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).     

Origin and release date assessment of environmental plutonium by isotopic composition

The origin and release date of environmental plutonium have been assessed by the measurement of plutonium and americium isotopic composition. The applicability and sensitivity of different plutonium isotope ratios, ²⁴⁰Pu/²³⁹Pu and ²⁴¹Pu/²³⁹Pu measured by inductively coupled plasma sector field mass spectrometry and ²³⁸Pu/²³⁹Pu analysed by alpha spectrometry, have been evaluated for origin determination in several types of environmental samples. With use of mixing models the contribution of different sources (e.g. global fallout or Chernobyl) can be calculated. By the measurement of the ²⁴¹Am/²⁴¹Pu isotope ratio, the release date (i.e. formation of ²⁴¹Pu by irradiation) can be estimated in environmental samples, which is an important parameter to distinguish recent plutonium release from previous (e.g. Chernobyl) emissions.     

In search of higher sensitivity: Pu isotopic analysis

Summary Thermal ionization mass spectrometry (TIMS) is an effective method for isotopic and ultra-sensitivity determination of plutonium. This project aims at improving the National Institute of Standards and Technology (NIST) TIMS system sensitivity for the analysis of plutonium from environmental samples. The TIMS detection limits for direct, electrodeposition, and resin bead source loading techniques were determined by systematically varying the amount of plutonium loaded on the rhenium filament. It has been shown in our preliminary work that the resin bead could produce a stable TIMS ion beam for as long as 6 h period with ?10⁸ Pu atoms loaded onto a single resin bead.     

The measurement of the isotopic composition of plutonium in an Irish Sea sediment by mass spectrometry

Samples from sediment cores, collected from a site close to the Sellafield outfall and dated using conventional - and -spectrometry, have been analysed using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Thermal Ionisation Mass Spectrometry (TIMS). This has provided information on the isotopic composition of plutonium in the liquid discharges from Sellafield; such information is not available using conventional -spectrometry alone. There was very good agreement between the²⁴⁰Pu/²³⁹Pu ratios obtained by the two MS methods. More recent discharges have been characterised by a progressive decrease in the relative proportion of²³⁹Pu.     

Calibration and use of a high-resolution,low-energy photon detector for measuring plutonium isotopic abundances

A low-energy photon detector was easily and accurately calibrated with plutonium sources of known isotopic contents after purification of the sources by anion exchange. Rapid data processing was attained by minicomputer calculations. Results obtained for plutonium abundances by λ-spectrometry and by mass spectrometry agreed within 1% for the ²⁴⁰Pu isotope, and within 10% for ²³⁸Pu and ²⁴¹Pu at the concentrations normally present. Alpha specific activities calculated from the abundances obtained by the two methods agreed within 0.5%.     

A combined method for the determination of the isotopic vector of plutonium isotopes in environmental samples

A combination of alpha-spectrometry, liquid scintillation counting (LSC) and accelerator mass spectrometry (AMS) was used for the determination of plutonium isotopes. ²³⁸Pu and ^239+240Pu were measured by alpha-spectrometry after separation of Pu by anion-exchange using ²³⁶Pu tracer as recovery monitor. After alpha-measurement, one part of the sample was dissolved for determining ²⁴¹Pu by LSC. Another part was used for the measurement of the ²⁴⁰Pu/²³⁹Pu atom ratio by AMS at VERA. Thus, it was possible to obtain complete information on the Pu isotopic composition of the samples. This method was applied to environmental reference samples and samples contaminated from nuclear reprocessing.     

Simultaneous analysis of uranium and plutonium using thermal ionization mass spectrometry

Simultaneous isotopic analysis of uranium and plutonium using thermal ionization mass spectrometer coupled to a multi-collector detection assembly with 9 Faraday cups has been reported earlier. Subsequently investigations have been carried out (1) to understand the applicability of correction methodologies available to account for the contribution of²³⁸Pu at²³⁸U and (2) to evaluate the effectiveness of these methodologies on the accuracy of²³⁵U/²³⁸U atom ratio being determined, particularly when samples containing different U/Pu atom ratios. Isotopic fractionation for both U and Pu in the simultaneous isotopic analysis has been compared with the results of the individual analysis of these elements. The different isotopic fractionation factors observed for U were attributed to different conditions of analysis. There was no significant difference in the isotopic fractionation patterns for Pu. The consideration to extend this method to actual samples from our observations on synthetic samples with diferent U/Pu atom ratios containing U and Pu isotopic reference standards is described.     

