that may reveal how and when the material was fabricated. The signatures contained in samples of dirt collected at two different uranium metal processing facilities in the United States were evaluated to determine uranium isotopic composition and compare results with processes that were conducted at these sites. One site refined uranium and fabricated uranium metal ingots for fuel and targets and the other site rolled hot forged uranium and other metals into dimensional rods. Unique signatures were found that are consistent with the activities and processes conducted at each facility and establish confidence in using these characteristics to reveal the provenance of other materials that exhibit similar signatures.     

Isotopic analysis of single uranium and plutonium particles by chemical treatment and mass spectrometry

The isotopic composition of single uranium and plutonium particles was measured with an inductively coupled plasma mass spectrometer (ICP-MS) and a thermal ionization mass spectrometer (TIMS). Particles deposited on a carbon planchet were first analyzed with an energy dispersive X-ray spectrometer (EDX) attached to a scanning electron microscope (SEM) and then transferred on to a silicon wafer using a manipulator. The particle on the silicon wafer was dissolved with nitric acid and the isotopic ratios of U and Pu were measured with ICP-MS and TIMS. The results obtained by both methods for particles of certified reference materials showed good agreement with the certified values within the expected uncertainty. The measurement uncertainties obtained in this study were similar for both mass spectrometric methods. This study was performed to establish the method of particle analysis with SEM, EDX, the particle manipulation and chemical preparation technique, and the measurement of isotopic ratios of U and Pu in a single particle by mass spectrometry.     

Use of data from environmental sampling for IAEA safeguards. Case study: uranium with near-natural 235U abundance

Environmental sampling (ES) is one of the measures applied in international nuclear safeguards. The detection capability of safeguards ES relies on a combination of highly sensitive analytical techniques and resourceful data evaluation. The evaluation process is dynamic, employing a variety of tools, information and analytical results. While the presence of uranium or plutonium may be a significant finding in itself, high quality isotopic measurements are essential to associate the material with a specific nuclear activity. This is illustrated in cases where the uranium detected appears to be “natural” or “near-natural”, but in fact can be identified with various nuclear processes.     

Characterization of uranium particles produced by hydrolysis of UF6 using SEM and SIMS.

Environmental sampling (ES) is a powerful technique used by safeguards inspectors of the International Atomic Energy Agency and the European Safeguards Office for the detection of undeclared nuclear activities. Since its implementation in the 1990s, ES has proven to be very sensitive and effective. Considering the consequences, the measurements should be carried out under a quality management programme. At the Institute for Reference Materials and Measurements, a new production method is under development for the preparation of reference uranium particles from well-certified UF6, allowing uranium particles with certified isotopic abundances to be prepared that are representative of those found in uranium enrichment facilities. Using an aerosol deposition chamber designed and built for the purpose, particles are formed by the hydrolysis of UF6 and their morphology and (isotopic) composition measured using SEM-EDX and SIMS. The SEM measurements show that by varying the relative humidity of the air in the reaction chamber, the morphology of the particles can be changed. By making a distribution map of the chemical composition of the particles, the relationship between fluorine and uranium as main constituents of the particle could be established. The presence of fluorine is a valuable indicator for the occurrence of nondeclared enrichment activities.     

An inter-calibration campaign using various selected Pu spike isotopic reference materials

In nuclear safeguards, precise and accurate isotopic analyses are needed for two major elements from the nuclear fuel cycle: uranium and plutonium. This can be achieved by Isotope Dilution Mass Spectrometry (IDMS), which is one of the most reliable analytical techniques for the determination of plutonium amount content to a high level of accuracy. In order to achieve reliable isotope measurements isotopic reference materials with certified amount of plutonium and isotopic composition are required. At the Institute for Reference Materials and Measurements (IRMM) various plutonium spike reference materials for isotopes ²³⁹Pu, ²⁴⁰Pu, ²⁴²Pu and ²⁴⁴Pu are available. This enabled the setup of an inter-calibration campaign inter-linking selected plutonium spikes on a metrological basis applying state-of-the-art measurement procedures. The aim of this campaign is threefold: firstly to perform measurements on selected plutonium spike isotopic reference materials for quality control purposes, secondly to verify the amount content and the isotopic composition of the recently produced IRMM-1027m large sized dried (LSD) spikes and thirdly to demonstrate IRMM’s measurement capabilities for plutonium analysis via external quality tools. The obtained results using various spike isotopic reference materials will be presented and discussed in this paper. The measurement uncertainties of the IDMS results were calculated according to the guide to the expression of uncertainty in measurement (GUM).     

