Production and Characterization of Plutonium Dioxide Particles as a

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Production and Characterization of Plutonium Dioxide Particles as a Quality Control Material for Safeguards Purposes Taeko Shinonaga,*,†,∥ David Donohue,*,† Helmut Aigner,† Stefan Bürger,† Dilani Klose,† Teemu Kar̈ kela,̈ ‡ Riitta Zilliacus,‡ Ari Auvinen,‡ Olivier Marie,§ and Fabien Pointurier§ †

International Atomic Energy Agency, Department of Safeguards, Wagramer Strasse 5, A-1400 Vienna, Austria VTT Technical Research Center of Finland, P.O. Box 1000, FI-02044 Finland § CEA, DAM-DIF, F-91297 Arpajon, France ∥ Helmholtz Zentrum München, German Research Center for Environmental Health, Institute of Radiation Protection, Ingolstädter Landstrasse 1, D-85764 Neuherberg, Germany ‡

ABSTRACT: Plutonium (Pu) dioxide particles were produced from certified reference material (CRM) 136 solution (CRM 136plutonium isotopic standard, New Brunswick Laboratory, Argonne, IL, U.S.A., 1987) using an atomizer system on December 3, 2009 after chemical separation of americium (Am) on October 27, 2009. The highest density of the size distribution of the particles obtained from 312 particles on a selected impactor stage was in the range of 0.7−0.8 μm. The flattening degree of 312 particles was also estimated. The isotopic composition of Pu and uranium (U) and the amount of Am were estimated by thermal ionization mass spectrometry (TIMS), inductively coupled plasma mass spectrometry (ICPMS), and α-spectrometry. Within uncertainties the isotopic composition of the produced particles is in agreement with the expected values, which were derived from the decay correction of the Pu isotopes in the CRM 136. The elemental ratio of Am to Pu in the produced particles was determined on the 317th and 674th day after Am separation, and the residual amount of Am in the solution was estimated. The analytical results of single particles by micro-Raman−scanning electron microscopy (SEM)−energy-dispersive X-ray spectrometry (EDX) indicate that the produced particles are Pu dioxide. Our initial attempts to measure the density of two single particles gave results with a spread value accompanied by a large uncertainty.

P

standard and characterized by thermal ionization mass spectrometry (TIMS) and secondary ion mass spectrometry (SIMS).8 The Pu isotopic composition of this material is, however, not representative of Pu which is relevant to nuclear safeguards and forensics. In the present study, Pu particles were produced from CRM 1369 solution (CRM 136plutonium isotopic standard, New Brunswick Laboratory, Argonne, IL, U.S.A., October 1, 1987). The atom amount ratios and expanded uncertainties (k = 2) as of October 1, 1987 were n(238Pu)/n(239Pu), 0.002624 ± 0.000082; n(240Pu)/n(239Pu), 0.14500 ± 0.00018; n(241Pu)/ n(239Pu), 0.022215 ± 0.000059; n(242Pu)/n(239Pu), 0.006801 ± 0.000035. The particle production was carried out on December 3, 2009, within a short time following Am separation on October 27, 2009. This isotopic composition represents Pu typical of a low burn-up of uranium (U) fuel in a power reactor. The objective of this study was to produce and characterize Pu particles to be used for quality control measurements of the

lutonium (Pu) particle analysis is one of the important tasks in the field of nuclear safeguards and forensics.1−3 The most important parameters to characterize in Pu particles are the Pu isotopic composition and age of the material as revealed by radioactive decay. The isotopic ratios of Pu provide information on the origin of the material,1 and the age of the material indicates the date when the Pu was purified and separated from its daughter nuclide 241Am.2,3 In addition, the chemical composition and the particle size distribution could provide information on the production processes. To support these investigations, standard well-characterized Pu particles are required as a reference material and for quality control purposes, although there are only a few Pu particle standards available. The certified reference material (CRM) 137, formerly distributed as NBS 947 (CRM 137plutonium isotopic standard, New Brunswick Laboratory, Argonne, IL, U.S.A., October 1, 1987)4 is one certified Pu standard available as a powdered sulfate form. However, the most common chemical form of Pu particles found in nuclear facilities in the environment and relevant to nuclear safeguards and forensic investigations is Pu oxide, mainly PuO2.5,6 One set of particulate isotopic standards containing Pu in an aluminosilicate matrix was produced and characterized without Am separation.7 Recently, Pu particles were produced using a 242Pu © 2012 American Chemical Society

