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Interviewing a Silent (Radioactive) Witness through Nuclear Forensic Analysis Nuclear forensics is a relatively young discipline in science which aims at providing information on nuclear material of unknown origin. The determination of characteristic parameters through tailored analytical techniques enables establishing linkages to the material’s processing history and hence provides hints on its place and date of production and on the intended use. Klaus Mayer,* Maria Wallenius, and Zsolt Varga European Commission Joint Research Centre−Institute for Transuranium Elements, 76125 Karlsruhe, Germany nature such as enriched uranium or plutonium, will carry traces of the environment it has been in contact with. Environment in this context may include the geological formation from which uranium has been mined or the process the material was exposed to. As a consequence, nuclear forensic science first needs to identify those measurable parameters in nuclear material or any combination thereof, which can be related to the material history. These parameters are referred to as “signatures”. Thus, nuclear forensics can “assist in the identification of the materials, as well as how, when, and where the materials were made, and their intended lawful use.”1 Hence, nuclear forensic science exploits information which is inherent to the nuclear material, turning the material into forensic evidence and getting this “silent witness” to reveal valuable information. To this end, signatures need to be identified, recorded, and documented and appropriate analytical methods have to be developed, optimized, and validated. The William Edmunds (Joint Research Centre, Institute for Transuranium Elements, Karlsruhe) underlying radiochemical principles and the analytical methods have been described in a textbook by Moody et al.2 The state of the art in nuclear forensic science has been reviewed more uclear and other radioactive materials are subject to recently.3 Hereafter we will discuss the application of analytical stringent control. Nuclear material (such as uranium or chemistry methods (including radioanalytical methods) for the plutonium) is kept under strict physical protection, it is identification of signatures and for the investigation of nuclear accurately accounted for and an international verification material of unknown origin. regime is in place to ensure that nuclear material is not diverted from the civil nuclear fuel cycle. Still, cases of illicit SIGNATURES incidents involving such material have been reported. Nuclear material out of regulatory control is a reason for concern to the Parameters such as the elemental composition, isotopic public and hence it was addressed at the Nuclear Security composition of major constituents (e.g., uranium or plutoSummits in Washington (2010), Seoul (2012), and The Hague nium), isotopic composition of minor constituents (e.g., Pb, Sr, (2014). The threat associated with nuclear materials out of or Nd), metallic impurities, anionic impurities, products of regulatory control is due to its radiological hazard and to the radioactive decay, microstructure, or molecular structure serve proliferation risk. Its potential use in malicious acts, possibly as nuclear forensic signatures. Signatures may be source even in acts of nuclear terrorism calls for appropriate measures material inherited (e.g., natural uranium carrying impurities originating from the uranium ore) or process inherited (e.g., Gd to prevent and deter such acts and to investigate and prosecute is added to nuclear fuel in order to allow higher burn-up). the unlawful possession, transfer, or use of nuclear material. Process inherited signatures may be due to chemical operations Whenever nuclear material is intercepted from illicit trafficking (such as dissolution, extraction, ion exchange, precipitation) or questions on the origin and on the intended use of the material they may be associated with physical processes (such as arise. Nuclear forensics is a fairly new discipline in science neutron irradiation). In addition to intentionally added which aims at answering exactly these questions. In essence, “impurities”, some unintentionally added impurities may nuclear forensic science shares the basis of all forensic provide useful information (e.g., corrosion products from the investigations, known as Locard’s Principle: “every contact reaction vessel material, residuals from reagents or solvents). leaves a trace”. According to Locard’s principle, nuclear material, be it of natural origin such as natural uranium or of anthropogenic
