Nuclear Forensic Science: Correlating Measurable Material

Review pubs.acs.org/CR

Nuclear Forensic Science: Correlating Measurable Material Parameters to the History of Nuclear Material Klaus Mayer,* Maria Wallenius, and Zsolt Varga European Commission Joint Research Centre Institute for Transuranium Elements, Postfach 2340, 76125 Karlsruhe Germany material carries signatures of the materials from which it was made and the processes to which it was subjected. Signatures are to be understood as measurable parameters or a combination of parameters that allow drawing conclusions on the provenance and history of the material. These signatures may be the result of chemical operations (such as dissolution, extraction, ion exchange, precipitation) or they may be associated with physical processes (such as neutron irradiation or radioactive decay). Signatures may be source material inherited (e.g., natural uranium carrying impurities originating from the uranium ore) or process inherited (e.g., Gd is added to nuclear fuel in order to obtain higher burn-up). In addition to such intentionally added “impurities”, some unintentionally CONTENTS added impurities may provide useful information (e.g., corrosion products from the storage or reaction with vessel 1. Introduction 884 2. Isotopic Signatures of Plutonium and Uranium 884 material). Uranium and plutonium both contain fissile isotopes, 3. Age Dating 887 substantiating the broad interest in the history, origin, and 3.1. Age Dating of Uranium 887 intended use of these materials. 3.2. Age Dating of Plutonium 889 Nuclear forensic science relies on principles of and combines 4. Chemical Impurities 891 methodologies of radiochemistry, nuclear physics, material 4.1. Anionic Impurities 892 sciences and other scientific disciplines. Radiochemical 4.2. Metallic Impurities 892 principles and radioanalytical techniques play a prominent 4.3. Isotopic Composition of Selected Impurities 893 role within the nuclear forensics toolset. The present review 5. Morphology of Nuclear Material Powders 894 provides an overview of the scientific discipline of nuclear 6. Conclusions and Outlook 897 forensics and summarizes the findings presented in published Author Information 897 literature giving some emphasis to the area of radiochemistry. Corresponding Author 897 Notes 897 Uranium and plutonium are the materials of primary concern Biographies 897 to nuclear forensic investigations. The physical basis of nuclear Nomenclature 898 forensic science, the radiochemistry and the analytical methods References 898 applied have been described by Moody et al.3 The monograph provides examples of the application of nuclear forensics, including the use of classical forensic methods on material 1. INTRODUCTION associated with the seized nuclear sample (such as packing Nuclear forensic science, referred to as nuclear forensics, has material, lead shielding etc). In the present review, we will focus been defined as the “analysis of intercepted illicit nuclear or on signatures and methodologies related to uranium and radioactive material and any associated material to provide plutonium. Signatures in uranium include the isotopic 1 evidence for nuclear attribution”. More recently, nuclear composition of uranium, the chemical impurities, and the forensics has been described as the “scientific analysis of nuclear date of the last chemical purification (referred to as “age” of the or other radioactive material, or of other evidence that is material), the morphology, and the isotopic signatures of contaminated with radioactive material in the context of legal selected trace elements. Signatures in plutonium include the proceedings, including administrative, civil, criminal or international law”.2 This scientific analysis aims at providing clues on isotopic composition of plutonium, the “age” of the material, the intended use of the material and on its history, providing and the morphology. investigative leads and possibly leading to a source attribution. In the first instance, nuclear forensic analysis reveals Special Issue: 2013 Nuclear Chemistry information inherent to nuclear material. Nuclear material has either been subject to technological processing to various extent Received: July 6, 2012 or is entirely of anthropogenic origin. Consequently, nuclear Published: November 29, 2012 © 2012 American Chemical Society

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dx.doi.org/10.1021/cr300273f | Chem. Rev. 2013, 113, 884−900

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Luksic7 concluded that the isotopic composition of reactor produced plutonium allows not only determination of the reactor class (242Pu compared to 240Pu), but also the date of discharge (241Pu relationship to 240Pu and 242Pu) and the time since the last chemical processing (241Am compared to 241Pu). The application of chemometric techniques for identification of the source of a plutonium sample is described by Alamelu and Aggarwal.8 They used Pu isotope abundances experimentally determined for Indian reactors and literature values for other types of reactors, thus covering a wide range of plutonium isotopic compositions ranging from very low burnup material (obtained from CIRUS or Dhruva research reactors) to high burn-up plutonium (as typically obtained from pressurized water reactors (PWR) or boiling water reactor (BWR)). Using the principal component analysis (PCA) methodology they could identify a strong correlation between 242 Pu and 238Pu and a negative correlation of 239Pu and 240Pu respectively with the other Pu isotopes. These observations confirm findings reported earlier by Mayer et al.9 Robel and Kristo10 also applied multivariate statistics for identifying the source of reactor produced plutonium using a combination of PCA and partial least-squares discriminant analysis (PLSDA). They conclude that all reactor types and fuel combinations are well resolved starting very early in the core life. PLSDA appears to be the superior tool for this classification problem, as it selects axes that maximize discrimination between classes. In either case (PCA or PLSDA), however, the best classification results were obtained using only plutonium isotopic data, that is, disregarding the respective uranium isotopic abundances. The neutronics calculations of Glaser11 focused on weapons grade plutonium (typically with higher than 93% 239Pu abundance) produced in natural uranium fuelled reactor types. He found that the calculated Pu isotopic compositions did not allow distinguish between the reactor types under investigation. Also Nicolaou12 used multivariate statistical analysis for discrimination between classes (of reactor produced plutonium) and for comparing “unknown” plutonium to these classes. That study used Pu isotopic compositions as calculated with ORIGEN-213 for different reactors types including a fast reactor. He found that plutonium isotope abundance serves as tools for determining the provenance of unknown plutonium. The sensitivity of the method was found to be sufficient for resolving fuels of similar initial uranium enrichment but subjected to different neutron energy distribution in the reactor used. A comparative analysis of light water reactor (LWR) and fast breeder reactor spent fuels for nuclear forensics evaluation was performed by Permana et al.14 They noted significant differences in the plutonium isotopic composition, as can be expected for reactors with significantly different neutron energy spectra. Correlations of a limited scope were investigated by Bignan et al.15 They analyzed plutonium data obtained for PWR, BWR, and Magnox reactors and a found consistent correlation for the two types of light water reactors which is distinctly different from Magnox plutonium. Similarly, Joe et al.16 examined isotope correlations of plutonium in high burnup PWR spent fuels from Korean power plants. They observed a linear relationship between the 238Pu/(239Pu + 240Pu) ratio and the burn-up of the fuel. The slope and intercept of the correlation depended on the initial 235U enrichment of the fuel