Received: September 27, 2011 Accepted: January 27, 2012 Published: February 28, 2012 2638

dx.doi.org/10.1021/ac202502z | Anal. Chem. 2012, 84, 2638−2646

Analytical Chemistry

Article

collected mass was in particles with aerodynamic diameter ranging from 0.4 to 1.4 μm. Aerodynamic mass median diameter (AMMD) of the particles was 0.6 μm. With the BLPI it was possible to classify the produced PuOx particles by their aerodynamic diameter on 11 separate collection stages. Working Area for Characterization of Produced Particles. The chemical treatment and measurements were performed in the clean area, classes 5 and 8 (ISO 14644)11 of the laboratory of the Environmental Sample Laboratory (ESL) of the IAEA Department of Safeguards in Seibersdorf, Austria. SEM−EDX−WDX. For the morphological and elemental analysis of the produced Pu particles, a scanning electron microscope (SEM) (JEOL JSM-6490LV) equipped with an energy-dispersive X-ray spectrometer (EDX), OXFORD INCA Penta FETX3, EDS7574 (Oxford Instruments), detector area 30 mm2, window, ATW2, resolution at 5.9 keV, 113 eV, and a wavelength-dispersive X-ray spectrometer (WDX), INCA Wave (fitted with WED PCB 51-1128-471) was used. The accelerating voltage of 20 keV and the slit size of 2.5 mm were used for the WDX analysis. A micromanipulator (Nanotek, Germany) was installed in the SEM sample chamber for relocation of Pu particles from the sample planchet to Si wafers. Chemical Reagents and Water. An original solution of the standard material CRM 136 was used for the Pu particle production. For the chemical separation, suprapure grade of reagents was used. For the isotopic dilution (ID) mass spectrometry, the certified standard solutions of IRMM-08512 (242Pu) and SRM 4332E13 (243Am) were used as spikes. TIMS Measurements. The atom amount ratios of Pu and U were measured by ThermoFisher Scientific Triton thermal ionization mass spectrometer following the established analytical procedure for isotope ratio measurements.14 A secondary electron multiplier with energy filter (RPQ) in peak jumping mode was used in combination with the total evaporation principle. The final purified fraction of Pu was drop-loaded (total Pu amount of 50−200 pg per filament) on a degassed single rhenium filament and dried by ohmic heating. For Pu analysis, a thin coating of graphite suspended in water was placed on top of the dried Pu sample and dried by ohmic heating. The filament temperature used for the Pu measurements was in the range of 1400−1500 °C. The Pu and U TIMS loading blanks are determined using 242Pu (IRMM-044)15 and 233 U (IRMM-040/1)16 spikes, respectively. All reported TIMS isotope ratios are mass fractionation corrected using linear law and traceable to NBL CRM 12817 or NBL CRM 137. Certified reference materials used were matched in concentration and matrix with those of samples as closely as possible. The TIMS Pu loading blank was insignificant compared to the total Pu amount loaded per filament. Additional CRMs such as NBL CRM 136 were analyzed for quality control purposes. The associated measurement uncertainty for all reported TIMS data was evaluated according to GUM principles.18,19 The following sources of uncertainty are included: uncertainty of mass fractionation and the certified value of the mass fractionation standard, measurement repeatability, atomic and molecular interferences, TIMS loading blank, detector nonlinearity, and peak tailing. ICPMS Measurements. The isotopic composition of Pu and U and the level of U and Am impurities were also measured with a Thermo Electron Element 2 ICPMS (ThermoFisher Scientific) equipped with magnetic sector field and a high-efficiency inlet system, Apex HF (Elemental

isotopic composition of Pu and the estimated time since the date of the last chemical purification. Although U particles are of major interest in swipe samples from enrichment facilities, Pu particles are of high interest for safeguarding reactors with hot cells, reprocessing, and research and development facilities.