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abundance ratio revealed relative differences as small as 0.04‰5 which can be understood by natural isotopic fractionation. Much larger variations are observed for the 234 U/238U isotope abundance ratios which are due to the increased mobility of the 234U isotopes as explained by Brennecka et al.5 The observed differences help distinguishing natural uranium from different origins and in some cases the measured isotope ratios may even point at the type of ore deposit the uranium was mined from. Some of the impurities in natural uranium (such as Pb, Sr, or Nd), though consisting of stable isotopes may also show small variations in the isotopic composition. These variations arise from radioactive precursors of some of the isotopes in these elements. For example the 143Nd/144Nd isotope ratio varies in nature due to the presence of the long-lived parent nuclide 147 Sm (T1/2 = 1.06 × 1011 years), which decays to 143Nd. They can be usefully exploited for nuclear forensic purposes as has been shown in a number of papers.6−9 Sr isotope ratios have been demonstrated to be a very robust signature showing low within mine variability and significant between mine differences, as can be seen from Figure 2. Apart from the isotopic signatures, also the chemical impurities may provide useful information. In particular, the rare earth elements were proven to be highly significant in identifying types of uranium deposits.10,11 Their chemical similarity (similar ionic radius, mostly trivalent oxidation state) minimizes segregation effects during chemical reactions and in consequence their relative concentration (i.e., the rare-earth element pattern) does not change even if the absolute concentration goes down by several orders of magnitude during chemical purification of uranium. In several real incidents this combination of signatures has been successfully applied for revealing the origin of illicitly trafficked uranium with one case described in some detail by Keegan et al.12 All of these signatures are source material inherited and provide hints on the type of geological formation (i.e., type of uranium ore deposit) from which the uranium has been mined. In addition we also observe process inherited signatures. The
Plutonium is an anthropogenic element which is formed in nuclear reactors through successive neutron capture reactions of uranium and subsequent decay. Different types of reactor, however, yield plutonium of varying isotopic composition. The model calculations shown in Figure 1 illustrate these differences and show how plutonium isotope ratios may serve as signature for determining in what type of reactor the plutonium was produced.
Figure 1. Plutonium isotope correlations for different reactor types as calculated using SCALE/ORIGEN, taken from Wallenius et al.4 Measured values are indicated on the graph by full circles: F19 denotes plutonium seized at Munich airport in 1994, SRM 946 and SRM 947 denote certified plutonium reference materials originating from light water reactors, as does RR. Sample R2 is of Russian origin and can be attributed to an RBMK type reactor. Sample R1 appears to be a blend of plutonium batches produced in different types of reactors. Reprinted with permission from ref 4. Copyright 2000 Springer.
Natural uranium shows only very small, though significant variations in the isotopic composition. High precision mass spectrometric measurements of the 235 U/ 238 U isotope
Figure 2. Measured 87Sr/86Sr isotope ratios of the investigated uranium ore concentrates. Uncertainties are presented at the 95% confidence level. Reprinted from ref 7. Copyright 2009 American Chemical Society. B
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Analytical Chemistry first parameter to be determined is the chemical composition of the material and thus also the molecular structure. Spectroscopic techniques such as Raman or infrared spectroscopy have be applied for determining the latter.13−18 Identification of the chemical compound will point at the process used for producing the material (e.g., uranium peroxide, ammonium uranyl carbonate, ammonium diuranate). Besides the molecular structure, the spectra indicate also the presence of some anionic species such as sulfate, nitrate, or carbonate which point at acids used for dissolution or back-extraction. The morphology of the material may also provide useful hints on the processing history of the material. The macroscopic appearance of nuclear fuel pellets is an important signature as the radius and the design may vary depending on the type of reactor they are intended for. Figure 3a,b shows
Figure 4. Radioactive decay of uranium isotopes and their respective progenies form the basis of radio-chronometry.