2. ISOTOPIC SIGNATURES OF PLUTONIUM AND URANIUM Plutonium is produced by neutron irradiation of uranium. The most prominent reaction is 238U capturing a neutron, transmutating to the short-lived 239U (T1/2 = 23.5 min) which undergoes β− decay to 239Np which subsequently decays (by β− decay) to 239Pu. Under neutron irradiation 239Pu can fission but it can also capture the neutron and form 240Pu. If the fuel remains for long enough time in the reactor, the plutonium produced will continue to capture neutrons (in competition to the neutron induced fission) and build up higher mass plutonium isotopes, such as 241Pu and 242Pu. Different routes lead to the formation of 238Pu: after having captured a neutron 238 U can also emit two neutrons in a so-called (n,2n) reaction, leading to the formation of 237U which rapidly decays to 237Np. The latter can then capture a neutron and after subsequent β− decay become 238Pu. Alternatively, (n,γ) reactions starting at 235 U will also result in 238Pu. Obviously, the longer nuclear fuel remains in the reactor and is exposed to neutron irradiation, the more plutonium and the more of the heavier plutonium isotopes in particular will be built up. The probability of the different neutron reactions (capture and fission) is a function of the neutron energy. While (n,γ) reactions are the prevailing nuclear reactions with thermal neutrons (i.e., neutrons with low kinetic energy), fast neutrons are required for (n,2n) reactions. As different types of reactor show different neutron energy distributions, the relative yield of plutonium isotopes generated by neutron capture will vary for different reactor types. As a consequence, the isotopic profile of plutonium present in the nuclear fuel after irradiation will serve as signature of the type of reactor it was produced from. Obviously, the difference in isotopic composition between different reactor types will increase with increasing irradiation time. At a minimum, this opens the possibility to discern spent fuels from groups of reactors (with a similar neutron energy spectrum) from each other. The ultimate isotopic composition is a function of several parameters, such as neutron energy spectrum, initial 235U enrichment and neutron flux. A first systematic study on the origin determination of reactor produced plutonium was presented by Wallenius.4 A correlation of the relative abundance of plutonium isotopes as shown in Figure 1 allows possible conclusions to be drawn on the type of reactor in which the plutonium was produced.

Figure 1. Plutonium isotope correlations for different reactor types as calculated using SCALE/ORIGEN,5 taken from Wallenius et al.6 885


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spike technique allows for even lower uncertainty values associated with the n(234U)/n(238U) ratios.23 The value of the n(234U)/n(238U) ratio as signature in nuclear forensic science has been investigated by several researchers and made use of in several publications.22,24 It has to be recognized however, that the variations in uranium isotope ratios of natural uranium material from different geographic (i.e., geological) origins do not allow a unique attribution to the uranium mine. The uranium isotopic signature may, however, enable attribution of an unknown natural uranium sample to a certain class of geological origins. While most textbooks describe natural uranium as being composed of the isotopes 234U, 235U, and 238U, high sensitivity measurement techniques, such as accelerator mass spectrometry (AMS), have proven the presence of ultratrace amounts of 236 U and 239Pu in natural uranium.25−27 Wilcken et al.25 have found n(236U)/n(238U) ratios in the range 12 to 242 × 10−12 and Steier et al.27 measured ratios in ore and yellow cake samples in the range 6 × 10−11 to 6 × 10−12. These ultratraces of 236U have been produced by the neutron capture reaction of 235 U (n, γ) 236U. The neutrons for this reaction are provided from the spontaneous fission of 238U or from (α, n) reactions in the matrix rock. Variations in the 236U abundance in uranium ores arise from differences in the U content of the ore and in the matrix material (which provides the moderation of the neutrons or the (α, n) reaction). The question of whether these variations may serve as an indicator in nuclear forensic investigations was addressed by Srncik et al.28 Mine-to-mine variations of the ore samples from mines in Canada, Australia, and Brazil were observed, suggesting that the n(236U)/n(238U) ratio might serve as a nuclear forensic indicator. They noted however that yellow cake samples would be more appropriate rather than uranium ores as this would eliminate to some extent the variability arising from inhomogeneity in the ore body. The presence of 236U in higher amounts than these background levels in uranium samples is a clear indication of previous neutron irradiation and reprocessing of the uranium. In most of today’s power reactor uranium fuels the n(236U)/ n(238U) ratio is well above the level found in unprocessed natural uranium or that produced solely by enrichment of natural uranium because such materials are often produced from uranium which has been blended with or contaminated with recycled uranium. As a consequence, the n(236U)/n(238U) ratio in fresh nuclear fuels may only serve as reliable nuclear forensic indicator if it can be compared against a database in which this information has been compiled. Varga and Suranyi developed an analytical method combining laser ablation ICP-MS and alpha spectrometry for quantifying 232U and 236U as indicators of neutron capture.29 The application of secondary ion mass spectrometry (SIMS) for determining the isotopic composition of uranium and plutonium in micrometer sized particles and its use for the detection of 236U have been described by Betti et al.30 The main isotope ratio n(235U)/n(238U) in fresh fuel provides a good indication of the intended use of the material. In combination with other parameters, such as the pellet geometry or the presence of neutron poisons (e.g., Er, Gd), a useful nuclear forensic signature is obtained. Because of the large variability of uranium isotopic compositions in fresh fuel and the spectrum of different reactor types, the isotopic composition of uranium after irradiation is of limited use in nuclear forensics as determined by Robel and Kristo.10