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EXPERIMENTAL SECTION Am Separation from Base Pu Standard Solution, CRM 136. Americium in the original CRM 136 solution was chemically separated using the ISO/TC 85/SC 5N 15366 method10 on October 27, 2009. The final fraction of purified Pu which was used for the particle production was converted to solid Pu nitrate for shipment. The chemical separation of Am from the Pu standard solution (CRM 136, ca. 35 mg of Pu) was performed in a glovebox in the Nuclear Material Laboratory (NML) of the IAEA Department of Safeguards in Seibersdorf, Austria. Pu Particle Production. Pu oxide aerosol particles were produced at the Technical Research Centre of Finland (VTT). Pu supplied by the IAEA was CRM 136 after Am separation and was shipped in the form of solid nitrate. Before the Pu particle production, the Pu nitrate powder (containing ca. 35 mg of Pu) was dissolved in 8 mL of deionized water. This solution was introduced in the atomizer (model TSI 3076) which was located at the inlet of the PuOx aerosol particle production facility as shown in Figure 1. The atomizer

Figure 1. Scheme of the facility for production of size-classified PuOx particles from Pu nitrate water solution. Sections from the condenser to impactor shown in red color were heated to 120 °C.

generated Pu nitrate containing water droplets in air flow (2 L/min, NTP: 0 °C and 101 325 Pa). At the atomizer outlet, the largest droplets were removed by impaction. Downstream the atomizer excess humidity was removed from the air flow by a water-cooled condenser. The line between the condenser and the reaction furnace was heated to 120 °C. During the preheating step, water was evaporated from the droplets, and the particles of solid Pu nitrate entered the reaction furnace, which was heated up to 500 °C. Pu nitrate was thermally decomposed within the furnace, and particles reacted with oxygen forming Pu oxide aerosol particles. Downstream of the furnace the line was heated to 120 °C. The flow was diluted with air (25 L/min, NTP) using a porous tube diluter in order to decrease the partial pressure of remaining water and to increase the total flow rate to be suitable for particle collection. The line beyond the diluter was divided into two parts. The first one was used to control the flow rate and to keep the pressure inside the facility slightly below atmospheric pressure. The second one was used to collect PuOx particles with a Berner-type low-pressure impactor (BLPI). Most of the 2639



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by SEM−EDX, each particle was picked up and relocated onto Si wafers for further chemical analysis. Each Si wafer with a Pu particle was transferred into a Teflon vial, and ca. 1 pg of spike IRMM-085, 100 μL of ca. 35% HNO3, and 10 μL 45% of HF were added. The further chemical treatment was performed as mentioned above for isotopic measurements of the produced particles for mass spectrometry. Half-Life of Nuclides. The half-lives of the isotopes used in this study were referred to the online version of the “Nuclear Wallet Cards” (http://www.nndc.bnl.gov/nudat2/indx_sigma. jsp) database, version 9-Nov-2011. The database is maintained by the National Nuclear Data Center (Brookhaven National Laboratory, Upton, NY, U.S.A.) on behalf of the International Network for Nuclear Structure. Data in this database are mostly taken from the Evaluated Nuclear Structure Data Files (ENSDF) or from more recent literature, such as Wellum et al.,25 if data in ENSDF have been superseded. The following half-lives of Pu isotopes were used (in year ± uncertainty): 238 Pu, 87.7 ± 0.1; 239Pu, 24 110 ± 30; 240Pu, 6561 ± 7; 241Pu, 14.325 ± 0.006; 242Pu, 375 000 ± 2000.