the actual measurement instruments may need to be specifically adapted for safely working with radioactive material. Material of unknown origin is always first subjected to nondestructive investigation which includes gamma spectrometry and microscopy. On the basis of the information obtained, the subsequent chemical analysis can be optimized. Mass spectrometric methods play a key role in nuclear forensic analysis because they combine features of versatility, selectivity, sensitivity, and precision. Thermal ionization mass spectrometry (TIMS) is widely applied for high-precision measurement of isotope ratios for elements with reasonably low ionization potential such as uranium or plutonium as extensively described in several textbooks.29,30 For high accuracy measurements of the minor abundant isotopes of uranium (i.e., 234U and 236U), a specific application was developed, the so-called modified total evaporation technique, which minimizes errors of systematic and of random nature and thus results in very low combined measurement uncertainty.31 TIMS can also be applied for measuring the isotopic composition of nonradioactive minor constituents, such as Nd or Sr. Inductively coupled plasma mass spectrometry (ICPMS) is at present the most frequently used mass spectrometric technique for both concentration and isotope ratio measurements, even if the analyte is present only in ultratrace amounts, hence at the level of femtograms per gram.32 This is achieved by double-focusing sector-field analyzers (ICP-SFMS) which are widely used. This powerful analytical technique is also increasingly used for the measurement of long-lived radionuclides, providing a complementary tool to the traditional radioanalytical techniques such as γ or α spectrometry. Use of multiple detectors (so-called multicollector ICP-SFMS instruments, MC-ICP-SFMS) improves the precision of the measurement as they detect the isotopes of interest simultaneously and eliminates time dependent effects arising from the sample-introduction and from the ion source.33 Compared to TIMS, it requires usually less tedious sample preparation and, due to its plasma ion source, it can be used also for elements with high ionization potential. Hence, it can be favorably applied to more than 60 chemical elements. ICPMS is the method of choice for measuring chemical
Figure 3. (a) Uranium fuel pellet showing a central hole (typically used in Russian type reactors) and a mechanical defect on the surface with traces of corrosion. (b) Uranium fuel pellet intended for western type light water reactor with markings on the top and a characteristic dishing.
examples of uranium fuel pellets intercepted from illicit trafficking where the macroscopic dimensions proved to be highly significant characteristics pointing at the intended use of the material.19 In contrast, powder samples do not show macroscopic characteristics and easily measurable dimensions. Here we rely on particle size, shape, and texture which points at the production process and on the chemical reaction used for precipitating the uranium (or plutonium) from solution.20,21 Nuclear material is subject to radioactive decay. While a radioactive isotope decays over time, its progeny, also referred to as daughter nuclide, is built up. Radioactive decay is described by an exponential law with the decay time being the main variable in this equation. In consequence, by measuring the number of parent atoms and the number of daughter atoms the decay time can be determined. The method is referred to as radiochronometry and the analytical methodology for uranium and plutonium age dating has been described by several authors (see also Figure 4).22−27 Although the half-lives of uranium isotopes are very long (some 105 up to 108 years compared to the time intervals considered in nuclear forensics, few decades) and hence only minute amounts of daughter nuclide are present in a huge excess of parent nuclides, analytical techniques have been developed which allow one to accurately date even small samples of material which is only a few years old.
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METHODS The analytical methods typically applied in nuclear forensic science have been reviewed by Stanley et al.28 It should be noted though that sample handling and preparation as well as C
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absence of other γ emitting nuclides (such as fission products) was verified. Subsequently, the samples were split for the different analytical techniques, such as electron microscopy and various mass spectrometric techniques. The isotopic composition of uranium was determined precisely by two different mass spectrometric techniques, TIMS and MC-ICP-SFMS. The analyses revealed no 236U present (above the detection limit), which is a clear indication that the samples originate from a facility where no reprocessed U materials had been handled. There was, however, a significant difference in the 234U abundance between the samples: 0.00577 ± 0.00011 wt % for the F35-1 and F35-3 and 0.00524 ± 0.00010 wt % for F35-2. This could indicate that the source of uranium of the sample F35-2 is different from the other two samples. Fourier transform-infrared spectrometry was used to determine the molecular composition of the materials as well as to detect possible molecular impurities. F35-1 showed similarities to the reference spectrum of UF4, whereas the other two samples were shown to be partly oxidized (i.e., being UO2F2) with higher H2O content and additional peaks of ammonia and sulfate present. The uranium content in the three samples varied from 20 wt % (for F35-3) to 55 wt % (for F351). This difference was also reflected in the impurity analysis performed by ICPMS, which showed much higher impurities in F35-2 and F35-3 compared to F35-1. As mentioned previously, the REE pattern can be characteristic for the source of uranium, i.e., the deposit type. The REE pattern of F35-1 was relatively flat shaped with a slightly negative Eu anomaly, hence similar to sandstone-tabular type deposits (see Figure 7). The age of the uranium was determined using the 230Th/234U daughter/parent pair, indicating a production date (i.e., date of the last uranium−thorium separation) around December 1978. The data allowed drawing the following conclusions: The samples, consisting largely of UF4 of natural isotopic composition, were intermediate products from the front-end of the nuclear fuel cycle, before the material is converted to UF6 to be enriched. The uranium had been mined from a sandstone type of uranium deposit (“tabular” subtype). The country of the origin of the scrap metal cargo started operating a sandstone deposit uranium mine at the end of 1970s; however, it was a “roll-front” subtype. A literature study revealed that this same country had imported uranium from Niger mined from a “tabular” subtype. In summary, the nuclear forensic signatures allowed tracing the greenish deposit detected on a piece of scrap metal back to its origin in an African uranium mine. The excellent radiation detection architecture in Rotterdam and generally at the scrap metal yards enables the detection of such
impurities (including the rare earth element patterns) and for determining the isotopic composition of, e.g., Sr, Pb, or Nd, hence, for many of the signatures outlined above. Also for age dating of uranium and plutonium, ICPMS can be usefully applied. The application of the different analytical techniques to determine nuclear forensic signatures in a real life incident is illustrated in the example below. It also shows how the processing history of the material and its geographical origin can be inferred based on the data obtained.
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EXAMPLE In April 2010, another cargo ship arrived at Europe’s largest sea port, the Port of Rotterdam. This particular shipment, like many others of close to 30 000 cargo vessels yearly, had arrived from Asia. Some of the cargo containers contained scrap metal. When unloading the container in a nearby scrap metal yard, multiple items were found that emitted increased levels of radiation and hence triggered corresponding alarms. One of the items, which was identified contained uranium (see Figure 5) as
Figure 5. Piece of scrap metal which triggered a radiation alarm. The greenish deposit is a uranium compound and was subject to a nuclear forensic analysis.
could be shown by γ spectrometric measurement. The analysis also revealed the uranium to be of natural isotopic composition, i.e., ∼0.7% 235U. After that, subsamples were taken (scraping off material at different spots) and sent to a specialized laboratory (i.e., the Joint Research Centre−Institute for Transuranium Elements, Karlsruhe) for nuclear forensics analysis. The three subsamples (named F35-1, F35-2, and F35-3; see Figure 6) were all different in color. As pointed out above, the first analysis involved nondestructive techniques. By highresolution γ spectrometry, the in-field measurement of the 235U abundance was confirmed under controlled conditions and the
Figure 6. Samples F35-1 (green), F35-2 (yellow), and F35-3 (brown) investigated by optical microscopy. D
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Figure 7. Rare earth element pattern of seized sample F35-1 showing similarity to “sandstone-tabular” type deposits (Beverly, Kerr-McGee, Homestake) and clearly dissimilar to “sandstone roll front” type deposits.
radioactively contaminated materials and prevents radioactivity being incorporated in the consumer goods.
nuclear forensics, obtaining her Ph.D. in 2001. She continued at ITU as a research scientist and developed new methods using mass spectrometry techniques (especially TIMS and ICP-MS) in the safeguards and nuclear forensics fields. Currently, she coordinates nuclear forensics casework and cooperative nuclear forensic projects at ITU.
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CONCLUSIONS Nuclear forensic science is a methodology that gets a silent witness (namely, the nuclear material under investigation) to talk. Information inherent to the material can be revealed by correlating measurable parameters (“signatures”) to the processing history of the material. A number of “signatures” have been established for different types of nuclear material. They provide hints on the place and date of production and its intended use and thus help preventing future diversion of material for malicious purposes. They also provide useful information to investigating authorities and to law enforcement. Analytical methods are of key importance for obtaining reliable results and for deriving credible conclusions. A variety of such methods have been developed, adapted, optimized, and validated for nuclear forensic applications. Ongoing research needs to further complement the nuclear forensics toolbox by identifying further signatures and by developing and perfecting analytical techniques.