and on the cooling time of the fuel following the end of irradiation. Overall, the plutonium isotopic composition has been found to be a very valuable signature for nuclear forensic purposes. It provides conclusions on the type of reactor in which the plutonium was produced and suggests on the initial enrichment of the uranium fuel. Such correlations have already been used successfully for provenancing unknown plutonium containing materials as described by Wallenius et al.17 The isotopic composition of uranium may vary widely. The rough categories usually referred to are depleted uranium (235U abundance less than 0.72%), natural uranium (235U = 0.72%), low enriched uranium (235U abundance higher than 0.72% and lower than 20%), and highly enriched uranium (235U abundance higher than 20%). Among the natural uranium materials uranium ore concentrates, often referred to as yellow cakes, play a special role as the first purified starting (feed) material for uranium fuel production. This term generally covers a wide range of uranium compounds (e.g., ammonium diuranate, oxides, peroxides), which are produced (milled) from uranium ores or secondary sources (e.g., phosphorites), and contain high amount of uranium (typically above 65% by mass). Throughout this document, we will exclusively use the term “uranium ore concentrate” as it provides the technically most appropriate material description. The 235U isotope abundance in natural uranium was taken for many years as a constant. More recent high precision, high accuracy measurement methods suggest that very small variations in the n(235U)/n(238U) ratio can be attributed to natural isotope fractionation.18 These minute differences have, however, can be exploited for nuclear forensic investigations. Brennecka et al.19 observed differences of 0.04‰ between low temperature uranium deposits and uranium deposited at high temperatures or by nonredox processes. In contrast to that, the n(234U)/n(238U) ratio shows much higher relative variations which are due to the preferential leaching of 234U from uranium ore bodies. This can be understood by the recoil the daughter nucleus experiences after alpha decay of the parent nuclide 238 U. As a consequence of this recoil, the daughter atom is less strongly bound in its chemical environment than the parent atom. As early as in 1963 Koide and Goldberg20 observed that the 234U/238U activity ratio in seawater is significantly higher than the equilibrium value (1.00) observed in uranium minerals. As a consequence of the preferential leaching of 234 U, the remaining uranium mineral will show a slightly reduced n(234U)/n(238U) ratio. Uranium deposits with (234U)/ (238U) activity ratios higher than the equilibrium value may have been generated by redeposition of leachates. A number of natural uranium samples of different geographic origins were investigated for small variations in n(234U)/n(238U) ratios by Ovaskainen21 using a thermal ionization mass spectrometer (TIMS) equipped with a retarding potential quadrupole energy filter for enhanced abundance sensitivity. Small, but significant differences between different groups of uranium ore samples could be identified. An improved version of this energy filter allowed Richter et al.22 to determine the n(234U)/n(238U) ratios with even better precision. Measuring samples from mines in Germany, France, Australia, Finland, Canada, Czech Republic, Namibia and Gabon he noted values ranging from 5.154(28) × 10−5 to 5.460(41) × 10−5. The very high value that was observed for the sample from Czech Republic (Straz pod Ralskem), can be explained by the nature of the ore deposit which is not a genuine ore but rather a redeposit. A double886


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Figure 2. Variation of major progeny-to-parent nuclide atom ratios for the most important nuclear radionuclides as a function of time.

3. AGE DATING Nuclear materials, just like other radioactive materials, contain radioactive nuclides, which decay over time. This unique property can be exploited to measure the time elapsed since the last chemical processing of the material. This time is generally referred to as the production date or age of the nuclear material. The age determination technique is based on the fact that progenies (decay products or “daughters”) of the nuclear materials are continuously growing-in within the nuclear material after the last chemical separation of the base material because of the decay of radioactive uranium or plutonium. The amounts of decay products are proportional to the amount of parent nuclide and the time elapsed since the last chemical separation (of parent and daughter nuclides). The daughter/ parent ratio used is often referred to as the chronometer of the age measurement. Thus, if the amount of decay products, which are present at trace-level in most cases, relative to their parent nuclide is measured, the time of their last chemical modifications (separation) can be calculated. These methods are based on the radioactive decay equations, and usually some assumptions or simplifications are necessary so that the unknown (the time elapsed since separation) can be analytically calculated. One assumed simplification is, for instance, the complete removal of the progenies. Figure 2 shows the concentration variation of main progenies for the most widely employed nuclear radionuclides as a function of the time. The chronometer used for the age calculation usually also defines which analytical techniques can be used for the analyses.