Determination of uranium isotopic composition and 236U content of soil samples and hot particles using inductively coupled plasma mass spectrometry

As a result of the accident at the Chernobyl nuclear power plant (NPP) the environment was contaminated with spent nuclear fuel. The 236U isotope was used in this study to monitor the spent uranium from nuclear fallout in soil samples collected in the vicinity of the Chernobyl NPP. Nuclear track radiography was applied for the identification and extraction of hot radioactive particles from soil samples. A rapid and sensitive analytical procedure was developed for uranium isotopic ratio measurement in environmental samples based on double-focusing inductively coupled plasma mass spectrometry (DF-ICP-MS) with a MicroMist nebulizer and a direct injection high-efficiency nebulizer (DIHEN). The performance of the DF-ICP-MS with a quartz DIHEN and plasma shielded torch was studied. Overall detection efficiencies of 4 x 10(-4) and 10(-3) counts per atom were achieved for 238U in DF-ICP-QMS with the MicroMist nebulizer and DIHEN, respectively. The rate of formation of uranium hydride ions UH+/U+ was 1.2 x 10(-4) and 1.4 x 10(-4), respectively. The precision of short-term measurements of uranium isotopic ratios (n = 5) in 1 microg L(-1) NBS U-020 standard solution was 0.11% (238U/235U) and 1.4% (236U/238U) using a MicroMist nebulizer and 0.25% (235U/238U) and 1.9% (236U/P38U) using a DIHEN. The isotopic composition of all investigated Chernobyl soil samples differed from those of natural uranium; i.e. in these samples the 236U/238U ratio ranged from 10(-5) to 10(-3). Results obtained with ICP-MS, alpha- and gamma-spectrometry showed differences in the migration properties of spent uranium, plutonium, and americium. The isotopic ratio of uranium was also measured in hot particles extracted from soil samples.     

A gamma–gamma coincidence spectrometric method for rapid characterization of uranium isotopic fingerprints

This paper reports on initial efforts for uranium isotopic analysis using gamma-rays and X-ray fluorescence coincidence. In this study, a gamma–gamma coincidence spectrometry was developed. The spectrometry consists of two NaI(Tl) scintillators and XIA LLC Digital Gamma Finder (DGF)/Pixie-4 software and card package. The developed spectrometry was optimized according to the considerations of output count rate and gamma peak energy resolution. It has been demonstrated that the spectrometry provides an effective method of assessing the content of uranium isotopes for nuclear materials. The main advantages of this approach over the conventional gamma spectrometry include the fact that ²³⁵U enrichment can be graphically characterized by its unique coincidence “fingerprints”. The method could be further developed for fast uranium isotope verification with an established gamma–gamma coincidence spectral imaging library by various nuclear materials.     

The use of SIMS and SEM for the characterization of individual particles with a matrix originating from a nuclear weapon.

The application of scanning electron microscopy (SEM) and secondary ion mass spectrometry (SIMS) for characterization of mixed plutonium and uranium particles from nuclear weapons material is presented. The particles originated from the so-called Thule accident in Greenland in 1968. Morphological properties have been studied by SEM and two groups were identified: a "popcorn" structure and a spongy structure. The same technique, coupled with an energy-dispersive X-ray (EDX) spectrometer, showed a heterogeneous composition of Pu and U in the surface layers of the particles. The SIMS depth profiles revealed a varying isotopic composition indicating a heterogeneous mixture of Pu and U in the original nuclear weapons material itself. The depth distributions agree with synchrotron-radiation-based mu-XRF (X-ray fluorescence microprobe) measurements on the particle (Eriksson, M., Wegryzynek, D., Simon, R., & Chinea-Cano, E., in prep.) when a SIMS relative sensitivity factor for Pu to U of 6 is assumed. Different SIMS identified isotopic ratio groups are presented, and the influence of interferences in the Pu and U mass range are estimated. The study found that the materials are a mixture of highly enriched 235U (235U:238U ratio from 0.96 to 1.4) and so-called weapons grade Pu (240Pu:239Pu ratio from 0.028 to 0.059) and confirms earlier work reported in the literature.     

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.     

Uranium isotope dilution mass spectrometry using NBL certified reference materials as spikes

The isotope dilution mass spectrometry method of analysis is used to determine the elemental uranium contents in a wide variety of uranium bearing materials. The method is based on the mass spectrometric analysis of a mixture prepared by diluting the sample to be analyzed with a spike of distinctly different isotopic composition to that of the sample. In this work, a beginning is made to identify suitable candidates among the multitude of certified reference materials (CRMs) available at the New Brunswick Laboratory to supplant the use of ²³³U which remains now as the preferred spike nuclide. The results of the study presented here identify CRM 112-A (of normal isotopic composition) and CRM 115 (depleted uranium composition) as suitable candidates to replace ²³³U as spike material for determining uranium in high enriched uranium materials, and CRM 116 (²³⁵U mass fraction of >90 %) for determining uranium in materials of low enrichment.     