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Dr. Zsolt Varga graduated from Eötvös Loránd University (Budapest, Hungary) in 2003 as a chemist (M.Sc.) and English technical translator. After university he did his Ph.D. studies at the Institute of Isotopes of the Hungarian Academy of Sciences as a research fellow of the Academy. His focus was on the measurement of long-lived radionuclides (226Ra, Th, U, and Pu isotopes) by inductively coupled plasma mass spectrometry (ICPMS) and radioanalytical methods, and their application for the analysis of environmental, safeguards, and nuclear forensic investigations. Recently, Z. Varga joined the Institute for Transuranium Elements (European Commission−Joint Research Centre) as a research scientist, developing novel tools for nuclear forensic investigations.
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REFERENCES
(1) Nuclear Forensics Support, Reference Manual, IAEA Nuclear Security Series No. 2, Technical Guidance, International Atomic Energy Agency: Vienna, Austria, 2006. (2) Moody, K. J.; Hutcheon, I. D.; Grant, P. M.; Nuclear Forensic Analysis; CRC Press: Boca Raton, FL, 2005. (3) Mayer, K.; Wallenius, M.; Varga, Z. Chem. Rev. 2013, 113, 884− 900. (4) Wallenius, M.; Peerani, P.; Koch, L. J. Radioanal. Nucl. Chem. 2000, 246, 317. (5) Brennecka, G. A.; Borg, L. A.; Hutcheon, I. D.; Sharp, M. A.; Anbar, A. D. Earth Planet. Sci. Lett. 2010, 291, 228. (6) Krajko, J.; Varga, Z.; Yalcintas, E.; Wallenius, M.; Mayer, K. Talanta 2014, 129, 499−504. (7) Varga, Z.; Wallenius, M.; Mayer, K.; Keegan, E.; Millet, S. Anal. Chem. 2009, 81, 8327−8334. (8) Svedkauskaite-LeGore, J.; Mayer, K.; Millet, S.; Nicholl, A.; Rasmussen, G.; Baltrunas, D. Radiochim. Acta 2007, 95, 601−605. (9) Fahey, A. J.; Ritchie, N. W. M.; Newbury, D. E.; Small, J. A. J. Radioanal. Nucl. Chem. 2010, 284, 575−581. (10) Mercadier, J.; Cuney, M.; Lach, P.; Boiron, M. C.; Bonhure, J.; Richard, A.; Leisen, M.; Kister, P. Terra Nova 2011, 23 (4), 264−269. (11) Varga, Z.; Wallenius, M.; Mayer, K. Radiochim. Acta 2010, 98 (12), 771−778. (12) Keegan, E.; et al. Forensic Sci. Int. 2014, 240, 111−121. (13) Lin, D. H. M.; Manara, D.; Varga, Z.; Berlizov, A.; Fanghänel, Th.; Mayer, K. Radiochim. Acta 2013, 101, 779−784.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. Biographies Dr. Klaus Mayer is group leader at the European Commission’s Joint Research Centre, Institute for Transuranium Elements (JRC-ITU) and in charge of the Institute’s activities on combating illicit trafficking and nuclear forensics. He has over 25 years experience in nuclear science and applications and is the author of more than 150 scientific publications in this field, including peer-reviewed articles, book chapters, and conference papers. He specialized in nuclear material analysis for safeguards and nuclear security purposes, and he has many interactions with IAEA and with Euratom safeguards. Since 2004 he is co-chairman of the Nuclear Forensics International Technical Working Group (ITWG). Dr. Maria Wallenius received an M.Sc. in radiochemistry at the University of Helsinki, Finland, where she worked as a research scientist in the safeguards project, focusing on analysis of uranium fuel pellets by potentiometric titration and thermal ionization mass spectrometry. From 1996 she conducted doctoral studies at the Institute for Transuranium Elements (ITU), in Karlsruhe, Germany, in E
