ARTICLE IN PRESS

Applied Radiation and Isotopes 67 (2009) 516–522

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Applied Radiation and Isotopes

0969-80

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/apradiso

Detection of previous neutron irradiation and reprocessing of uraniummaterials for nuclear forensic purposes

Zsolt Varga a,�, Gergely Suranyi b

a Radiation Safety Department, Institute of Isotopes, Hungarian Academy of Sciences, Konkoly-Thege ut 29-33, H-1121 Budapest, Hungaryb Research Group of Geology, Geophysics and Space Research Sciences, Hungarian Academy of Sciences, Pazmany Peter setany 1/C., H-1117 Budapest, Hungary

a r t i c l e i n f o

Article history:

Received 14 March 2008

Received in revised form

23 October 2008

Accepted 8 December 2008

Keywords:

Uranium

Inductively coupled plasma mass

spectrometry

Laser ablation

Illicit trafficking

Nuclear forensics

Reprocessing

43/$ - see front matter & 2008 Elsevier Ltd. A

016/j.apradiso.2008.12.006

esponding author. Tel.: +36 1392 2222/1220;

ail address: varga@iki.kfki.hu (Z. Varga).

a b s t r a c t

The paper describes novel analytical methods developed for the detection of previous neutron

irradiation and reprocessing of illicit nuclear materials, which is an important characteristic of nuclear

materials of unknown origin in nuclear forensics. Alpha spectrometry and inductively coupled

plasma sector-field mass spectrometry (ICP-SFMS) using solution nebulization and direct, quasi-non-

destructive laser ablation as sample introduction were applied for the measurement of trace-level 232U,236U and plutonium isotopes deriving from previous neutron irradiation of uranium-containing nuclear

materials. The measured radionuclides and isotope ratios give important information on the raw

material used for fuel production and enable confirm the supposed provenance of illicit nuclear

material.

& 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Identification and characterization of confiscated or foundnuclear materials (e.g. uranium-oxide or plutonium-oxide ma-trices) form integral part of combating nuclear smuggling andillicit trafficking (Mayer et al., 2002, 2005; Wallenius et al., 2006).Several characteristics of the material can be used to reveal itsprovenance, such as shape, geometric dimensions, U or Pucontent, isotopic composition or trace impurities (Grant et al.,1998; Mayer et al., 2002; Pajo et al., 2001; Varga and Suranyi,2007; Wallenius et al., 2002). By the measurement of theseparameters, the origin of the nuclear material can be possiblyidentified, thus it provides fundamental data for the forensicinvestigation. Among the various characteristics of the nuclearmaterial, determination of trace-level impurities plays an im-portant role in nuclear forensics, as these parameters giveinformation on the production location, the nuclear fuel fabrica-tion methodology and also on the raw materials employed.

Several types of minor components originating from thedifferent production steps can be present in the investigatedmaterials. Besides stable elements (e.g. transition metals or rare-earth elements) and progenies of uranium or plutonium basematerial (e.g. 231Pa, 230Th), activation (e.g. 236U, 239Pu or 240Pu)

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and fission products (e.g. 137Cs) can also be present in a nuclearmaterial, if it was previously irradiated or mixed with irradiateduranium in the course of fuel production (e.g. reprocessing). Mainactivation products originate from the neutron capture of eitherthe radionuclides of the base material (e.g. 236U is formed by (n, g)reaction of 235U) or from the neutron capture of the progenies ofmajor radioisotopes (e.g. 232U is formed by decay of 236Pu which isa product of (n, 2n) reaction of 237Np). The presence of theseradionuclides can be exploited for the identification of the startingraw materials (i.e. use of enriched or reprocessed fuel) and toreveal the production method. Moreover, as the isotopic composi-tion of trace-level activation products (e.g. 236U/235U, 240Pu/239Pu)depends on the irradiation conditions, accurate assignation to theoriginal material and irradiation conditions can be performed.

The measurement techniques generally used for the determi-nation of trace-level activation products present in nuclearmaterials can be divided into two groups: non-destructivetechniques (e.g. gamma spectrometry) and destructive methods(e.g. alpha spectrometry or mass spectrometric methods). Outof the non-destructive methods, gamma spectrometry is the mostcommonly applied technique for the measurement of roughisotopic composition and the assessment of the radiologicalhazard of the material. It also allows the determination of someactivation and fission products. However, in most cases gammaspectrometry is used only for the first screening and categoriza-tion of the nuclear material due to the relatively high detectionlimits (Mayer et al., 2002; Nguyen and Zsigrai, 2006). Some

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Table 1Optimized solution-based and laser ablation ICP-MS operating parameters.