Polyaniline-functionalized porous adsorbent for Sr2+ adsorption

To support the nuclear safeguards particle analysis, a method was developed to produce micrometer-sized uranium oxide microspheres with a known uranium content and isotope ratio, which are intended to be characterized as certified reference material. To simplify handling of the produced particles, the collected particles have been transferred into ethanol suspensions. In addition to a demonstration of the suitability and new capabilities of such particle suspensions, such as the preparation of particle mixtures, the stability of the particles has been investigated and demonstrated with regard to dissolution and isotopic exchange.     

SIMS Analysis of Uranium and Actinides in Microparticles of Different Origin

In the framework of the projects Environmental Sampling and High Performance Trace Analysis in support to the International Atomic Energy Agency (IAEA) and the Euratom Safeguards Office (ESO), the research has been focused on the identification and characterisation of particles of interest containing mainly U and other actinides.Multiple analyses, elemental and isotopic, of single radioactive particles in the µm-size range are key-tools in environmental research and in nuclear forensics. The main purpose in the analysis of single particles by SIMS is the determination of their isotopic composition that has been demonstrated successfully for uranium and plutonium particles. From these results, the enrichment of the material produced, the process used for the enrichment as well as the starting material employed (natural or reprocessed uranium) can be identified.     

In situ liquid SIMS analysis of uranium oxide

Trace analysis of nuclear materials in solid particles collected in the environment or particles in liquid slurry generated in nuclear material manufacturing processes can pinpoint elemental, organic, and isotopic signatures of nuclear fuel cycle activities and processes. Such information can support nuclear safeguards programs by increasing our ability to detect undeclared nuclear materials, routine activities for safeguarding at declared facilities, and illicit activities. However, trace radioactive material analysis in liquids and slurries is challenging using bulk approaches. For example, one drawback of sensitive analysis such as inductively coupled plasma mass spectrometry (ICP-MS) is that sample is consumed or destroyed as a result of the technical approach. We developed a vacuum compatible microfluidic interface to enable surface analysis of liquids and solid–liquid interactions using time-of-flight secondary ion mass spectrometry (ToF-SIMS). In this work, we illustrate the initial results from the analysis of liquid uranium oxide standard solutions using in situ liquid SIMS. Because the liquid SIMS analysis is almost nondestructive, the same sample can then be analyzed by other analytical techniques or saved for future reference. Consequently, multimodal analysis is possible. Our results demonstrate that in situ liquid SIMS can be used as a new approach to analyze radioactive materials in liquid and slurry forms of relevance to diverse applications.     

Distribution coefficients for <Superscript>85</Superscript>Sr and <Superscript>137</Superscript>Cs in Japanese agricultural soils and their correlations with soil properties

The performance of several commercially available portable radiation spectrometers containing small NaI(Tl) scintillation detectors has been studied. These devices are used by field inspection personnel to detect and identify illicit radioactive materials. The detection and identification of enriched uranium is an important deterrent to undeclared nuclear proliferation and nuclear terrorism. This study was conducted using a variety of shielded and unshielded uranium sources in a simulated maritime environment. The results indicate adequate identification capability for various uranium enrichments using the manufacturer’s spectral analysis firmware. More sophisticated methods for analyzing the spectra can be applied to these short field measurements to determine the isotopic enrichment.     

Determination of uranium, thorium and plutonium isotopes by ICP-MS

Quantitative and isotopic measurement of actinide elements is required in many circumstances in the nuclear industry. For example, determination of very low levels of these alpha emitters in human urine samples is used to assess the internal committed dose for nuclear workers. Quantifying actinide isotopes in radioactive waste from nuclear processing and nuclear facility decommissioning provides important information for waste management. Accurate determination of the uranium isotopic ratios in reactor fuels provides fuel burnup information. Inductively coupled plasma mass spectrometry (ICP-MS) has been used for the determination of Th, U, and Pu in various samples including urine, nuclear waste, and nuclear fuel in our laboratory. In order to maximize the capability of the technique and ensure quality analyses, ICP-MS was used to analyze samples directly, or after pre-treatment to separate complicated matrices or to concentrate the analyte(s). High-efficiency sample introduction techniques were investigated. Spectral interferences to minor isotopes caused by peak tails and hydride ions of major actinide isotopes were studied in detail using solutions prepared with light and heavy waters. The quality of the isotopic ratio measurement was monitored using standard reference materials.     