Scientific, U.S.A.) for enhanced sensitivity. All measurements were carried out using low-resolution mode and a PFA microflow nebulizer in self-aspirating mode. α-Spectrometry. An α-spectrometry system (Canberra Alpha Analyst, model 7200-12) with α-passivated implanted planar silicon detectors (A450-18AM) having an active area of 450 mm2 and GENIE 2000 α-spectroscopy software was used for the analysis of 238Pu and Am. The efficiency of the detectors under the condition used in this study was estimated to be typically 25% for all detectors. Chemical Treatment for Isotopic Measurement of Produced Pu Particles. A subset of the particles (ca. 1 ng of Pu) on the surface of the Al foil from the impactor was cut and put in a Teflon vial together with 1 mL of isopropyl alcohol in a closed plastic glovebox. The vial was treated in an ultrasonic bath for 10 min. The Al foil was removed from the vial again in a closed plastic glovebox, heated to dryness, and then 1 mL of ca. 70% of HNO3 and 100 μL of 45% HF were added. The combined solution in the vial was then heated without cap to dryness. The residue was fumed with ca. 70% of HNO3, and the aliquots were prepared for TIMS, ICPMS, and α-measurements. Chemical Separation of Pu, U, and Am. Chemical separation of U and Am from Pu in the CRM 136 particle solution was performed to check the isotopic abundances of U and the amount of Am using a small aliquot (picogram to nanogram level) of the solution. Pu, U, and Am were separated on a three-column system consisting of AG MP-M1 ion exchange (Bio-Rad Laboratories, Inc.) at the upper position, UTEVA (Eichrom Technology LLC) at the second position, and TRU (Eichrom Technology LLC) at the lowest position conditioned with 8 M (mol/L) HNO3. Each nuclide fraction was eluted with HI in 9 M HCl (Pu) from the AG MP-M1 resin and 0.1 M HCl from the UTEVA resin (U). The separated U and Pu fractions were purified further by the ionexchange method using AG MP-M1 resin (U and Pu) in 9 M HCl media. U and Pu were eluted with 0.1 M HCl and 45% HBr, respectively. The Am fraction was eluted with 0.1 M HCl from the TRU resin. The chemical procedure for Pu is described in detail elsewhere.3,20−22 The sample sources for the α-spectrometry were prepared as NdF3(PuF4) using the microcoprecipitation method.23 Micro-Raman Spectrometer−SEM−EDX. The chemical form of the Pu particles was analyzed by micro-Raman spectrometry (MRS) coupled with SEM−EDX. The microRaman analysis was carried out for 12 particles. A coupling device between the MRS (“InVia”, Renishaw PLC, WottonUnder-Edge, Gloucestershire, U.K.) and the SEM called SEM− SCA (SEM−Structural and Chemical Analyzer, Renishaw PLC) equipped with an EDX analyzer and GSR software (FEI XL-30 Esem, Eindhoven, The Netherlands) was used. This system allows the Raman analysis to be performed within the SEM chamber thereby avoiding relocation of the particles from the SEM to the MRS. Relocation from SEM to MRS is not precise enough to allow efficient MRS analysis for actinide particles where the size is below 5 μm.24 Density of Single PuO2 Particles. The density of two single PuO2 particles was estimated by combining the particle size measured by SEM with the Pu amount determined by ID ICPMS. The volume of the particle was estimated assuming a spherical shape with the average value of the radius obtained from four different diameters of one particle. After checking the presence of Pu as a major constituent in the selected particles

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RESULTS AND DISCUSSION Morphological and Elemental Analysis and Size and Flattening Distribution of Produced Pu Particles. Among the 11 impactor stages, stage 8, which is the fourth position from the particle slot, was used for all characterization studies. For the study on Pu chemical composition particles from stage 7 were used. The material on all stages of the impactor should be the same isotopic and chemical composition because it comes after the aerosol generator and calcination furnace. Elemental analysis of the produced particles was performed using SEM−EDX and WDX. The peaks at 3.34 keV (Mα), 3.37 keV (Mβ), and 3.53 keV (Mγ) obtained by the EDX and the same peaks by WDX confirm that Pu is the major component of these particles. The SEM image and SEM−EDX and WDX spectra are shown in Figure 2. The scale is given on the picture.

Figure 2. (a) Pu particle, (b) EDX spectrum of the particle, and (c) WDX spectrum of the particle. 2640



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Figure 3. (a) Size distribution of single plus aggregated particle (312 particles), (b) single particles only (187 particles), and (c and d) flattening distribution. The particle size was estimated with the average value of the long length (L) and the short length (S). The flattening degree FD was calculated with the equation FD = 1 − S/L.

products. The major aim of these validation measurements was to confirm that no contamination had occurred and that, therefore, the CRM 136 certified values can be adopted as best estimate of the isotopic composition of the Pu contained in the produced particles after appropriate decay correction. The uncertainties of the measured values were generally estimated according to the uncertainty propagation model of the GUM,18 based on an assessment of all known sources of uncertainty as stated by the measurement specialists. It should be noted, however, that counting statistics were found as the dominating uncertainty component for measurements by ICPMS and αspectrometry. For comparing the measurement results with the expected values, which were directly derived from the CRM 136 certificate9 by decay correction, also the repeatability standard deviation observed on replicate measurements was calculated and used for the test statistics, if its value was smaller than the standard uncertainty based on the propagation of uncertainty components according to the GUM.18 Although the pure repeatability certainly underestimates the true measurement uncertainty, this dual approach appeared justified for two reasons: (1) The aim was to verify that the measured values do not deviate significantly from the decaycorrected certified values. Should the verification succeed with a smaller denominator for the test statistics, it would safely