Liquid sample

introduction

Laser ablation

Optimized operating parameters

Forward power (W) 1250 1334

Cooling gas flow rate

(L min�1)

15.95 15.70

Auxiliary gas flow rate

(L min�1)

1.19 0.88

Nebulizer gas flow rate

(L min�1)

0.978 0.868

Sample introduction conditions

Solution uptake rate

(mL min�1)

100 NA

Spray chamber temperature

(1C)

105 NA

Membrane temperature

(1C)

160 NA

Sweep gas flow rate

(L min�1)

2.85 NA

Data acquisition

Z. Varga, G. Suranyi / Applied Radiation and Isotopes 67 (2009) 516–522 517

activation products, such as 241Am can also be measured innuclear materials by high resolution gamma spectrometry(HRGS), however, its detection limit is inferior to that of thedestructive methods due to the high background and the low-energy gamma emission of 241Am decay (59.5 keV). For instance,Wallenius and Mayer (2000) used HRGS for the direct measure-ment of 241Pu/241Am ratio for age determination of plutoniummaterials. For more precise results and lower detection limits, useof destructive techniques, such as alpha spectrometry or massspectrometric techniques is necessary. Alpha spectrometryand inductively coupled plasma sector-field mass spectrometry(ICP-SFMS) have been successfully applied for the measurementof trace-level 236U, plutonium and 241Am from uranium oxidematrix (nuclear fuel or penetrator sample) (Boulyga et al., 2001;Desideri et al., 2002), or for the measurement of uranium fractionseparated from plutonium materials for the determination ofproduction date (Wallenius et al., 2002). The measurementtechniques applied are relatively robust and can be easily carriedout. However, the chemical sample preparation methods, whichare necessary to obtain a pure analyte fraction for the measure-ment, are tedious and generate relatively high amount of (radio-active) waste, even if less chemical-consuming extractionchromatography is used instead of ion exchange chromatography.Moreover, these techniques involve the dissolution of the sample,which should be avoided or can be even impossible for confiscatednuclear materials, as these samples serve as evidences during theforensic investigation.

For the forensic analysis of radioactive materials, direct, quasi-non-destructive laser ablation coupled to inductively coupledplasma mass spectrometry could be an ideal tool, as it does notrequire the dissolution of the sample and only a tiny amountof the material (ng to mg amount) is consumed for the analysis byevaporating it from the surface of the material (Becker, 2005;Varga and Suranyi, 2007). It offers an easy possibility for isotoperatio measurement in various types of samples, when dissolutionof the sample is not possible or should be avoided, and in thosecases, when limited sample amount is available. However,expensive instrumentation, calibration problems, sample inho-mogeneity and inferior detection capabilities to those of liquidsample introduction ICP-SFMS after pre-concentration and che-mical separation encumber its wide use.

The aim of this study was to develop inductively coupledplasma sector-field mass spectrometric (ICP-SFMS) and alphaspectrometric methods for the measurement of trace-levelactivation products (232U, 236U, 239Pu, 240Pu and 241Pu), whichare characteristic for previous neutron irradiation and reproces-sing of illicit uranium-based nuclear materials. The informationobtained in addition to the other characteristics of the sample canbe used to identify the origin of confiscated nuclear materials.Besides the destructive techniques, quasi-non-destructive laserablation ICP-SFMS methods have been developed for the detectionof previous neutron irradiation. The comparison of the differentmethodologies together with advantages and limitations arediscussed.

Resolution 300 4000

Runs and passes 5�8 20�1

Mass window (%) 5 120

Samples per peak 100 20

Search window (%) 60 60

Integration window (%) 5 80

Integration type Average Average

Scan type E-Scan E-Scan

Sampling time (s) 0.003 (238U) 0.02 (238U)

0.05 (233U) 0.07 (235U)

0.10 (235U, 239Pu,242Pu)

0.30 (234U, 236U, 239Pu,240Pu)

0.15 (234U, 236U, 240Pu)

0.30 (241Pu)

2. Experimental

2.1. Instrumentation

Alpha sources were counted by a PIPS type alpha Si detectorwith a surface area of 450 mm2 attached to an alpha spectrometer(Canberra Inc., USA). Counting time was typically 3–30 h, thedetector background was less than 1–5 cps in the applied3.5–9 MeV energy range. The detector efficiency was 0.11–0.21depending on geometry.

