Nuclear forensic investigations: Two case studies
M. Wallenius
*
, K. Mayer, I. Ray
European Commission, Joint Research Centre, Institute for Transuranium Elements, Postfach 2340, 76125 Karlsruhe, Germany Received 19 August 2004; received in revised form 15 December 2004; accepted 15 December 2004
Available online 25 March 2005
Abstract
This paper describes the methodology and analytical methods used in nuclear forensic investigations. Two case studies are taken as examples to illustrate this. These examples represent typical cases that have been analysed at the Institute for Transuranium Elements (ITU) since last 10 years, i.e. the beginning of the illicit trafficking of nuclear materials. Results of the various analytical techniques are shown, which, together with other type of information, reveal the origin of the material. # 2005 Elsevier Ireland Ltd. All rights reserved.
Keywords: Illicit trafficking; Nuclear forensics; Uranium pellet; Highly enriched; Uranium
1. Introduction
Since the beginning of the 1990s, cases of illicit traffick-ing involvtraffick-ing nuclear material were started betraffick-ing reported. As a result, nuclear material has become a part of the forensic investigations and a new discipline – nuclear for-ensic science – was developed. Obviously, the question on the origin of the material, its intended used and the last legal owner needs to be answered. The methodology developed in nuclear forensics may also be applied for source attribution of nuclear material in contaminated scrap metal or environ-mental samples, e.g. illegal dumping of nuclear waste or accidental release. The source attribution can be achieved using the characteristics inherent to the nuclear material. For each seized sample a specific analytical strategy needs to be developed, taking into account the particular conditions of the seizure, the very nature of the material and of its packing and other evidence. The analytical strategy is based on a step-by-step approach, where experimental results are com-pared to information on nuclear material of known origin contained in a relational database. Based on the actual
findings, the next step is defined and performed. Numerous analytical techniques are used in the investigations, includ-ing radiometric and mass spectrometric techniques as well as electron microscopy.
The Institute for Transuranium Elements (ITU) has worked on the field of nuclear forensic science from the beginning and has contributed significantly to its develop-ment. The instrumentation available in the laboratories is specifically adapted for work with nuclear material. The instruments are routinely applied for nuclear material ana-lysis in several areas, e.g. safeguards, material science, contractual work, method development, environmental stu-dies. Since 1992, around 30 samples of seized nuclear material (from natural uranium to weapons grade plutonium, from particles to bulk material) have been analysed in the context of nuclear forensic investigations. The present paper describes two case studies, which shall illustrate the meth-odology applied, the extensive work and the specific know-how required to solve a case.
2. Methodology
Nuclear forensic investigations are carried out in three reporting periods that were recommended by the Interna-www.elsevier.com/locate/forsciint Forensic Science International 156 (2006) 55–62
* Corresponding author. Tel.: +49 7247 951373; fax: +49 7247 95199373.
E-mail address: [email protected] (M. Wallenius).
0379-0738/$ – see front matter # 2005 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.forsciint.2004.12.029
tional Technical Working Group (ITWG) on nuclear smug-gling[1]. These are a 24-h report, a 1-week report and a 2-months report, to be delivered after arrival of the material in the laboratory. The purpose of the 24-h report is to categorize the material, i.e. to determine whether the material consists of a radioactive source (as used, e.g. for medical purposes) or whether it involves nuclear material, natural, reactor grade or weapons grade. In this case the quality and quantity of the seized material can be sufficiently well characterised by non-destructive techniques, e.g. gamma spectrometry. Within 1 week normally the main information on the material can be obtained, and some first conclusions can already be drawn. However, in case of non-typical material, additional, espe-cially for nuclear forensic purposes, developed analyses, as well as search of information from the open sources are required in order to deduce the origin. The techniques used in the investigations at ITU and the information obtained by them are summarised inTable 1.