This signature was found to be highly valuable in nuclear forensic investigations as being a predictive characteristic (i.e., no comparison sample or information is necessary) in comparison to the other parameters used for origin assessment.31,32 3.1. Age Dating of Uranium

During the past decade several age dating methods have been reported for uranium bearing nuclear materials using various chronometers, such as 214Bi/234U, 230Th/234U, or 231Pa/235U. Among the nondestructive methods high resolution gamma spectrometry (HRGS) was found to be applicable for the age measurements of highly enriched uranium samples.33,34 The age of the material can be measured without alteration or dissolution of the material using the 214Bi/234U ratio, even at on-site inspections. Advantages of this technique are the relatively low instrumentation costs, virtually no sample preparation is needed, and the quantification can be performed using intrinsic efficiency calibration, so no calibrator or standard is required. A disadvantage of this chronometer is the longer ingrowth time of 214Bi compared to other chronometer systems: as 214Bi is several steps away from the parent 234U nuclide with a few longer-lived intermediate progenies in the decay chain, such as 230Th and 226Ra, longer time is necessary for 214Bi can be formed in measurable quantities. Therefore, the technique is more suitable for higher enriched and older uranium materials. Gamma spectrometry has also been applied to the dating of fissile 233U using the 229Th/233U chronometer.35 The method was compared with inductively coupled plasma mass 887


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counting statistics, the reported figures of merit in terms of the sample amount and chemical separations are comparable.39 In the last decades, several methods have been described using ICP-MS not only for the general impurity analysis but also for the high-precision age measurement of uraniumbearing materials. Initially the multicollector ICP-MS (MCICP-MS) was studied to measure the 234U/230Th parent/ daughter ratio directly without chemical separation.39 The reported overall uncertainty is comparable to that for achieved by TIMS methods,38,39 and the decreased sensitivity of 230Th caused by high uranium concentrations can be corrected by empirically measured values using synthetic Th/U mixtures. The sensitivity of the method, and thereby the minimum age and amount of the material, can be significantly decreased using isotope dilution with 229Th or 232Th as isotopic tracers and by subsequent chemical separation and matrix elimination.36,41 By this means it is also possible to date natural or depleted uranium materials. The direct measurement can be improved by the use of isotope dilution to eliminate the matrix effects or by using spectral deconvolution to decrease the effects of the spectral interference (i.e., tailing) of the neighboring highly abundant uranium peaks on the 230Th signal.41 An important aspect of the methods discussed above is that they are all based on the assumption of the complete parent/ daughter chemical removal at the time of manufacture. The measured age values of the uranium materials are thus referred to as model ages (i.e., calculated and derived age values based on the measured parent/daughter ratio) and were found to be significantly higher than ages calculated from the archive data on the preparation of these materials.42 This discrepancy was attributed to the incomplete separation of the uranium material, and a small systematic bias caused by the residual 230Th present. The same phenomena, but higher extent, was observed for uranium ore concentrates (yellow cakes), where the 230 Th/234U chronometer gave unrealistically high ages due to the excess residual 230Th present. 43 In this work, an alternative chronometer, 228Th/232Th was applied to determine the age of such materials. As this chronometer is based on the variation of the isotope ratio of the same element, the completeness of Th/ U separation is not a necessary condition. The effect of residual 228 Ra, which can cause a systematic bias, was corrected using gamma-ray spectrometric measurement; however, it was found to be too low in the samples investigated, which was attributed to the low solubility and separation of RaSO4 during the material production. The method developed was demonstrated to be applicable for the dating of uranium ore concentrates in several cases, but it was limited by the higher detection limits of the alpha spectrometry measurement. Its use is limited to uranium-bearing samples with an age of less than approximately 30 years due to the lower half-life of 228Th (T1/2 = 1.91 a). An important aspect of the age dating of uranium materials is that currently there are no certified uranium age dating reference materials are available.44 This role is more emphasized as the materials are forensic evidence specimens, where the credibility of measurement is of utmost importance. In contrast to plutonium, the available Th/U chronometers are limited, thus using different parent/daughter ratios and comparing them is not possible. To address this concern uranium age dating reference materials are currently under preparation using different approaches: either by the recertification of uranium isotope reference materials for 230 Th/234U ratio45 or by the assured complete separation of