The size distribution of both single and aggregated particles gathered on an impactor stage is shown in Figure 3. The flattening degree (FD) which was estimated with the equation of FD = 1 − S/L, where L is the long length and S is the short length of the particles, is also shown in Figure 3. The highest density of the distribution of the particle size obtained with 312 particles was in the range of 0.7−0.8 μm in diameter. The flattening degree was found to be smaller than 0.1 for 194 particles and between 0.1 and 0.2 for 100 particles among 312 particles in total. A combination method of backscattered electron imaging to identify particles containing “heavy” elements followed by EDX measurement of the particle’s elemental composition was used. Obviously, measurement of small particles in the vicinity of large features such as other particles or irregularities in the substrate could lead to “shadowing” effects for some particles, in that the path of Xrays from the particle of interest to the EDX detector could be blocked. This is not a problem in the case of particles sparsely distributed on one of our standard substrates, although it sometimes occurs for real samples where the particle loading is much higher. Validation and Verification Measurements. Validation measurements were carried out at selected stages throughout the entire production process on intermediate and final 2641



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Table 1. Results of Pu Isotopic Measurements Measurements on Base CRM 136 Solution Nov 2009 variable n(238Pu)/n(239Pu)

n(240Pu)/n(239Pu)

n(241Pu)/n(239Pu)

n(242Pu)/n(239Pu)

item

TIMS

ICPMS

expected value ± uncertainty no. of measurements relative bias % U(relative bias %) expected value ± uncertainty no. of measurements 3 relative bias % −0.03 U(relative bias %) 0.47 expected value ± uncertainty no. of measurements 3 relative bias % −0.13 U(relative bias %) 0.84 expected value ± uncertainty no. of measurements 3 relative bias % 0.02 U(relative bias %) 1.87 Measurements on Particles item

n(238Pu)/n(239Pu)

expected value ± uncertainty no. of measurements relative bias % U(relative bias %) expected value ± uncertainty no. of measurements relative bias % U(relative bias %) expected value ± uncertainty no. of measurements relative bias % U(relative bias %) expected value ± uncertainty no. of measurements relative bias % U(relative bias %)

n(240Pu)/n(239Pu)

n(241Pu)/n(239Pu)

n(242Pu)/n(239Pu)

TIMS

TIMS

α

ICPMS

0.002204 ± 0.000082 6 1.02 3.72 0.14475 ± 0.00018 3 3 −0.24 −0.01 1.13 0.27 0.007620 ± 0.000059 3 3 0.92 −0.39 1.16 0.96 0.006805 ± 0.000035 3 3 −0.02 −0.32 1.87 1.47

3 −0.06 0.44 3 0.40 1.96 3 0.04 1.32 Single Particle

June 2010 variable

after second Am separation (Nov 2009)

after Am separation (Sept 2010)

ICPMS

TIMS

ICPMS

α

Mar 2011 ICPMS

0.002190 ± 0.000082 3 −0.15 3.79 6 4 0.01 −0.39 0.14 0.75 0.007409 ± 0.000059 6 4 0.13 4.13 0.82 2.75 6 −0.30 0.60

4 0.26 2.44

succeed with a larger one, too. (2) Because reference materials from the same origin (NBL and formerly NBS) are also used

0.14474 ± 0.00018 5 7 0.19 0.23 0.33 0.23 0.007326 ± 0.000059 5 7 0.75 1.46 1.20 1.59 0.006805 ± 0.000035 5 7 0.33 0.91 1.00 1.14

1 0.34 15.30 0.007146 1 49.89 23.32 1 1.44 19.74

where u(observed average)

for calibration and quality control purposes in the laboratories

= min[u(based on repeatability), u(based on “GUM”)]

performing the measurements, the biases of measurements and CRM 136 are perhaps not independent from each other.

(2)

Relative biases (in percent) and associated expanded uncertainties, U, are presented in the tables for better readability. These were calculated according to eqs 3 and 4.