3. Case studies
The two recent examples whose analyses have been completed were chosen for this in order to illustrate the methodology, and to provide some factual information. The other criterion for the choice was ‘‘the starting point’’ of the analysis. Even if both of the cases are real seizures, we did not obtain the samples from legal authorities (e.g. police), like usually, but from a national laboratory of the relevant country, where the seizure had taken place. Thus, these analyses also demonstrated the ‘‘functionality’’ of the so-called joint analyses that has been established between ITU and the new member states (formerly called ‘‘accession countries’’) of the EU. The rationale of the joint analysis is the following: if the country, where the incident has happened, does not have its own capabilities to perform the analysis itself or if complementary analysis are needed, the state can send the sample to ITU where appropriate facilities for handling and analysing the material are
avail-able. Typically, the investigations are carried out with parti-cipation of a national laboratory expert. This enables the expert to present the results and conclusions of the nuclear forensic investigations in a national court, if necessary. 3.1. First example: uranium pellets
In June 2003, ITU received four uranium pellets from Lithuania. Uranium dioxide pellets are used as fuel in nuclear power reactors. All the pellets showed identical geometry; they had a central hole and they were dished (Fig. 1). The pellets were weighed and their dimensions were measured (Table 2).
3.1.1. Uranium content and isotopic composition
All the four pellets were measured individually by high-resolution gamma spectrometer for the first indication of the isotopic composition. The spectra showed gamma lines Table 1
Information that can be obtained from nuclear (U, Pu) material
Parameter Information Analytical technique
Appearance Material type (e.g. powder, pellet) Optical microscopy
Dimensions (pellet) Reactor type Database
U, Pu content Chemical composition Titration, HKED, IDMS
Isotopic composition Enrichment) intended use; reactor type HRGS, TIMS, ICP-MS, SIMS
Impurities Production process; geolocation ICP-MS, GDMS
Age Production date AS, TIMS, ICP-MS
18
O/16O ratio Geolocation TIMS, SIMS
Surface roughness Production plant Profilometry
Microstructure Production process SEM, TEM
HKED, hydrid K-edge densitometry; IDMS, isotope dilution mass spectrometry; HRGS, high-resolution gamma spectrometry; TIMS, thermal ionisation mass spectrometry; ICP-MS, inductively coupled plasma mass spectrometry; SIMS, secondary ion mass spectrometry; GDMS, glow discharge mass spectrometry; AS, alpha spectrometry; SEM, scanning electron microscopy; TEM, transmission electron microscopy.
belonging only to uranium (e.g. 110, 144 and 186 keV). The MGAU code was used for the spectrum deconvolution[2]. The analysis gave an average 235U enrichment of 1.986 0012 (mass%). As the pellets were confirmed to be identical also from the isotopic composition of U (besides the dimensions), only one of them was dissolved and taken for further analysis.
The isotopic composition of uranium was determined by mass spectrometry. Mass spectrometry techniques are able to provide accurate results for the minor abundant isotopes (234U and236U), which is not the case with gamma spectro-metry. The measurement technique routinely used for U and Pu isotope analysis is thermal ionisation mass spectrometry (TIMS). An inductively coupled plasma mass spectrometer with multi-collector detection system (MC-ICP-MS) was used to compare the accuracy and precision between these two methods (Table 3). The uncertainties in this paper (if not otherwise mentioned) are calculated using GUM Workbench programme, i.e. standard uncertainty U = k uc, where ucis
the combined uncertainty and k is the coverage factor, k = 2
[3].
The uranium content in solution was determined by three different methods, namely: by potentiometric titration, by hydrid K-egde densitiometry (HKED) and by isotope dilu-tion mass spectrometry (IDMS) (Table 4) [4–6]. It can be seen that the uranium content corresponds the stoichiometry of uranium dioxide (UO2) whose theoretical value is 88%.