spectrometry (ICP-MS) and was also used under field conditions. Laser ablation coupled to inductively coupled plasma mass spectrometry (LA-ICP-MS), as a quasi-nondestructive technique, has also been successfully applied for the age measurement of uranium materials.36 The technique uses a high energy laser to evaporate a small portion of the material from the surface, which is then transferred to the mass spectrometer by a carrier gas (argon) in the form of a fine aerosol. The technique consumes only a small amount of sample (less than a few micrograms in total) and does not require the dissolution of the material for the mass spectrometric measurement. The quantification of the 230Th/234U chronometer used in this technique was carried out using the relative sensitivity factor method: thus it requires a standard material with known 230Th content and similar chemical composition to that of the sample so it can be analyzed. The method has a higher sensitivity than gamma spectrometry and can be applied to lower-enriched materials, but it has a higher detection limit compared to the destructive mass spectrometric methods. Moreover, the method can be prone to molecular interferences, which, if not eliminated, for example, by higher mass resolution, may lead to inaccurate results. However, as the analysis can be performed rapidly, within a few hours without significant alteration of the evidence sample it was found to be a highly useful tool in nuclear forensics. Of the destructive techniques, various mass spectrometric methods and alpha spectrometry have been widely applied for the age determination of uranium materials. Using these techniques, low-level measurement of the longer-lived progenies (e.g., 230Th or 231Pa) can be used to date the uranium-bearing materials.37 Thermal ionization mass spectrometry (TIMS) was first used to determine the age using the 230 Th/234U chronometer system.38,39 As 230Th is present at trace-level, it has to be chemically separated from the matrix to avoid matrix effects. Either ion exchange separation38 or extraction chromatographic separation.39 can be applied for the 230Th separation, and 230Th/234U chronometer can be quantified by isotope dilution using 233U and 229Th tracers. Although very precise and accurate results can be obtained compared to the nondestructive gamma spectrometric technique or to LA-ICP-MS, the chemical separation is usually tedious and labor-intensive. Another important issue from nuclear forensic aspects is that for lower-enriched uranium materials larger sample amounts (up to gram amount in the case of natural uranium) was reported to be necessary. Alpha spectrometry can also be effectively used for the measurement of the age of uranium materials either by the 230Th/234U or by the 231Pa/235U chronometer.39,40 As the activity of the parent uranium material are typically several orders of magnitude higher than those of the measurand decay products, thorough separation of the analyte has to be performed either by extraction chromatography39 or adsorption on silica gel.40 Alpha spectrometry (AS) was also successfully applied for the age measurement by the 231Pa/235U chronometer, where the chemical recovery estimation of the 231Pa decay product is hampered by the lack of availability of a longer-lived tracer. In this case, use of gamma spectrometry for the 233Pa chemical recovery measurement combined with the alpha spectrometry analysis for the 231Pa/235U ratio can be a method of choice.40 The alpha spectrometric analysis is typically a lower-cost option for the age measurements, and although the overall precision of the technique is slightly lower due to the limitations of the 888


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43 5−10% 35 mg AS and ICP-MS Th/232Th 228

uranium ore concentrates

Th/234U 230

MC-ICP-MS

10−100 mg

0.5%

high sensitivity and good precision, applicable also for depleted uranium applicable for incompletely separated uranium materials

tedious chemical separation

42

41 3−5% about 500 μg Th/234U 230

dissolved uranium oxides uranium oxides

Th/234U 230

ICP-MS

a few percent

Pa/235U 231

ICP-MS

5%

10−100 mg

high sensitivity and good precision, applicable also for depleted uranium rapid, high sensitivity, no chemical separation is needed

worse precision and detection limit than with separation tedious chemical separation

36

40

tedious chemical separation, chemical recovery estimation difficult tedious chemical separation

39 tedious chemical separation accurate and precise 1−5%

above a few milligrams (HEU) 10 mg Th/234U 230

TIMS, AS, MCICP-MS AS, ICP-MS

rapid, quasi nondestructive, applicable for LEU, HEU accurate and precise 15% a few percent few micrograms up to 1 g LA-ICP-MS TIMS Th/234U Th/234U 230

230

HRGS, ICP-MS Th/233U 229

few milligrams to grams

1−8%

HRGS, nondestructive, field-use; ICP-MS, better precision

molecular interferences tedious chemical separation

36 38

35

applicable only for highly enriched or old materials nondestructive, no calibrant needed 10−15% several grams HRGS

advantages typical uncertainty sample amount used analytical technique chronometer used

Bi/ U

highly enriched uranium 233 U solution and 233 U3O8 uranium oxides natural uranium oxide highly enriched uranium highly enriched uranium uranium oxides

refs 889

sample type

Table 1. Overview of Methods for the Production Date Determination of Uranium-Bearing Nuclear Materials