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Analytical Chemistry (14) Kips, R.; Kristo, M. J.; Hutcheon, I. D.; Wang, Z.; Johnson, T. J.; Gerlach, D. C.; Amonette, J. E.; Olsen, K. B.; Stefaniak, E. Proc. Radiochim. Acta 2011, 1, 7−11. (15) Klunder, G. L.; Plaue, J. W.; Spackman, P. E.; Grant, P. M.; Lindvall, R. E.; Hutcheon, I. D. Appl. Spectrosc. 2013, 67 (9), 1049− 1056. (16) Plaue, J. W.; Klunder, G. L.; Hutcheon, I. D.; Czerwinski, K. R. J. Radioanal. Nucl. Chem. 2013, 296, 551−555. (17) Varga, Z.; Wallenius, M.; Mayer, K.; Meppen, M. Proc. Radiochim. Acta 2011, 1, 1−4. (18) Varga, Z.; Ö ztürk, B.; Meppen, M.; Mayer, K.; Wallenius, M.; Apostolidis, C. Radiochim. Acta 2011, 99, 807−813. (19) Wallenius, M.; Mayer, K.; Ray, I. Forensic Sci. Int. 2006, 156, 55. (20) Ho, D. M. L.Study on the applicability of structural and morphological parameters on selected uranium compounds for nuclear forensic purposes. Dissertation University Heidelberg, Heidelberg, Germany, 2015; http://www.ub.uni-heidelberg.de/archiv/18595 (last accessed April 14, 2015). (21) Plaue, J. Forensic Signatures of Chemical Process History in Uranium Oxides. Ph.D. Dissertation, University of Nevada, Las Vegas, NV, 2013; http://digitalscholarship.unlv.edu/thesesdissertations/1873 (last accessed April 14, 2015). (22) Wallenius, M.; Mayer, K. Fresenius' J. Anal. Chem. 2000, 366, 234. (23) Wallenius, M.; Tamborini, G.; Koch, L. Radiochim. Acta 2001, 89, 55. (24) Wallenius, M.; Morgenstern, A.; Apostolidis, C.; Mayer, K. Anal. Bioanal. Chem. 2002, 374, 379. (25) Varga, Z.; Wallenius, M.; Mayer, K. J. Anal. At. Spectrom. 2010, 25, 1958−1962. (26) Williams, R., Hutcheon, I., Kristo, M., Gaffney, A., Eppich, G., Goldberg, S., Morrison, J., Essex, R. Radiochronometry by Mass Spectrometry: Improving the Precision and Accuracy of Age-Dating for Nuclear Forensics, LLNL-CONF-655059; https://e-reports-ext.llnl. gov/pdf/775834.pdf (last accessed April 14, 2015). (27) Stanley, F. E. J. Anal. At. Spectrom. 2012, 27, 1821−1830. (28) Stanley, F. E.; Stalcup, A. M.; Spitz, H. B. J. Radioanal. Nucl. Chem. 2013, 295, 1385−1393. (29) de Laeter, J. R. Applications of Inorganic Mass Spectrometry; John Wiley: New York, 2001. (30) Platzner, I. T. Modern Isotope Ratio Mass Spectrometry; John Wiley: New York, 1997. (31) Richter, S.; Kühn, H.; Aregbe, Y.; Hedberg, M.; HortaDomenech, J.; Mayer, K.; Zuleger, E.; Bürger, S.; Boulyga, S.; Köpf, A.; Poths, J.; Mathew, K. J. Anal. At. Spectrom. 2011, 26, 550. (32) Becker, J. S.; Dietze, H.-J. Spectrochim. Acta, Part B 1998, 53 (11), 1475−1506. (33) Montaser, A., Golightly, D. W., Eds. Inductively Coupled Plasmas in Analytical Atomic Spectrometry, 2nd ed.; VCH: New York, 1992.
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