3.1.2. Chemical impurities
Impurities in the sample were determined after complete dissolution by sector-field ICP-MS using103Rh as an internal standard. The list of the main impurities (>1 mg/g) and their concentrations (relative to uranium) are seen inTable 5. 3.1.3. Age determination
The age of the material is important in order to know the date when the material was produced and thus identify the production campaign or batch. The radioactive decay of the uranium isotopes provides a unique chronometer which is
inherent to the material. This clock is reset to zero each time the decay products (daughter nuclides) are chemically sepa-rated from the uranium.
235U!231Pa T
1=2¼ 7:038 108a
234U!230Th T
1=2¼ 2:455 105a
The half-life (T1/2) of the uranium isotopes in question is very
long, therefore the short periods between the preparation of uranium fuel and the seizure of the material will generate only extremely minute amounts of daughter nuclides. Neverthe-less, the age can be determined from these parent/daughter ratios. The age of the uranium was calculated using the equation of radioactive decay and its derivatives:
N¼ N0 elt (1) NU-234 NTh-230 ¼ N0;U-234 e lU-234t N0;U-234 N0;U-234 elTh-230t (2) t¼ lnð1 R=KÞ B (3)
where R, measured230Th/234U atom ratio; K, activity ratio lU-234/(lTh-230lU-234); B, a factor combining the
234
U and
230
Th decay constants (lTh-230lU-234).
The sample solution was spiked with 228Th and 233U prior to the U/Th separation[7]. The amount of234U and M. Wallenius et al. / Forensic Science International 156 (2006) 55–62 57 Table 2
Macroscopic data of the pellets
Pellet no. Weight (g) Height Dimensions (mm) Diameter Hole 1 14.6672 14.12 11.45 2.1 2 14.7614 14.26 11.44 2.1 3 15.3979 14.91 11.45 2.1 4 14.8626 14.46 11.45 2.1 Table 3
Isotopic composition of uranium by mass spectrometry in mass%
Technique 234U 235U 236U 238U
TIMS 0.0147 0.0010 2.0005 0.0010 0.0071 0.0067 97.9778 0.0019
MC-ICP-MS 0.0142 0.0002 2.0005 0.0001 0.0071 0.0000 97.9782 0.0010
Table 4
Uranium content in mass%
Technique U-content HKED 87.43 0.32 Titration 87.90 0.13 IDMS 87.99 0.24 Table 5 Impurities in mg/g U by SF-ICP-MS Element Concentration Al 6.08 0.73 Ca 18.4 2.2 Cr 6.12 0.73 Cu 1.80 0.22 Fe 91.9 7.4 K 44.7 3.6 Mg 4.71 0.57 Mn 1.13 0.14 Na 17.9 2.1 Ni 5.14 0.62 Zn 3.40 0.41
230
Th was determined using the isotope dilution technique, i.e. relative measurements against the (known amount of) spike isotope. The age of the material was 12.6 0.8a (at the measurement date 16.06.2003); thus the pellets had been produced in the end of 1990. Only the234U/230Th parent/ daughter ratio could be used this time for the age determina-tion, due to the long half-life of the235U and consequently the very small amounts of daughter nuclide (231Pa) that are built up.
3.1.4. Reference information
As mentioned above, a relational database has been established at ITU, which contains data from several nuclear fuel manufacturers (including most of the Western Europe and Russia)[8]. The data include, e.g. dimensions of pellets,
235
U enrichment and typical impurities. Besides the com-mercial reactor fuels, the database also contains information on few research reactor fuels and information acquired from open literature. Additionally, results of old findings are introduced in the database for a comparison with future cases.