disadvantages

The decay of plutonium isotopes can be utilized in two ways in nuclear forensics. First, as discussed already in section 2, the isotopic composition of freshly discharged reactor plutonium is different than, for instance, that of plutonium that has been stored for several years due to the rapid decay of the shortest living plutonium isotope 241Pu (T1/2 = 14.35 a), and therefore, the time of discharge can be determined.7 Second, the amount of decay products, that is, daughter products, can be measured and based on their relation to the respective parent plutonium isotopes, the “age” of the plutonium can be determined. This latter case will be discussed in detail in the following. The separation of decay products from plutonium could have taken place either when the material was reprocessed or, for separated plutonium, after a long storage period, when the material was purified from the ingrown 241Am. Americium is not a desired product in Pu because it is a strong gamma emitter and therefore causes a high radiation dose to workers. Thus, the determined age does not necessarily reflect the original date of chemical processing of the plutonium, but the last purification process. The age of plutonium can be determined, in principle, using five different parent/daughter relations: 238Pu/234U, 239Pu/235U, 240 Pu/236U, 241Pu/241Am, and 242Pu/238U. However, in practice, only the first four ratios are used (Figure 3, blue and red lines). The reason for this is that 242Pu has a very long half-life (T1/2 = 3.75 × 105 a), which results in only a minute amount of ingrown 238U in a relative short time, for instance in 50 years. In addition, the daughter nuclide 238U is the most abundant uranium isotope, therefore even a small amount of crosscontamination (e.g., with natural uranium) would cause positively biased results. Besides the above-mentioned parent/ daughter ratios, also parent/granddaughter relations can be utilized (Figure 3 green lines), however typically their amounts are too low to be quantified precisely. The prerequisite for a correct age result is that the daughters and granddaughters were removed completely when the chemical separation took place. If this was not the case, the determined result will be positively biased as shown by Mayer et al.47 In this study, blending calculations were performed for weapons and reactor grade Pu to demonstrate the degree of bias in case of residual uranium resulting from incomplete U/ Pu separation during the processing. The greatest influence was found for Pu of weapons grade composition with increasing amount of uranium impurity and with decreasing decay time (i.e., time after separation). Because of the multiple parent/daughter ratios available for the Pu age determination, including chronometer systems containing two different elements (Pu/U and Pu/Am), it is fairly easy to detect inconsistencies. Especially the aforementioned incomplete separation of daughter nuclides can be perceived easily. If all four ratios give the same age, one can be reasonably sure that the separation of daughter from parent was complete when the material was processed. Because of the two different parent−daughter elemental ratios available (i.e., Pu/U and Pu/Am) and the isotopes of Pu having orders of magnitude differing half-lives, there are

234

3.2. Age Dating of Plutonium

214

thorium from uranium materials at a known and welldocumented date.46 Table 1 provides an overview of different chronometers that can be used for uranium age dating, the respective measurement methods, suggested sample size and advantages as well as disadvantages.

33, 34

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Figure 3. Amount (in number of atoms) of the parent (blue line) and most abundant daughter (red line) and granddaughter (green line) nuclides for plutonium as a function of time.

different techniques on hand for the age determination of plutonium. Initially, a method for the age determination of plutonium was developed for nuclear safeguards purposes, where the age was determined by high resolution gamma spectrometry (HRGS) from the 241Pu/241Am ratio by Parus et al.91 Later a similar method was applied to determine the age of plutonium−beryllium neutron sources by Nguyen.48 The results obtained from the 241Am/241Pu activity ratio agreed in most cases (within the method uncertainty of a few percent) with the date of the neutron source certificates, which were regarded as reference dates. The advantage of the age dating, when performed by HRGS, besides being a nondestructive technique, is that it is also quick and does not require any chemical sample preparation. The main disadvantage, on the other hand, is as it is based only on one parent-daughter ratio, that one cannot know if the determined age is positively biased if any Am was left in the material when processed. However, Keegan et al. have developed a method, which can overcome this problem.92 This method is also based on HRGS measurement, but the gamma-ray emission of 233Pa (which is in secular equilibrium with 237Np) is measured together with the 241Pu and 241Am. The amount of 237Np at t = 0 (N30) can be calculated based on the measurements and the age is then obtained with the help of a graphical function f(t), where the time of separation is at the minimum of the curve (Figure 4). Age determination methods based on multiple parent/ daughter ratios utilize typically various mass spectrometry

Figure 4. Graph of f(t) versus time. The minimum on the curve is the age of a Pu sample.50

techniques (TIMS, ICP-MS) combined with chemical separation of the elements. Wallenius et al.49 developed a method using four ratios, namely 238Pu/234U, 239Pu/235U, 240Pu/236U, and 241Pu/241Am. The method tested two different chemical separation procedures, anion exchange (Dowex 1 × 8) and ion 890


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one should note that the HRGS is applicable only for larger samples, that is, >100 mg. Chen et al.53 compared different methods combining various techniques, such as MC-ICP-MS, AS, and liquid scintillation counting (LSC), for the age determination from the 240Pu/236U and 241Pu/241Am ratios after spiking and ion chromatography separations (using TEVA and TRU resins). The study showed that for ng sized Pu samples the MC-ICP-MS provides most accurate and precise results with typical precision of 1.5−3%. The second method, combination of MC-ICP-MS and AS (for 241 Am) from the 241Pu/241Am ratio, could compete in accuracy with the “pure” MS method. However, the third method, a combination of AS and LSC produced 5−10% negatively biased results. The reason for this difference was attributed to the LSC measurement of the 241Pu. Due to the moderately short half-lives of Pu isotopes 238Pu, 239 Pu, and 240Pu (87.74, 24110, and 6563 a, respectively), the age determination can be performed also for plutonium particles (μm sized grains correspond to pg amount of Pu in weight) as detectable amounts of daughter nuclides are grownin even in a few years decay time. Wallenius et al.54 developed a method, which uses direct measurement (i.e., without chemical separation) of 238Pu/234U, 239Pu/235U, and 240Pu/236U ratios by SIMS. To compensate for the ionization efficiency difference of U and Pu in such measurements, a reference sample of known age needs to be available to determine so-called relative sensitivity factor (RSF) for the Pu/U ratio beforehand. The method was also tested for a mixed U- and Pu-oxide (MOX) powder sample. In such a case the age from the 240Pu/236U ratio was observed to be closest to the true value, whereas the ages determined from the other two ratios (238Pu/234U and 239 Pu/235U) were suffering from U interference (i.e., 238U interfered with the 238Pu causing too low age and the 235U from U particles interfered with the ingrown 235U causing too high age). Another method for Pu particle age determination, however utilizing chemical separation of Pu and Am, was developed by Shinonaga et al.55 In that method, the elemental Am/Pu ratio was first determined by wavelength dispersive X-ray spectrometry (WDX). Later, the single particles were picked up by micromanipulators and chemically separated using a combination of anion exchange (AG MP-M1) and ion chromatography (TRU) resins. The separated Pu fraction was then measured by TIMS. Typically precision of 2−3% was achieved using this method, which is comparable with the method developed by Wallenius et al.54 Age determination of plutonium has been demonstrated also in a few case studies. For instance Schwantes et al.56 could determine the age for the oldest man-made 239Pu sample and Wallenius et al.17 reported several real cases of nuclear forensic investigations of plutonium-containing materials.