The mass spectrometric analysis was carried out usinga double-focusing inductively coupled plasma sector-fieldmass spectrometer equipped with a single electron multiplier(ELEMENT2, Thermo Electron Corp., Bremen, Germany). Measure-ments using liquid sample introduction were carried out in low-resolution mode (R ¼ 300) with a low-flow microconcentricnebulizer operated in a self-aspirating mode (flow rate was100mL min�1) in combination with a desolvation unit (Aridus,CETAC Technologies Inc., Omaha, NE, USA) in order to decreasehydride interferences (e.g. 235U1H+ and 238U1H+). Optimizedoperating parameters are summarized in Table 1. Prior to analysisof samples the instrument was tuned using a 1 ng g�1 multi-element solution (Merck, Darmstadt, Germany). The optimizationwas carried out with respect to maximum uranium sensitivity andlow UO+/U+ ratio. Sensitivity was approximately 1.3�106 cps for1 ng g�1 238U, the UH+/U+ and UO+/U+ ratios were less than6.2�10�6 and 2�10�4, respectively.

Laser ablation measurements of the samples were carried outusing an UP-213 laser ablation system (New Wave, Freemont,USA). The ablated material was transported by argon as a carriergas into the plasma. Optimization was carried out using a NIST612 glass reference material (NIST, Gaithersburg, USA) withrespect to maximum 238U+ intensity, good precision and mini-mum UO+/U+ ratio. Laser ablation was performed with a Nd:YAGlaser at a wavelength of 213 nm. For the measurements line scanwas used (scan speed: 5mm s�1, repetition rate: 10 Hz, laser beamdiameter: 20mm, laser energy: 40% (0.15 mJ)). Other optimizedICP-SFMS and laser ablation parameters are summarized in Table1. For the laser ablation ICP-SFMS measurements medium(R ¼ 4000) mass resolutions were employed in order to enhance

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selectivity and abundance sensitivity. Mass calibration of theinstrument was verified by the analysis of the NIST 612 referencematerial. ICP-SFMS data acquisition parameters for liquid sampleintroduction and laser ablation are shown in Table 1.

Fig. 1. A typical laser ablation ICP-SFMS chromatographic measurement of the

UOX-LEU sample in medium-resolution mode (R ¼ 4000).

2.2. Reagents and materials

The 242Pu (NIST 4334F, USA) and 233U (New BrunswickLaboratory, USA) were used for the isotope dilution ICP-SFMSmeasurements. Natural uranium solution was used to correct forinstrumental mass discrimination. The TEVATM (100–150mmparticle size, active component: aliphatic quaternary amine)extraction chromatographic resin was supplied by EichromTechnologies Inc. (Darien, Illinois, USA). For the analysis, 1.8 mLof the resin was placed in plastic Bio-Rad holders (diameter:8 mm, length: 40 mm). All reagents used were of analytical grade.Acids used in the final steps prior to ICP-SFMS analysis wereSuprapurTM grade (Merck, Darmstadt, Germany). For dilutionultrapure water was used (Milli-Q System, Millipore, USA). Allsample preparation and measurement procedures were carriedout under clean room conditions (Class 100 000).

2.3. The investigated samples

Three uranium-oxide pellets or materials confiscated in Hungarywere analyzed in this study. The samples have different 235Uenrichment: HU-DEP is a depleted uranium pellet (235U/238Uisotope ratio is 0.0025870.00004), HU-NAT is a natural uraniumcontaining material, presumably a nuclear by-product (235U/238Uisotope ratio is 0.0071370.00014), while HU-LEU is a low-enricheduranium pellet (235U/238U isotope ratio is 0.025570013). In order tovalidate the methods, highly enriched uranium-oxide powder froma Round Robin interlaboratory exercise (RR-HEU) organized by theNuclear Smuggling International Technical Working Group (ITWG)was used. For the destructive analyses the known amounts ofuranium-oxide materials were dissolved in 6 M ultrapure nitric acidwhile heating slightly in a water bath. The uranium concentration inthe dissolved aliquots was between 30 and 120 mg mL�1.