In this case the database gave very unambiguous answer. Already the pellet dimensions and enrichment were enough to identify the reactor type, which is an RBMK-1500, a Russian type water-cooled, graphite-mod-erated reactor. There are two models of the RBMK reac-tors, namely, 1000 and 1500. The 1000 is the older model and is more widely distributed. For the 1500 model there is only one reactor in the world and is Ignalina Unit 2 in Lithuania. The reactor started in August 1987 and it is still operational. Furthermore, there is only one manufacturer for this type of fuel, namely MZ Electrostal in Moscow, Russia. The measured impurities (seeTable 5) were below the maximum values given in the manufacturer specifica-tions and they also agreed with the experimental data from the old findings of the same fuel. Last confirmation para-meter was the age, which fitted with the production data of the manufacturer (start of the fuel production: December
1989). The information contained in the nuclear materials database proved to be essential for the attribution of the material.
3.1.5. Summary
The IAEA database on illicit trafficking of nuclear and other radioactive materials and of some other open source information, report a case of theft of a fresh fuel assembly that was stolen from the Ignalina power plant in 1992. These four pellets under investigation do definitively originate from Electrostal and most probably from that stolen assem-bly. This kind of fuel assembly contains about 110 kg of uranium. Between 1994 and 1997 more than 100 kg of pellets has been confiscated in several seizures; thus the greater part of the material has been recovered[9].
The material itself is not useable for nuclear weapons, because the235U enrichment of 2% is far too low. However, what makes this case spectacular is the amount of the material that was stolen. Efforts have been undertaken to improve the physical protection at nuclear power plants and other storage facilities for nuclear material in the former Soviet Union.
3.2. Second example: uranium powder
In April 2003 ITU received four powder samples of uranium from the Czech Republic. The samples looked identical (Fig. 2) and they were subsamples from the larger batches, seized at several occasions during 1994–1995 (Table 6).
3.2.1. Uranium content and isotopic composition
First, the samples were measured non-destructively by gamma spectrometry. The measured average enrichment of
235U was 89.59 0.43 (mass%). The sample Cl was
excluded from the average, as it gave a significantly dis-crepant result (<88 wt.%) due to the small sample size.
After dissolution of part of the samples the isotopic composition was determined also by TIMS and MC-ICP-MS. There were no significant differences between the samples and both techniques gave similar results (the indi-vidual results were pooled and the average values are shown in Table 7). The isotopic composition obtained by the previous-mentioned techniques is, of course, valid only for the bulk material. If the material is a mixture of several components, which could be the case with powder sample, then the result of the isotopic composition is an average of the (potentially present) different components. In order to see, e.g. if the smaller particles had different composition from the bigger ones, we used SIMS[10,11]. 10–15 smaller (1 mm) and bigger (10 mm) particles were measured, respectively, per sample. No significant differences in the isotopic composition of the individual particles were detected. Hence, the material proved to be isotopically homogeneous.
The U content was determined by potentiometric titration (not for sample Cl) and by IDMS. The content varied between 82.26 and 86.09 mass% of U, being lowest for the smallest sample (Cl) and highest for the largest samples (Al and Bl). This could indicate that the samples have been partly oxidised during storage. A surface oxida-tion will affect the smallest samples most, as its ‘‘free’’ surface area is relatively large. Thus, the stoichiometry of the material ranges between UO2(88.0 wt.%) and U3O8
(84.6 wt.%). Additionally, the material may have adsorbed humidity, which further reduces the apparent uranium content.
3.2.2. Impurities
Impurities in solutions (100 mgU/g) of samples Al, Bl, Cl, and Dl, respectively, were determined by sector-field ICP-MS using103Rh as an internal standard. The list of the main impurities and their average concentrations are seen in
Table 8. The quantities of the impurities are fairly close to the values that were found in the pellets; thus this could
indicate that the same type of process was used in both the cases.
3.2.3. Age determination
The age of all the four samples was determined by using the method described above in the first example[7,12]. The average result obtained by alpha spectrometry gave a pro-duction date of January 1976 1.2 years (1s). Besides this, also a direct parent/daughter ratio measurement (i.e. no separation or spiking) by ICP-MS was performed. This gave a production date of August 1976 3.0 years (1s). Addi-tionally, two other methods were used for the sample Al for a comparison (Table 9). The alpha spectrometry using the
234
U/230Th ratio is the most precise method as expected. However, for the first indication of the age, the direct ICP-MS determination is sufficiently accurate and, moreover, much faster than alpha spectrometry.