Rapid separation and purification of uranium and plutonium from dilute-matrix samples

This work describes a streamlined approach to the separation and purification of trace uranium and plutonium in environmental swipe samples that contain a small amount of collected bulk material. We describe key modifications to conventional techniques that result in a relatively rapid, safe, cost-effective, and efficient U and Pu separation process. Simulated samples were produced by loading appropriate ²³⁵U, ²³⁸U, and ²⁴⁰Pu onto high purity cotton swipes. Uranium concentration and isotopic composition were measured by multi-collector inductively coupled mass spectrometry. Corresponding plutonium measurements were conducted with a three stage thermal ionization mass spectrometer. Quantitative U and Pu recoveries were observed with this method.     

Low-level determination of plutonium by gamma and L X-ray spectroscopy

We have developed an analytical method for detection of²³⁹Pu in aqueous samples at concentrations as low as 10^–10M. This nuclear counting technique utilizes the uranium L X-rays, which follow the alpha-decay of plutonium. Because L X-rays are specific for the element and not for the individual isotope, the isotopic composition of the plutonium sample must be known. The counting efficiency in the 11–23 keV range is determined from a plutonium standard, and the concentration of the sample is then calculated from the L X-ray count and the isotopic composition. The total L X-ray count is corrected for possible contributions from other radionuclides present as impurities by measuring the low-energy gamma-spectrum for each contaminant to establish specific photon/X-ray ratios. The ratios are important when²⁴¹Pu and²⁴²Pu are measured, because the respective decay chain members produce non-U L X-rays. This new method can replace the use of labor-intensive radiochemical separation techniques and elaborate activation methods for analysis of²³⁹Pu in aqueous samples. It is also applicable for assaying plutonium in liquid wastes that pose possible hazards to the environment.     

Determination of isotopic ratios of plutonium and curium by internal conversion electron spectrometry

The isotopic ratios²⁴⁰Pu/²³⁹Pu in plutonium samples purified freshly and allowed to stand for a long time were determined by using a high resolution internal conversion electron spectrometer. As a result, it was proved that the above ratios can be determined accurately and precisely. The method was also examined further through a similar determination with curium samples.     

A gamma-spectrometric method for the determination of plutonium isotope ratios

A gamma-spectrometric method using an intrinsic high resolution germanium detector has been developed for the determination of isotope ratios of plutonium from samples in solution form. The method is based on the assay of low energy gamma-rays of²³⁸Pu,²³⁹Pu,²⁴⁰Pu and²⁴¹Pu and does not require the use of branching intensities or the knowledge of detection efficiencies for different gamma rays. Since low energy gamma-rays are used, the effect of²⁴¹Am has also been studied. It is found that results are not affected up to 0.5 wt% of²⁴¹Am in plutonium samples. An accuracy of 3% is achievable in the determination of²⁴⁰Pu/²³⁹Pu and²⁴¹Pu/²³⁹Pu atom ratios as demonstrated by carrying out measurements on isotopic standards of plutonium.     

Procedures to define Pu isotopic ratios characterizing a contaminated area in Palomares (Spain)

Plutonium isotopic ratios have been calculated in soils contaminated by the Palomares accident which occurred in 1966 (Almería, Southeastem Spain). Contrasted techniques have been used to determine the radionuclide activities:²³⁸Pu and^238+240Pu were analysed by -spectrometry prior purification on anion-exchange resins, the ratio²³⁹Pu/²⁴⁰Pu was estimated by -spectra deconvolution and²⁴¹Pu was directly measured by liquid scintillation counting and indirectly through quantification of in-grown²⁴¹Am from aged plutonium discs. The mean activity ratios²³⁸Pu/²³⁹Pu,²³⁹Pu/²⁴⁰Pu,²⁴¹Pu/²³⁹Pu, backdated to 1966, were 0.027±0.002 (1), 4.5±0.2 (1) and 8.2±0.8 (1), respectively, characterizing the accident of Palomares as the source term of the measured plutonium.     