2.4. Laser ablation ICP-SFMS analysis of the nuclear materials

For the laser ablation investigation, uranium-oxide standard(referred to as UOX-STD) was prepared from natural uranyl-acetate (Lachema Ltd., Czechoslovakia) by ashing. 1.8 g ofUO2(CH3COO)2 �2H2O was placed in a porcelain crucible. Thesample was dried at 110 1C for 2 h, and ashed thereafter bygradually increasing the temperature to 400 1C and holding at thattemperature for 3 h. The uranium-oxide, presumably as U3O8 wasthoroughly mixed and homogenized. For the laser ablation works,the uranium-oxide powders (the Round Robin HEU sampleand the laboratory-prepared natural uranium-oxide powder) werepressed hydraulically into a disk-shaped pellet with a diameterof 5 mm and an approximate thickness of 1 mm.

The spectra during the laser ablation measurements wererecorded in chromatographic mode. The intensity ratios werecalculated by the integration of the isotope signals over theselected ranges. For the integration the built-in software of theELEMENT2 instrument (Thermo Electron Corp., software version2.41) were used. The intensities of the analyte (234U, 235U, 236U,238U, 239Pu and 240Pu) signals were recorded as a function of time.The intensity ratio was calculated by the integration of the signalsof the isotopes of interest over the selected ranges. For example,

the intensity ratio of 235U/238U was calculated as follows:

IU-235

IU-238¼

R t3

t2ISMPU-235

t3 � t2�

R t1

0 IBKGU-235

t1R t3

t2ISMPU-238

t3 � t2�

R t1

0 IBKGU-238

t1

(1)

where ISMPU�235 and ISMP

U-238 are intensities of 235U and 238U during theablation period, and IBKG

U-235 and IBKGU-238 are the intensities of 235U

and 238U during the background measurement, respectively.Integration allows the elimination of possible drifts and spikes.The first approximately 100 seconds of data acquisition beforeswitching on the laser (t1) was used for the backgroundcorrection. The integration limits (t2 and t3) for the ablationperiod were selected within the stabilized signal range. A typicalchromatographic measurement of UOX-LEU sample in medium-resolution mode is shown in Fig. 1.

For the analysis three parallel measurements were usedat different sampling positions. The uranium isotope ratioswere calculated from the intensity ratios after correction forthe instrumental mass discrimination using linear correction(Heumann et al., 1998). The uncertainties were calculated as thestandard deviations of the three parallel measurements takinginto account the measurement uncertainties and uncertainty ofmass discrimination.

2.5. Solution-based ICP-SFMS analysis of the nuclear materials

2.5.1. Sample preparation for 236U measurement

Approximately 10 mg aliquot of the dissolved uranium-oxidesample was used for the analysis. The sample was diluted threetimes successively with 2 m/m% ultrapure nitric acid solution.The uranium concentration in the sample aliquot subjected to theICP-SFMS analysis was approximately 10 ng g�1. This high degreeof dilution is necessary to protect the instrument from contam-ination and from accidental detector overload. The uranium levelin the method blank was negligible compared to that of thesample (below approximately 3 pg g�1).

2.5.2. Sample preparation for Pu determination

The sample preparation scheme developed for the plutoniumanalysis is based on the procedure used for production datedetermination by the measurement of 230Th progeny in uranium-oxide matrices (Varga and Suranyi, 2007). The procedure

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Fig. 3. A typical alpha spectrum of a HU-LEU sample.

Z. Varga, G. Suranyi / Applied Radiation and Isotopes 67 (2009) 516–522 519

proposed can also be extended for the simultaneous separation of230Th (production date measurement) and Pu isotopes (for thedetection of previous neutron irradiation or reprocessing), thus itminimizes analysis time, sample amount required and wastegenerated. Approximately 100 mg aliquot of the dissolved ura-nium-oxide sample was placed in a polyethylene Eppendorf tube.The sample was spiked with approximately 200 pg of 242Pu tracergravimetrically and 1 mL of 3 M HNO3 was added to the sample. Inorder to adjust the oxidation state of Pu to Pu(IV), 10 mg of NaNO2