M. Wallenius et al. / Forensic Science International 156 (2006) 55–62 59 Table 6
Sample information
Sample Sample mass (g) Total mass of uranium in seizure (g) Date and place of the seizure
A1 3.01 1388 14.12.1994, Prague, Czech Republic
B1 3.01 1342 14.12.1994, Prague, Czech Republic
C1 0.13 0.4 06.06.1995, Prague, Czech Republic
D1 2.01 17 08.06.1995, Ceske Budejovice, Czech Republic
Table 7
Summary of the isotopic composition of U by mass spectrometry in mass%
Technique 234U 235U 236U 238U
TIMS 1.079 0.001 87.775 0.018 0.211 0.001 10.937 0.018
MC-ICP-MS 1.076 0.001 87.789 0.015 0.214 0.005 10.922 0.017
SIMSa 1.085 0.023 88.141 0.254 0.214 0.020 10.559 0.219
a
Sample Cl was not measured by SIMS.
Table 8
Average values of the impurities by SF-ICP-MS
Element Concentration (mg/g U 1s) Al 7.87 2.21 Ba 1.23 0.59 Ca 45.1 18.0 Cr 6.52 1.20 Cu 2.24 1.46 Fe 69.5 22.5 K 26.7 7.3 Mg 10.6 1.6 Mn 1.25 0.16 Na 19.1 4.2 Ni 3.66 0.38 P 21.0 l.9 Pb 3.91 2.09 Zn 6.55 2.24 Zr 24.5 1.1
3.2.4. Electron microscopy
Specimens of the powders were prepared for examination by scanning electron microscopy (SEM) by fixing a small, dispersed sample on an aluminium specimen stub and lightly coating with carbon to ensure good electrical conductivity. The SEM used was a Philips XL40, which has been specially modified for the examination of radioactive and contami-nated specimens by mounting the microscope column and pumping system in a glovebox separated from the control console. The microscope is equipped with an EDAX energy dispersive X-ray analysis system.
The powder consists of irregularly shaped particles in the approximate size range 100 mm to 1 mm (Fig. 3). A high magnification image of the surface of a single particle revealed that the particle consists of an agglomerate of very fine particles with an average size of about 0.25 mm (Fig. 4). Some of the particles were cross-sectioned to reveal the internal structure. The internal structure of the particles was identical to the surface structure; thus
con-firming that the particles are agglomerates of very fine grained material.
3.2.5. Summary
The investigated uranium powder is very near the enrich-ment that is required from uranium used in nuclear weapons; thus material can be categorised as ‘‘weapons-usable’’. The material consists of granulates, which are normally com-pacted to produce fuel for fast breeder reactors. This so-called vibro-compacting process was created in 1975 at Scientific Research Institute for Atomic Reactors (SRIAR) in Dimitrovgrad, Russia and it was intended for MOX fuel production.
Besides these three seizures in the Czech Republic, 0.8 g of HEU was found also in Landshut, Germany, in June 1994. The results of that finding fit perfectly with the other findings; thus all the seized materials originate most likely from the same HEU batch. The exact origin of the material is rather difficult to deduce without confirmation from the Table 9
Results of the age determination for the sample Al Method
AS AS ICP-MS MC-ICP-MS
234
U/230Tha 235U/231Paa 234U/230Tha 234U/230Tha
Age on the measurement date 26.52 0.7 25.21 0.8 29.9 5.0 25.32 3.5
Production date October 1976 Febraury 1978 June 1973 January 1978
a
Parent/daughter ratio.
suspected party. Several speculations about the origin (e.g. Ozersk, Odessa, Minsk) are found in the literature, but without any real evidence[13–15]. However, the cardinal point is always the same, i.e. the former Soviet Union.