However, the covariances among various CRMs in general and between the subject measurements and CRM 136 in particular are unknown. Not taking these covariances into account may

certified values.

relative bias% (observed average) − (expected value) = 100 (expected value)

In summary, a conventional t test statistic was calculated from standard uncertainties, u, according to eqs 1 and 2:

U (relative bias%) = k100

potentially lead to an overestimation in the propagation of the joint uncertainty of measurement results and CRM 136

t̂ =

×

|bias| u(observed average)2 + u(expected value)2

(1)

(3)

u(observed average)2 + u(expected value)2 (expected value) (4)

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Figure 4. Results of isotopic measurement by TIMS, ICPMS, and α-spectrometry. Blue asterisks and black bullets show independent results and average values, respectively. Bars indicate the range of uncertainty.

where the coverage factor k was either set to 2, or an appropriate two-tailed value of Student’s t for a level of confidence of 95% was used, if the degrees of freedom of U(bias) were known and smaller than six. Hence, any U(relative bias %) > |relative bias %| in the tables indicates that the observed average measurement result is not significantly different from the respective expected value. Isotopic Composition of Pu in Base CRM 136 Solution after Am Separation on October 27, 2009 Estimated by TIMS, ICPMS, and α-Spectrometry. The Pu isotopic composition of the base CRM 136 solution was determined by TIMS, ICPMS, and α-spectrometry within 5 days of the Am separation and also after a second purification (separation of residual Am, if any). Each result is shown in Table 1 as relative bias (in percent) with respect to the expected value (decaycorrected certified value), together with the information on the number of analyses, the expected value used, and the uncertainty of the relative bias. See eqs 3 and 4 for details. None of these biases is statistically significant. Hence, all atom amount ratios can be considered as consistent with the respective expected values. Isotopic Composition of Pu in the Produced CRM 136 Particles in December, 2009 Measured by TIMS, ICPMS, and α-Spectrometry. After the production of Pu particles, the Pu isotopic composition of the CRM 136 particles was measured by TIMS and ICPMS before and after Am separation during June 2010 and September 2010, respectively, and n(238Pu)/n(239Pu) was measured by α-spectrometry only after Am separation in September 2010. The results are shown in Table 1. Similar to the results of the CRM 136 solution shown in Table 1, the observed biases are generally not significant, except for the ratio n(241Pu)/n(239Pu) measured by ICPMS on particles without Am separation. The high value for n(241Pu)/ n(239Pu) by ICPMS on the particles without preceding

chemical purification (+4.1% relative bias) can be explained by the presence of 241Am formed from the decay of 241Pu (about 3% of the 241Pu decayed into 241Am between the purification of the CRM 136 solution in October 2009 and the measurements of the particles in June 2010). The count rate at mass 241 gives the total amount of 241Pu and 241Am by ICPMS. On the other hand, the TIMS measurements are less biased by the presence of 241Am, because 241Am is preferentially evaporated at a lower temperature during filament heating, compared with 241Pu (Am, 1200−1300 °C; Pu, 1400−1500 °C). On the basis of these considerations and particularly based on the fact that no TIMS result indicated any significant difference between measured and expected values, it was confirmed that, within uncertainties, the Pu isotope ratios in the particles are the same as those shown in the certificate of NBL CRM 136, taking decay corrections into account. Hence, it was concluded that no contamination changing the Pu isotopic composition had occurred during the particle production and chemical treatment steps In addition, the isotopic composition of a single Pu particle was measured in March 2011 by ICPMS without chemical separation. Even though the results are not very conclusive due to their high uncertainty, they are also shown in Table 1 for completeness. A plot of all results is shown in Figure 4. Isotopic Composition of U in the Produced CRM 136 Particle after U Purification by TIMS and ICPMS. The U isotopic composition in the produced Pu particles was also estimated by TIMS and ICPMS. If no contamination of unknown origin had occurred, then the U found in the particles stems from two sources, namely, U from the decay of Pu and U from blank. The following assumptions have been made: (1) references indicate that the material which was later distributed as NBL CRM 136 (previously as NBS SRM 946) was purified by anion exchange and recrystallization in April 1970,26 and it is 2643



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Table 2. Isotopic Measurement of U in CRM 136 Particles after U Purification Measured by TIMS and ICPMSa isotopic composition of U in natural U U from decay particles after U purification measured by TIMS particles after U purification measured by ICPMS

measured expectedb relative bias % measured expectedb relative bias %

n(234U)/n(238U)

n(235U)/n(238U)

5.4 × 10−5 1623 (9.27 ± 0.58) × 10−4 9.96 × 10−4 −6.9 (9.40 ± 0.60) × 10−4 9.90 × 10−4 −5.1

7.15 × 10−3 2287 (1.562 ± 0.012) × 10−2 0.848 × 10−2 84.3 (1.564 ± 0.055) × 10−2 0.847 × 10−2 84.7

n(236U)/n(238U)

mass [pg] of U in particles by ICPMS

1218 (7.07 ± 0.40) × 10−4 measured = pred. (7.03 ± 0.24) × 10−4

74 ± 8

measured = pred.