Determination of isotopic composition and concentration of uranium,plutonium and neodymium by mass-spectrometric isotope dilution in the irradiated fuel of the Czechoslovak atomic power station A-1

Determination of the isotopic composition and concentration of uranium, plutonium and neodymium by mass-spectrometric isotope dilution is described. Isotopes²³³U,²⁴²Pu and¹⁵⁰Nd were used as spikes. Isotopic composition was measured with a Varian-TH 5 mass spectrometer. Optimum amounts loaded onto the filament were 2–5 μg U, ∼0.1 μg Pu and <0.1 μg Nd. The accuracy and reproducibility of the isotopic ratio and concentration measurements were evaluated.     

Simultaneous Plutonium and Uranium Isotopic Analysis from a Single Resin Bead - A Simplified Chemical Technique for Assaying Spent Reactor Fuels

Abstract

The method uses basic anion resin to adsorb plutonium and uranium from 7–8 M HNO₃ solutions containing dissolved spent reactor fuels. After equilibrating the resin with the solution, a single bead is used to determine the isotopic composition of plutonium and uranium on sample sizes as small as 10^?9 to 10^?10 g of each element per bead. Isotopic measurements are essentially free of isobaric interferences and fission product contamination in the mass spectrometer is eliminated. A very small aliquot of dissolver solution containing 10^?6 g of U and 10^?8 g of Pu is sufficient sample for chemically preparing several resin beads. A single prepared bead is loaded onto a rhenium filament and analyzed in a two-stage mass spectrometer using pulse counting for ion detection to obtain the high sensitivity required. Total quantity of the elements, in addition to isotopic abundances, can be determined by isotope dilution. Other areas where the method may be useful are: in plutonium production, isotope separations, and for trace detection of contamination on reactor parts.     

Determination of the plutonium concentration by isotope dilution mass spectrometry using239Pu as a spike

A method is described for the determination of plutonium concentration in the presence of a bulk of other impurities by isotope dilution mass spectrometry /IDMS/ using²³⁹Pu as a spike. The method involves the addition of²³⁹Pu spike / 90 atom%/ to samples with²³⁹Pu / 70 atom%/ and vice versa. After ensuring chemical exchange between the sample and the spike isotopes, plutonium is purified by conventional anion exchange procedure in 7M HNO₃ medium.²³⁹Pu/²⁴⁰Pu atom ratio in the purified spiked sample is determined with high precision /better than 0.1%/ using a thermal ionization mass spectrometer. Concentration of plutonium in the sample is calculated from the changes in²³⁹Pu/²⁴⁰Pu atom ratio in the spiked mixture. Results obtained on different plutonium samples using²³⁹Pu as a spike are compared with those obtained by the use of²⁴²Pu spike. Precision and accuracy comparable to those achieved by using²⁴²Pu are demonstrated. The method provides an alternative in the event of non-availability of enriched²⁴²Pu or²⁴⁴Pu required in IDMS of plutonium and at the same time, offers certain advantages over the use of²⁴²Pu or²⁴⁴Pu spike.     

Determination of the balance of the nuclear reactor fuel burn-out process by gamma-spectrometry of fission products III

The possibility for the determination of heavy water reactor fuel burn-up on the basis of gamma-spectrometric measurements of the activity quotients¹⁰⁶Ru/¹³⁷Cs and¹³⁴Cs/¹³⁷Cs has been experimentally investigated. The investigation has been carried out on the non-enriched uranium metal fuel of the Czechoslovak Nuclear Power Plant Al. A spectrometer with germanium detector has been used for spectrum analysis of the irradiated fuel gamm-radiation. Burn-up has been determined (1) by the applied here procedure, and (2) from the results of mass-spectrometric determination of the isotopic composition and content of U, Pu and Nd. Two groups of the values obtained have been compared and the influence of the errors of the measured activity quotients on the established deviations has been evaluated.     