was added to the sample. After thorough mixing and approxi-mately 10 min waiting, the Pu was separated from uranium byextraction chromatography. For the column preparation 1.8 ml ofTEVATM resin was placed in plastic Bio-Rad holders (diameter:8 mm, length: 40 mm). After conditioning of the resin with 15 mLof 3 M HNO3 the sample was loaded on the column. Flow rate wasapproximately 1 mL min�1. After rinsing the Eppendorf tube twicewith 1 mL of 3 M HNO3 and adding the rinsing solution to thecolumn, the uranium was stripped from the column with 15 mL of3 M HNO3. Finally, the thorium and plutonium fractions wereeluted together with 20 mL of 9 M HCl, 2.5 mL of 4 M HCl and20 mL 0.1 M HNO3/0.1 M HF into a PFA beaker. In order to furtherpurify the sample, the extraction chromatographic separation wasrepeated. The eluate was evaporated to almost dryness followedby the addition of 2 mL concentrated HNO3 to destroy organicresin residual and to remove remains of HF that may precludeplutonium retention on the resin. The residue was dissolved in2 mL 3 M HNO3 while slight heating. After cooling to roomtemperature and adding 10 mg of NaNO2 to the sample, theextraction chromatographic separation with TEVATM resin wasrepeated, but in this second separation the Th fraction (elution by20 mL of 9 M HCl and 2.5 mL of 4 M HCl) and Pu fraction (elutionby 20 mL 0.1 M HNO3/0.1 M HF) were collected separately, andonly the plutonium part was used in the forthcoming step. Theplutonium fraction obtained was evaporated to almost drynessfollowed by the addition of 2 mL concentrated HNO3 to destroyany organic resin residual. Finally, the residue was dissolved in

Fig. 2. The schematic diagram

three times 300mL of 2 m/m% nitric acid while heating slightly.The schematic diagram of the sample preparation is shownin Fig. 2.

2.6. Alpha spectrometric analysis of the nuclear materials

Approximately 50 mg aliquot of the dissolved uranium-oxidesample was used for the analysis. Followed by a 100-fold dilutionwith 2 m/m% ultrapure nitric acid solution, a sample aliquotcontaining approximately 10mg of uranium was electrodepositedon a stainless steel disc according to the Talvitie (1971) method,and was subjected to alpha spectrometric analysis. A typical alphaspectrum of a HU-LEU sample is shown in Fig. 3. The overall

of the sample preparation.

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uncertainty was calculated taking into account the measuredintensities and half-lives of the isotopes of interest.

3. Results and discussion

3.1. Comparison of the different analytical methods

The main characteristics of the different techniques for thedetection of previous neutron irradiation by the measurement ofmain activation products present in illicit uranium-based nuclearmaterials are shown in Table 2.

The main advantage of laser ablation ICP-SFMS over theother techniques is that it does not require the dissolutionof the sample, and only a tiny amount of material is required forthe measurement (ng to mg). This feature is of vital importance innuclear forensics, as the material serves as evidence in the courseof the investigation, thus alteration of the sample should beavoided. Moreover, as usually several other destructive measure-ments should be performed on the material for the reliableidentification of origin and intended use (e.g. uranium contentdetermination, measurement of trace impurities), higher sampleamount is kept for such analyses. Another advantage of thistechnique is the rapidity: the isotopic composition of the materialcan be obtained within hours, which is vital for forensicinspection. However, in some cases, LA-ICP-SFMS cannot bedirectly applied, e.g. if the material surface is uneven, the variablelaser ablation efficiency results in unstable signal, thus causinghigh uncertainty in the measurement. Moreover, as laser ablationICP-SFMS takes sample only from the upper approximately0.1–10mm depth of material, if the sample is not homogeneousand the analyzed fraction does not represent the total sample, itleads to inaccurate results. Although for fresh, non-irradiatednuclear fuel pellets the sample inhomogeneity is negligible,it is crucial for spent fuel or technological nuclear materials.The detection limits achievable by laser ablation ICP-SFMS areappropriate for nuclear forensic purposes: not only the measure-ment of main uranium isotopes (234U, 235U, 238U), which arepresent as major components in the nuclear material, can beperformed, but also the minor isotopes (236U, Pu-isotopes or otherlong-lived radionuclides) can be measured down to the mg g�1–pgg�1 level. The detection limit of the minor uranium isotope, 236U,is comparable to that of achievable by the conventional solution-based ICP-SFMS method, which derives from the fact that the 236Umeasurement capabilities are limited by the presence and

Table 2The main characteristics of the applied analytical methods for the determination of ac

Laser ablation ICP-SFMS

Approximate sample amount consumed ng to mg

Sample preparation duration Less than 30 min

Analysis time 1 h

Detection limitsa 236U: 1.3mg g�1

239Pu: 8.8mg g�1

Typical precision (95% confidence level, k ¼ 2) 5–15%

Advantages Rapid

Quasi-non-destructive

Disadvantages Expensive instrumentation

Prone to surface roughness and inho

a Calculated for natural uranium. In case of the analysis of 239Pu by LA-ICP-SFMS it is

interference of the major 235U and 238U isotopes in both cases.These isotopes cause interference on the 236U peaks by over-lapping to the adjacent peaks (also called abundance sensitivity) ifthey are present in high quantity compared to the 236U analyte.Therefore, the increase in the analyte signals (e.g. by using higherlaser power or laser diameter) does not improve the detectionlimits, as it also increases the 235U+ and 238U+ signal causingelevated background.