U content in the process blank was 63 ± 8 pg. bExpected for a hypothetical mixture of natural U with decay U, using 236U as pivot isotope (hence, for the isotope 236U, the measured value is confined to equal the predicted value). a

October 27, 2009. The measurement of the ratio Am/241Pu can be used to determine the “age” of the Pu, i.e., the time elapsed since the most recent chemical separation of Am from Pu. Figure 5 shows the expected ratio 241Am/Putotal

assumed that the material was free from U after this processing, (2) U was not separated from the CRM 136 solution during the purification for removing the Am (October 2009), and (3) the blank U is essentially natural U, which is not expected to contain 236U. Under these model assumptions, the amount of blank U can be calculated from the U isotopic composition measured in particles, by using the U from the Pu decay as “spike” and applying a kind of isotope dilution calculation. 236U from the “spike” and 238U from the blank are used as the pivot isotopes for this exercise. This method provides at least an indication whether the measured results are consistent with the model assumptions. Measurement and calculation results are shown in Table 2. The “IDMS calculation” indicates that the amount of blank U is about 0.76 relative to Pu (molar ratio). This value would overestimate the actual blank, if parts of the U in CRM 136 had been removed during the chemical purification of the master solution. It should also be noted that this method cannot distinguish between U actually contained in the particles and blank U added during the treatment and preparation of particles for measurement. The measured ratios n(234U)/n(238U) are within the same order of magnitude as the calculated ones, i.e., about 0.001, whereas the measured ratios n(235U)/n(238U) of about 0.016 are about twice as large as the predicted values. This indicates consistency with the model assumptions for the two isotopes 234U and 236U, while the higher values for 235U indicate that parts of the blank U are actually low-enriched U, which is also prevalent in the laboratory facilities, or that some enriched U was already present in the original CRM 136 material. It should be noted that the underlying mathematical model is extremely sensitive to variations of the assumed isotopic composition of the blank U. For example, the relative bias of the n(235U)/n(238U) measured values with respect to the calculated predicted values would drop from 84% to zero when assuming a n(235U)/ n(238U) ratio of 0.0143 in the blank U instead of using the value 0.00715 for natural U. The practical applicability of this approach remains subject to further studies. We realize that there is some risk with the model assumptions due to lack of precise information about the early history of NBL CRM 136, which was previously distributed as NBS SRM 946. The calculations were added to the paper to test a hypothesis. The results indicate that a U background (of 1−2% enriched U) may have been present, thus contradicting the initial hypothesis. The conclusions relating to the U, however, might be different among laboratories if the experiments are performed under different conditions. Amount of Am in the Produced Pu particles on the 317th and the 674th Day after Am Separation on

241

Figure 5. Ingrowth curve of Am and measurement results from September 2010 and September 2011.

as a function of time/date, assuming the (decay-corrected) isotopic composition of Pu in CRM 136 and a concentration of zero for Am in October 2009, the date of the chemical purification of the CRM 136 base solution. An “age” determination would rely on the inverse of this decay function. The figure tabulates results of 241Am measurements by ICPMS (September 2010) and α-spectrometry (September 2011) following spiking with 243Am. Two sets of such measurements were performed, i.e., 317 and 674 days after Am separation, respectively. The results of Am quantitative analysis are shown in Figure 5 together with the expected ratio of 241Am/Putotal. The measurement results are reproducible but come out somewhat higher than the expected values for the respective dates. Biases are seen in the range of +(27 ± 2)% and +(25 ± 3)% on the 317th and the 674th day, respectively, compared to the expected values for both experiments. Thus residual Am was present after chemical separation in 2009, and this biases the zero value of the 241Am/241Pu chronometer. Using actual measurement results for this bias preserves the usefulness of this material for age determinations or calibration of SEM−XRS measurements. Oxidation State of Single Pu Particles. A Raman band attributed to polycrystalline PuO2 was identified at 478 cm−1 by 2644