Determination of uranium isotopic composition and 236U content of soil samples and hot particles using inductively coupled plasma mass spectrometry

As a result of the accident at the Chernobyl nuclear power plant (NPP) the environment was contaminated with spent nuclear fuel. The ²³⁶U isotope was used in this study to monitor the spent uranium from nuclear fallout in soil samples collected in the vicinity of the Chernobyl NPP. Nuclear track radiography was applied for the identification and extraction of hot radioactive particles from soil samples. A rapid and sensitive analytical procedure was developed for uranium isotopic ratio measurement in environmental samples based on double-focusing inductively coupled plasma mass spectrometry (DF–ICP–MS) with a MicroMist nebulizer and a direct injection high-efficiency nebulizer (DIHEN). The performance of the DF–ICP–MS with a quartz DIHEN and plasma shielded torch was studied. Overall detection efficiencies of 4×10^–4 and 10^–3 counts per atom were achieved for ²³⁸U in DF–ICP–QMS with the MicroMist nebulizer and DIHEN, respectively. The rate of formation of uranium hydride ions UH⁺/U⁺ was 1.2×10^–4 and 1.4×10^–4, respectively. The precision of short-term measurements of uranium isotopic ratios (n = 5) in 1 μg L^–1 NBS U-020 standard solution was 0.11% (²³⁸U/²³⁵U) and 1.4% (²³⁶U/²³⁸U) using a MicroMist nebulizer and 0.25% (²³⁵U/²³⁸U) and 1.9% (²³⁶U/²³⁸U) using a DIHEN. The isotopic composition of all investigated Chernobyl soil samples differed from those of natural uranium; i.e. in these samples the ²³⁶U/²³⁸U ratio ranged from 10^–5 to 10^–3. Results obtained with ICP–MS, α- and γ-spectrometry showed differences in the migration properties of spent uranium, plutonium, and americium. The isotopic ratio of uranium was also measured in hot particles extracted from soil samples.     

Unique challenges with recent gamma spectroscopy measurements at Los Alamos National Laboratory

A variety of unique radioactive samples have been measured recently at Los Alamos National Laboratory (LANL) using an electrically-cooled high-purity germanium detector. In each case the purpose of the measurements included one or more of the following objectives: (1) an accurate determination of the isotopic weight fractions of different plutonium or uranium materials; (2) an accurate determination of the isotopic quantity in the absence of relevant calibration standards; and (3) a qualitative determination of various sample impurities for additional forensic information. This paper discusses how simple modifications to the PC-FRAM parameter sets enabled a better determination of the isotopic content of the following samples: (1) high-purity plutonium metal, (2) plutonium-beryllium (PuBe) neutron sources, (3) neutron-irradiated natural uranium, and (4) re-processed HEU fuel with elevated ²³⁶U content. The isotopic quantity in a variety of samples was determined using a combination of the Spectral Nondestructive Assay Platform (SNAP™) routine from Eberline Services and the Monte Carlo Neutral Particle (MCNP) code developed at LANL. The non-traditional sources that were quantified with these gamma ray modeling codes included dozens of neutron-irradiated targets of natural uranium, several plutonium-beryllium neutron sources, and three high-purity samples of plutonium metal.     

Verification of a Fissile Material Cutoff Treaty: The case of enrichment facilities and the role of ultra-trace level isotope ratio analysis

One challenge to a potential verification regime for a Fissile Material Cutoff Treaty (FMCT) would be to assure that enrichment plants are not producing highly enriched uranium (HEU) for weapons purposes. Namely in some older enrichment plants, operated in nuclear weapon states, environmental sampling techniques might detect particles from historic HEU production. Determination of the age of these particles would be the most direct confirmation of treaty-compliance. While methods are available to determine the age of nuclear materials based on the concentrations of decay products, micron-sized uranium particles are particularly difficult to analyze. We will review the sensitivity requirements for age determination of HEU particles in an FMCT, and assess the potential of advanced measurement techniques available for this application.     

Isotopic abundance measurements on solid nuclear-type samples by glow discharge mass spectrometry

A double-focusing Glow Discharge Mass Spectrometer (GDMS) installed in a glovebox for nuclear sample screening has been employed for isotopic measurements. Isotopic compositions of zirconium, silicon, lithium, boron, uranium and plutonium which are elements of nuclear concern have been determined. Interferences arising from the matrix sample and the discharge gas (Ar) for each of these elements are discussed. The GDMS results are compared with those from Thermal Ionization Mass Spectrometry (TIMS). For boron and lithium at microg/g-ng/g levels, the two methods gave results in good agreement. In samples containing uranium the isotopic composition obtained by GDMS was in agreement with those from TIMS independently of the enrichment. Attempts for the determination of plutonium isotopic composition were also made. In this case, due to the interferences of uranium at mass 238 and americium at mass 241, the GDMS raw data are complementary with those values obtained from physical non-destructive techniques.