As uranium-oxide standards with certified Pu-content werenot available and such samples need to be handled under specialcircumstances (e.g. by the use of special glove box), the detectionlimits of the Pu measurements by LA-ICP-SFMS can only beestimated assuming that the sensitivities of uranium andplutonium are similar. The determination of Pu in the uraniumoxide samples is hindered by the very high 238U+ interference(overlapping to the adjacent m/z ¼ 239 mass peak). As theoverlapping (abundance sensitivity) is typically 5�10�7 for theELEMENT2 instrument in medium-resolution used for the LA-ICP-SFMS work, the maximum 239Pu content, which can be deter-mined in natural UO2 (238U content is 87.5% by weight) isapproximately 8.8mg g�1.

Solution-based ICP-SFMS method provides the most preciseuranium isotope ratios among the techniques applied andinformation on the total sample can be obtained, as it eliminatesthe inhomogeneity by the dissolution of the sample. The typicalachievable isotope ratio uncertainties are lower than 1–5% forthe major isotopes (above 10�3 abundance) using a single-collector type ICP-SFMS instrument. Due to counting statisticsand decreased signal-to-background ratio the isotope ratiouncertainty is higher for the low-abundant minor isotopes(Becker, 2005; Wallenius et al., 2006). The detection limit of236U is slightly better than that of the laser ablation ICP-SFMSmethod and is determined by the contribution of 235U+, 238U+ and235U1H+ signals to the background. Similarly to the laser ablationICP-SFMS method, if the 235U+ and 238U+ signals are too high(above approximately 5�106 and 107, respectively) their con-tribution to the m/z ¼ 236 has to be taken into account. As 236Ucannot be chemically separated from the interferences, separationor pre-concentration does not improve the detection capabilities,thus the advantages of the destructive methods cannot beexploited. Use of the desolvation unit in the sample introductionsystem highly decreases the hydride interferences by producinga dry aerosol in comparison to the conventional nebulization(the 238U1H+/238U+ ratios are 6.2�10�6 and 4�10�4 using micro-flow nebulizer with and without desolvation unit, respectively),

tivation products from uranium-oxide materials.

Solution-based analysis by ICP-SFMS Alpha spectrometry

Uranium analysis: 1–10mg 1–50mg

Plutonium analysis: 1–10 mg of U

Uranium analysis: 3 h 3 h

Plutonium analysis: 1 day

Uranium analysis: 1 h 3–30 h

Plutonium analysis: 1 h236U: 1.1mg g�1 232U: 61 pg g�1

239Pu: 0.41 pg g�1 236U: 13mg g�1

240Pu: 0.23 pg g�1

241Pu: 0.30 pg g�1

1–5% (major isotopes) 2–10%

1–15% (minor isotopes)

Precise Low-cost instrumentation

Low detection limits

Destructive Destructive

mogeneity High detection limits

assumed that plutonium measurement has the same sensitivity to that of uranium.

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thus it highly improves the detection limits and reliability.However, it has to be emphasized that the achievable detectionlimits for 236U by ICP-SFMS (either after dissolution or using alaser ablation as sample introduction) are predominantly deter-mined by the uranium isotopic composition of the material,(i.e. 235U and 238U content), as the background is determined bythese isotopes.

For the determination of plutonium from uranium-oxidematrices very low detection limits can be achieved by thedestructive ICP-SFMS method developed (down to pg g�1 con-centration level), due to the matrix removal and pre-concentra-tion of the analyte. The decontamination factor of uranium,calculated as the ratio of the weight of the 238U before the samplepreparation in the sample and thereafter in the measured fractionwas higher than 2�105. The effective separation of uranium fromplutonium is essential to eliminate its negative effect onionization efficiency, which occurs if uranium is present in highamount (above mg g�1 level (Wallenius et al., 2002)) and it isalso crucial to minimize the 238U1H+ interference formation.The chemical recovery of the sample preparation ranged between63% and 89%. Although the plutonium concentration can also bemeasured in the final fraction by alpha spectrometry, ICP-SFMShas the advantage of lower detection limits and the possibility tomeasure the 240Pu/239Pu ratio, which bears information on theorigin and previous irradiation conditions.