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Begun et al.27 In their study, polycrystalline PuO2 was prepared by calcination of trivalent Pu oxalate in air at 1000 °C for 12 h. The laser excitation wavelength was 514.5 nm, and the laser power was 100 mW. No other characteristic band was observed for PuO2. In the present study, the produced particles (from impactor stage 7) were prepared on an adhesive carbon planchet for the SEM observation. Twelve Pu particles were found by the SEM equipped with EDX spectrometry. The micro-Raman analysis was performed for these 12 particles using specifically the SEM−SCA instrument. All particles had nearly spherical geometry with diameter between 1.4 and 2.4 μm. The band positions were observed in the range of 474.9− 476.4 cm−1 with a bandwidth in the range of 23.3−28.9 cm−1. This is consistent with the band estimated by Begun et al.27 mentioned above. The integrated peak area was in the range of 9000−17 300 counts. The spectra were obtained with a 514 nm laser, an attenuation of 0.5% of the laser power (i.e., a power of 0.25 mW), and an irradiation time of 120 s. The particles were identified as PuO2. One of the micro-Raman spectra measured with a ca. 2 μm diameter particle is given in Figure 6. The spectrum includes no background subtraction for fluorescence or for any other phenomenon.

experimental density estimation. Our initial attempts to measure the density of the particles gave only two results with a spread between 4 and 9 g/cm2 accompanied by more than 20% of uncertainty. It was not possible to estimate the uncertainty exactly because it was not possible to calculate the uncertainty of the particle diameter estimation in this study. Because of the difficulty of successfully performing this experiment, measurements to resolve this discrepancy were not made, but this will be the subject of a further study and report. More sophisticated methods of size estimation under SEM and check of void in the particles are also required to obtain reliable information on the density. It might also be worth to note that, according to Golovnin, a value of the Pu dioxide density close to the theoretical value can be obtained only by growing a regular PuO2 crystal.32

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CONCLUSIONS The Pu dioxide particles were produced and characterized for a quality control material. The characterization includes Pu and U isotopic ratios, Am amount of the produced Pu particles, elemental ratio of 241Am to Pu, and chemical form and density of produced single Pu particles. The results for these Pu particles obtained in this study demonstrate that they are a valuable material for quality control of Pu particle analysis in the field of nuclear safeguards and forensics, especially for the determination of Pu isotopic composition and age dating of single Pu dioxide particles. The production of “standards” or “reference materials” should be left to reference material providers such as IRMM and NBL. These organizations should collect the needs of the user community and then produce materials that would allow them to meet these needs. Lessons learned from the present work could be used to alert the reference material providers to the challenges of such a production and certification process. The difficulties encountered in this study point to the importance of certain parameters such as particle size, sphericity, voids, density variations, etc. that need to be overcome before a successful reference material can be provided.

Figure 6. Raman spectrum of a Pu particle (ca. 2 μm diameter).

Theoretical Approach for Density Estimation of PuO2 Particles. The density of the produced spherical PuO2 particles can be estimated when the physical diameter (dp) and the aerodynamic diameter (da) of the particles on a specific stage of a BLPI impactor28 are known. The physical diameter of particles can be measured with SEM. The aerodynamic diameter is based on the characteristic cutoff diameter of each collection stage. Particles greater than the cutoff diameter are collected on the stage while smaller particles pass through. The aerodynamic diameter of the collected particles is defined as the geometric mean of the cutoff diameters belonging to the collection stage and the stage preceding it. The relationship of dp and da is defined in eq 5. ⎛ ρp ⎞1/2 da = ⎜⎜ ⎟⎟ dp ⎝ ρ0 ⎠

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.S.); d. [email protected] (D.D.). Phone: +49 89 3187 2239 (T.S.); +43 1 2600 28550 (D.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the assistance of Konomi T. Esaka of the University of Tokyo in the morphological and the chemical analysis of the studied materials. We are grateful to Norbert Doubek of the IAEA for the Am separation from the original CRM 136 solution. Cheol Su Kim of the IAEA is gratefully acknowledged for the discussion on the ICPMS measurements. We acknowledge the contribution of the late Andrzej Ciurapinski for discussions on particle analysis by SEM.

(5)

where ρ0 is the standard density of a particle (1.0 g/cm3). The real density of a particle, ρp, can be solved from the equation.29 If the diameter of a particle is significantly less than 1 μm, it is necessary to take into consideration that the relative velocity of gas at the surface of a particle is not zero.29 The estimated density of the produced PuO2 particles on stage 8 was close to the reference value of 11.5 g/cm3.30,31 To achieve an accurate estimation of the density with this method, it is very important to know the exact prevailing conditions (e.g., temperature, pressure) during the sampling of particles. Experimental Trial of Density Estimation with Single PuO2 Particles. Two single PuO2 particles were used for

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