4. Conclusions
The two examples presented here are typical cases analysed at ITU since the first seizure in 1992. As demon-strated, the pellet case is often much easier to solve than a powder case, because of the information of the commercial nuclear fuels available in our database. Powder is usually not a final product, but an intermediate product and/or from a pilot-plant and not from a commercial production cycle. Thus in this case we have to rely also on the help from the suspected party.
To make the origin determination more ‘‘waterproof’’, we are continuously doing research on samples of known origins. As at the moment, we use so-called exclusion principle in origin determination (i.e. we exclude the country or facility as a potential source of the material), the goal is to find a new parameter that would point to a certain process or facility without doubt.
Acknowledgments
The authors would like to thank S. Abousahl, A. Mor-genstern, A. Nicholl, G. Rasmussen, G. Tamborini and T. Wiss for their contribution in the analyses of these samples.
References
[1] S. Niemeyer, L. Koch, The nuclear smuggling international technical working group: making a difference in combating illicit trafficking, in: Proceedings on Advances in Destructive and Non-Destructive Analysis for Environmental Monitoring and Nuclear Forensics, IAEA-CN-98/3, Karlsruhe, Germany, 21–23 October, 2002, pp. 17–19.
[2] S. Abousahl, A. Michiels, M. Bickel, R. Gunnink, J. Verplancke, Applicability and limits of the MGAU code for the determination of the enrichment of uranium samples, Nucl. Instrum. Methods Phys. Res. A 368 (1996) 443– 448.
[3] GUM Workbench Version 2.3, Metrodata GmbH, Grenzach-Wyhlen, Germany (2001).
[4] W. Davies, W. Gray, A rapid and specific titrimetric method for the precise determination of uranium using iron(II) sulphate as reductant, Talanta 11 (1964) 1203–1211.
[5] H. Ottmar, H. Eberle, The hydrid K-edge/K-XRF densit-ometer: principles–design–performance, Forschungszentrum Karlsruhe, KfK Report 4590, 1991.
[6] K.G. Heumann, Isotope dilution mass spectrometry, Int. J. Mass Spectrom. Ion Proc. 118–119 (1992) 575– 592.
[7] M. Wallenius, A. Morgenstern, C. Apostolidis, K. Mayer, Determination of the age of highly enriched uranium, Anal. Bioanal. Chem. 374 (2002) 379–384.
[8] Y. Dolgov, Y. Bibilashvili, N. Chorokhov, L. Koch, R. Schenkel, A. Schubert, Case studies with a relational database system for identification of nuclear material of unknown origin, in: Proceedings of the Russian International Conference on Nuclear Material Protection, Control and Accounting, Obninsk, Russia, 9–14 March, 1997, pp. 116– 120.
M. Wallenius et al. / Forensic Science International 156 (2006) 55–62 61
[9] IAEA database on illicit trafficking of nuclear materials and other radioactive sources. Open information 2002-04-23.
[10] G. Tamborini, M. Betti, V. Forcina, T. Hiernaut, B. Giovan-none, L. Koch, Application of secondary ion mass spectro-metry to the identification of single particles of uranium and their isotopic measurement, Spectrochim. Acta, Part B 53 (1998) 1289–1302.
[11] G. Tamborini, M. Betti, Characterisation of radioactive parti-cles by SIMS, Microchim. Acta 132 (2000) 411–417.
[12] A. Morgenstern, C. Apostolidis, K. Mayer, Age determination of highly enriched uranium: Separation and analysis of Pa-231, Anal. Chem. 74 (2002) 5513–5516.
[13] More uranium finds, Intell. Newslett. 256 (1995).
[14] M. Hibbs, Germany won’t answer questions on smuggling probe, Minatom says, Nucleonics Week (30 March 1995) 13–14.
[15] M. Hibbs, Smuggled HEU seized in Germany, Prague came from Mayak stockpile, police say, Nuclear Fuel 21 (September 1998).