Alpha spectrometry is especially useful for the measurementof shorter-lived uranium isotopes (232U and 234U isotopes), as forthe determination of the longer-lived 235U, 236U and 238U isotopeslong counting time is necessary to decrease the detection limitand to improve precision. The typical detection limit of 236Uby alpha spectrometry (13mg g�1) is approximately one order ofmagnitude higher than those of the mass spectrometric methods,which is the consequence of longer half-life (lower specificactivity) and slight energy difference between the 235U and 236Upeak (the energy difference between the most intensive alphapeaks is 95 keV). The small energy difference between the 235Uand 236U also implies that the determination of 236U is becomingdifficult or even impossible for higher-enriched uranium materialsdue to the overlap of the peaks. Moreover, the precision of the

Table 3Uranium isotope ratios in the analysed nuclear materials measured by the mass spectr

Sample Laser ablation ICP-SFMS

234U/238U 235U/238U 236U/235U

HU-DEP (1.3570.10)�10�5 (2.6170.32)�10�3 (2.4570.22)�10�2

HU-NAT (5.3470.48)�10�5 (7.0870.33)�10�3 o1.9�10�4

HU-LEU (3.2170.20)�10�4 (2.6170.10)�10�2 (1.6670.18)�10�1

RR-HEU (1.1470.18)�10�1 10.8070.44 (7.7870.70)�10�3

The combined uncertainties are given with a coverage factor of 2 (95% confidence leve

Table 4Uranium isotope ratios (atom ratios) in the analyzed nuclear materials measured by al

Sample Alpha spectrometry

234U/238U 235

HU-DEP (1.1470.05)�10�5 (2.

HU-NAT (5.0570.20)�10�5 (7.2

HU-LEU (3.1770.32)�10�4 (2.

RR-HEU 45.4�10�2 4RR-HEU Round Robin exercise average (1.0870.16)�10�1 (1.0

The combined uncertainties are given with a coverage factor of 2 (95% confidence leve

isotope ratios is usually worse than that of the mass spectrometricmethods due to the limiting counting statistics. However, as 232Ucannot be measured by ICP-SFMS due to possible interferencefrom the natural 232Th, for the comprehensive characterization ofthe material use of both techniques is required. Furthermore,alpha spectrometry has the advantage that the analysis can becarried out with relatively cheap and widely available instrumen-tation.

3.2. Analysis of confiscated nuclear materials

The procedures developed were applied for the analysis of theconfiscated uranium-oxide materials and the Round Robinsample. The obtained isotope ratios, which are characteristic tothe materials, are summarized in Tables 3 and 4. The alphaspectrometric results (activity ratios) were converted to atomratios for the easier comparison. The average values of thereported isotopic ratios from the participating laboratories inthe Round Robin exercise are shown in Table 4 (Dudder et al.,2002). The isotope ratios obtained by the different techniquesagree well within the measurement uncertainty and are also inagreement with values reported following the inter-laboratorycomparison (Dudder et al., 2002). In all cases, the precision of the235U/238U and 236U/235U measured by alpha spectrometry wereinferior to those of obtained by mass spectrometry, especially fordepleted or natural uranium samples. If the 235U enrichmentis high, the accurate alpha spectrometric measurement of 235Ucan be problematic, as also 234U is present in high amount causingelevated background. The measurement of 236U was morefavorable by mass spectrometry due to the significantly lowerdetection limits. Out of the samples investigated, 236U wasdetected in all cases with the exception of the HU-NAT samples.Therefore, these materials were produced from (or were mixedwith) not only isotopically enriched or depleted, but alsopreviously irradiated and reprocessed uranium material. It isnoteworthy that for depleted uranium samples (e.g. HU-DEP)the detection capabilities of 235U and 236U by alpha spectrometryare limited by the low counting statistics and overlapping of the

ometric methods.

Solution-based ICP-SFMS

234U/238U 235U/238U 236U/235U

(1.31970.075)�10�5 (2.65670.053)�10�3 (2.2470.11)�10�2

(5.5770.14)�10�5 (7.36170.088)�10�3 o1.9�10�4

(3.4770.14)�10�4 (2.6470.11)�10�2 (1.59470.080)�10�1

(1.1370.05)�10�1 10.1970.30 (7.7770.25)�10�3

l).

pha spectrometry.