Determination of femtogram quantities of 239Pu and 240Pu in bioassay samples by thermal ionization mass spectrometry

Summary A thermal ionization mass spectrometry (TIMS) method is described for the determination of ultra-trace levels of plutonium isotopes in human urine samples. The method has been validated through the analysis of artificial urine samples spiked with known amounts of ²³⁹Pu ranging from 2.5 fg to 50 fg (6-115mBq). A slight positive bias of 1.7%-2.7% was determined, with a relative precision of 2.2% at 50 fg, increasing to 2.7% for 5-25 fg ²³⁹Pu. The detection limit of the method was 0.53 fg (1.2mBq) ²³⁹Pu, and the instrumental detection limit was at least 0.1 fg. The determination of the isotopic signature of the sample with ²³⁹Pu, ²⁴⁰Pu, and ²⁴¹Pu amounts of several femtograms is possible, and was demonstrated with the determination of the 240 to 239 ratio in an inter-laboratory sample comparison. The method is relatively free from interferences, 95% of sample preparations were acceptable both in terms of chemical recovery and lack of isobaric interference. The isotopic abundance of the ²⁴²Pu SRM 4334E of the National Institute of Standards and Technology (NIST) was also determined by TIMS and was found to be 99.99967 atom% ²⁴²Pu.     

Xenon isotopic signature study of the primary coolant of CANDU nuclear power plant to enhance CTBT verification

To support interpretation of observed atmospheric ¹³⁵Xe, ¹³³Xe, ^133mXe and ^131mXe, a database of xenon radioisotope in the primary coolant of CANDU reactors has been established. This database is comprised of 40000 records of high-quality xenon radioisotope analyses. Records from the database were retrieved by a specifically designed data-mining module and subjected to further analysis. Results from the analysis were subsequently used to study isotopic ratios of observed xenon radioisotopes in the CANDU reactor primary coolant. These studies provided novel and practical information on the characterization of CANDU reactor xenon radioisotope releases, which can be used to discriminate between reactor effluence and underground nuclear test releases.     

Krypton isotopic signature study of the primary coolant of CANDU nuclear power plant

To support interpretation of observed atmospheric krypton radioisotopes, a database of krypton radioisotope in the primary coolant of CANDU reactors has been established. This database is comprised of 40,000 records of high-quality ⁸⁹Kr, ⁸⁷Kr, ⁸⁸Kr and ^85mKr analyses. Records from the database were retrieved by a specifically designed data-mining module and subjected to further analysis. Results from the analysis were subsequently used to study isotopic ratios of observed krypton radioisotopes in the CANDU reactor primary coolant. These studies provided practical information on the characterization of CANDU reactor krypton radioisotope, which can potentially be used to discriminate between reactor effluent and fuel reprocessing for nuclear safeguard ⁸⁵Kr monitoring applications (Kalinowski et al., J Environ Radioact 73:203, 2004). The study also has some potential application to Fissile material cut-off treaty.     

Determination of Pu isotopes in sediment cores in the Sea of Okhotsk and the NW Pacific by sector field ICP-MS

Summary Studies on the environmental behavior of plutonium in the marine environment require an analytical method with high sensitivity and capability to provide the isotopic composition of Pu in marine samples. In this work, as part of our on-going project on Pu environmental behavior in the Pacific Ocean, a sector field ICP-MS method combined with an off-line anion-exchange chromatography system was optimized for the determination of Pu and its atomic ratio of ²⁴⁰Pu/²³⁹Pu in sediment core samples. Using a conical concentric nebulizer and 150-second counting time, we were able to lower the detection limit of Pu down to 0.35 fg. The mass discrimination effect was evaluated using a mixed Pu isotope standard solution with certified a ²⁴⁰Pu/²³⁹Pu isotope ratio (NBS-947). The overall performance of the analytical method was validated by the determination of Pu and its isotope composition in an ocean sediment reference material (IAEA-368). Both the ^239+240Pu activity and ²⁴⁰Pu/²³⁹Pu atomic ratio were found to be in good agreement with the certified and/or literature values. As an important application, we employed the analytical method to investigate the vertical profiles of ^239+240Pu activity and ²⁴⁰Pu/²³⁹Pu atomic ratio in sediment cores in the Sea of Okhotsk and the NW Pacific. It was found that the Bikini close-in fallout Pu could be transported as far as the Sea of Okhotsk. The results provided evidence to support our hypothesis on the oceanic current transportation of Bikini close-in fallout Pu in the NW Pacific and its marginal seas.