U/238U 236U/235U 232U/238U

6970.32)�10�3 (2.2270.26)�10�2 o6.1�10�11

570.93)�10�3 o1.8�10�3 o6.1�10�11

9370.87)�10�2 (1.5770.49)�10�1 (8.3371.95)�10�10

5.3 (7.4071.98)�10�3 o6.1�10�11

070.14)�101 (7.3770.45)�10�3

l).

ARTICLE IN PRESS

Table 5The measured 239Pu, 240Pu and 241Pu concentrations in the investigated materials.

Sample 239Pu

concentration

(pg g�1)

240Pu

concentration

(pg g�1)

241Pu

concentration

(pg g�1)

HU-LEU 6.8370.82 0.40070.076 o0.3

RR-HEU 6.6370.27 1.85270.074 0.27670.030

The combined uncertainties are given with a coverage factor of 2 (95% confidence

level).

Z. Varga, G. Suranyi / Applied Radiation and Isotopes 67 (2009) 516–522522

peaks. In case of HU-LEU sample the previous neutron irradiationwas also verified by the presence of 232U measured by alphaspectrometry. In the HU-NAT sample the investigated activationproducts were not detected, which implies that neither enrichednor reprocessed uranium was used for fuel fabrication.

Previous reprocessing was also identified in the HU-LEU andRR-HEU samples by the detection of trace-level Pu content. Themeasured 239Pu, 240Pu and 241Pu concentrations in the investi-gated samples are shown in Table 5. The 239+240Pu activityconcentration in the Round Robin sample (calculated as the sumof 239Pu and 240Pu activity concentrations) is 30.870.88 Bq g�1,which agrees with the value of 3575 Bq g�1 measured by Malekand Sus (2002). The plutonium concentration was below detectionlimit in the HU-DEP sample, though by the presence of 236U it wasfound to have been produced from previously irradiated uranium.Thus, for the detection of previous neutron irradiation 236U isa better indicator than the presence of plutonium, as U/Puseparations are carried out effectively in the course of reproces-sing. However, if plutonium can be measured, its isotopiccomposition bears valuable information on the origin of thematerial and irradiation conditions (e.g. such as productionreactor type) by comparing the measured isotopic compositionwith those of calculated by reactor-production computer codes(Wallenius et al., 2000). Thus, a better assignation of the materialto its provenance can be performed.

4. Conclusions

The paper describes novel analytical methods developed forthe detection of previous neutron irradiation and reprocessingof illicit nuclear materials by the determination of trace-levelactivation products (232U, 236U, 239Pu, 240Pu and 241Pu). Themeasured isotopes give fundamental information on the rawmaterial used for fuel fabrication. Different analytical methods(alpha spectrometry, ICP-SFMS using solution-based nebulizationand laser ablation) were applied for the measurements. Laserablation ICP-SFMS has the advantage that major isotopes andthe activation products characteristic to the previous neutronirradiation can be determined rapidly and without the need fordissolving the material. Solution-based ICP-SFMS provides themost precise isotope ratios. Very low detection limits can beachieved (down to sub-pg g�1 level) for the plutonium impuritiesand their isotopic composition can be obtained, if the ICP-SFMSmeasurement is preceded by the chemical separation methodproposed. However, 236U was found to be a better indicator ofprevious neutron irradiation or reprocessing. The detection limitsand precision of alpha spectrometry are inferior to those of themass spectrometric methods for most of the investigated radio-nuclides with the exception of 232U, which cannot be determinedby ICP-SFMS. Furthermore, alpha spectrometry has the advantage

that the analysis can be carried out with cheap and widelyavailable instrumentation. However, in several cases the determi-nation of 236U can be problematic due to the overlap of the 235Uand 236U alpha peaks (e.g. for depleted or highly enriched uraniumsamples). By the analytical techniques developed the raw materialused for fuel fabrication and the production conditions (e.g.irradiation conditions) can be identified by comparison withsuitable databases, thus it offers tool for authorities to reveal theprovenance of the illicit material.

Acknowledgments

This study was financially supported by the Hungarian AtomicEnergy Authority. Dr. Tamas Bıro (Institute of Isotopes) is thankedfor helpful discussions.

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