chromatography (UTEVA) for the Pu/U/Am separation. The chemically separated and spiked Pu and U fractions were measured subsequently by TIMS, whereas the 241Am/241Pu ratio was measured directly by HRGS without chemical separation. All three Pu/U parent/daughter ratios gave consistent results with the typical precision of 3500 for Pu in the Amfraction and ∼700 for Am in the Pu-fraction with a recovery between 60 and 75%. The measurements of the spiked Pu, U, and Am fractions to calculate the ages from the 240Pu/236U and 241 Pu/241Am ratios were performed by ICP-SFMS. The obtained results for two reference materials and two environmental samples were very accurate having typical precision of ∼2% for the age determined from the 241Pu/241Am ratio. Spencer et al.51 reported interlaboratory comparison results on age determination for submilligram sized Pu metal samples. The method used is based on chemical separation of the elements by anion exchange and measurement of the separated Pu and U fractions by TIMS and the Am fraction by ICP-MS and/or by HRGS. The interlaboratory comparison demonstrated the best agreement on results from the 239Pu/235U and 240 Pu/236U ratios (relative difference 750 °C) is seen in the increased number of small size particles (Figure 14). However, the mass distribution of particles is not significantly affected. No growth in grain volume occurred during calcination, therefore the principle parameters of the particle size distribution for the oxide were defined by the physicochemical conditions of the oxalate precipitation, similar to the previous studies of U powders. However, the grain shape and porosity were subjected to modifications, for example, higher calcination temperature increased the proportion of smooth grains. This study also confirmed the fact that the geometry of the grains obtained depends on the synthesis procedure. The synthesis of PuO2 by thermal decomposition of tetravalent plutonium oxalate did not produce crystals other than irregular and truncated octahedral ones, whereas the crystals obtained by slow crystallization in molten salts (e.g., Li2·0.2MoO3) have been reported to have octahedral shapes. White et al. have studied the effect on thorium oxalate precipitation conditions of thoria powder.88 The study revealed that the precipitation temperature had the most graphic effect upon morphology, that is, the higher temperature, larger particle size (Figure 15). The agitation type (mechanical vs mechanical combined with ultrasonic) had hardly any effect on the platelet size, however, the platelet shape was different (cubes versus anhedral, respectively). When uranium was coprecipitated with thorium, the morphology was found to be identical to that of thoria particles. Hingant et al. used two different wet chemistry methods for the preparation of U/Th dioxides.89 The morphology of the obtained dried precipitates showed large differences depending on the preparation method (open system in a beaker vs closed system in a Parr acid digestion bomb). Also the consequent heating to 673 K to transform the oxalate to oxide affected the morphology of hydrothermally formed (i.e., closed system) crystals drastically (Figure 16). For the product obtained by the open system, the heating did not change the morphology. In summary, the above-discussed studies have shown that the morphology of U, Pu, or Th oxide powders is extensively determined by the initial morphology of the starting material. The precipitation conditions (e.g., time, temperature, concentration) affect drastically the morphology (e.g., particle size, shape) of the obtained product. The calcination temperature

was noticed to have lesser if any effect on the particle size and shape, though the porosity was often modified. However, these studies have also demonstrated that the powder morphology can be affected relatively easily. Therefore, one should consider the above-discussed factors more as a help to obtain additional information on the material rather than “written in stone”-type of information when drawing conclusions about the possible processes the nuclear material has gone through.

6. CONCLUSIONS AND OUTLOOK Nuclear forensic science is a new discipline in nuclear research, which includes aspects of radiochemistry, material science, nuclear physics, and analytical chemistry in combination with knowledge of the nuclear fuel cycle. Investigations on nuclear material intercepted from illicit trafficking started in the early 1990s. In parallel, systematic studies were launched in order to develop a better scientific understanding of the mechanisms leading to the signatures inherent to the material and to establish and improve the analytical methods for identifying these signatures. In consequence, nuclear forensic science has matured and is today getting a lot of attention due to its potential application in nuclear security, nonproliferation, nuclear safeguards and law enforcement contexts. Still, significant research and development activities, as well as efforts in training and education are being undertaken to further advance the discipline of nuclear forensic science. “Nuclear Forensics: Role, State of the Art, Program Needs”, a 2008 study90 by the Joint Working Group of the American Physical Society (APS) and the American Association for the Advancement of Science (AAAS) underlined the need for continued R&D and for increasing the workforce in this important area. AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies

Klaus Mayer has over 20 years experience in nuclear science and applications and is the author of more than 120 scientific publications in this field, including peer-reviewed articles, book chapters, and conference papers. He specializes in nuclear material analysis for safeguards purposes, and he has many interactions with IAEA and with Euratom safeguards. In recent years, the analysis of nuclear material of unknown origin, also referred to as “nuclear forensics” has been in the focus of his research. From 1997 to 2010, he chaired the ESARDA 897


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ICP-MS

inductively coupled plasma mass spectrometry laser ablation coupled to inductively coupled LA-ICP-MS plasma mass spectrometry LIBS laser induced breakdown spectrometry LSC liquid scintillation counting LWR light water reactor MC-ICP-MS multicollector inductively coupled plasma mass spectrometry MOX mixed oxide fuel PCA principal component analysis PWR pressurized water reactor PLSDA partial least-squares discriminant analysis REE rare-earth elements SEM/EDX energy-dispersive X-ray spectroscopy SIMCA soft independent modeling of class analogy SIMS secondary ion mass spectrometry TIMS thermal ionization mass spectrometry UOC uranium ore concentrate XRD X-ray diffraction

Working Group on Destructive Analysis. Today, he is in charge of the activities on combating illicit trafficking and nuclear forensics at the European Commission’s Joint Research Centre, Institute for Transuranium Elements (ITU). Since 2004, he has been the cochairman of the Nuclear Forensics International Technical Working Group (ITWG).

REFERENCES Maria Wallenius started her radiochemist career in Finland at the University of Helsinki in the nuclear safeguards project. In 1996, she moved to Institute for Transuranium Elements (ITU) to do research on characteristic parameters for the origin determination of plutonium in nuclear forensics, and she obtained her Ph.D. in 2002. Since then she has been involved in various research and training activities in the nuclear forensics field. She is also the coordinator of the nuclear forensics analysis at ITU.

(1) Nuclear Forensics Support, 2006. IAEA Nuclear Security Series No. 2. http://www-pub.iaea.org/MTCD/publications/PDF/ Pub1241_web.pdf. (2) Nuclear Forensics in Support of Investigations, IAEA Nuclear Security Series No.2, to be published. (3) Moody, K. J., Hutcheon, I. D., Grant, P. M. Nuclear Forensic Analysis; CRC Press: Boca Raton, FL, 2005; ISBN 978-0-8493-1513-8. (4) Wallenius, M. Origin determination of reactor produced plutonium by mass spectrometric techniques: Application to nuclear forensic science and safeguards, 2001. http://ethesis.helsinki.fi/ julkaisut/mat/kemia/vk/wallenius/originde.pdf. (5) Hermann, O. W.; Westfall, R. M. ORIGEN-S: Scale System Module to Calculate Fuel Depletion, Actinide Transmutation, Fission Product Buildup and Decay, and Associated Radiation Source Terms, CCC-545; Radiation Safety Information Computational Center, Oak Ridge National Laboratory: Oak Ridge, TN, 1998. (6) Wallenius, M.; Peerani, P.; Koch, L. J. Radioanal. Nucl. Chem. 2000, 246, 317. (7) Luksic, A. T.; Collins, B. A.; Friese, J. I.; Schwantes, J. M.; Starner, J. R.; Wacker, J. F. Proceedings of the 51st Annual Meeting of the Institute of Nuclear Materials Management, July 11−15, 2010, Baltimore, Maryland; Institute of Nuclear Materials Management: Deerfield, IL, 2010. (8) Alamelu, D.; Aggarwal, S. K. Int. J. Nucl. Energy Sci. Technol. 2011, 6, 30. (9) Mayer, K.; Wallenius, M.; Ray, I. L. F. Analyst 2005, 130, 433. (10) Robel, M.; Kristo, M. J. J. Environ. Radioact. 2008, 99, 1789. (11) Glaser, A. Nucl. Sci.Eng. 2009, 163, 26. (12) Nicolaou, G. J. Environ. Radioact. 2008, 99, 1708. (13) Croff, A. G. Nucl. Technol. 1983, 62, 335. (14) Permana, S.; Suzuki, M.; Su’ud, Z. AIP Conf. Proc. 2011, 1448, 142−152, DOI: 10.1063/1.4725449. (15) Bignan, G.; Ottmar, H.; Schubert, A.; Ruhter, W.; Zimmermann, C. ESARDA Bull. 1998, 28, 1 http://esarda2.jrc.it/bulletin/bulletin_ 28/28ART_1.PDF. (16) Joe, K.; Jeon, Y.-S.; Song, B.-C.; Han, S.-H.; Jung, E.-C.; Song, K. Appl. Radiat. Isot. 2010, 68, 505. (17) Wallenius, M.; Lützenkirchen, K.; Mayer, K.; Ray, I.; Aldave de las Heras, L.; Betti, M.; Cromboom, O.; Hild, M.; Lynch, B.; Nicholl, A.; Ottmar, H.; Rasmussen, G.; Schubert, A.; Tamborini, G.; Thiele, H.; Wagner, W.; Walker, C.; Zuleger, E. J. Alloy. Comp. 2007, 444− 445, 57. (18) Weyer, S.; Anbar, A. D.; Gerdes, A.; Gordon, G. W.; Algeo, T. J.; Boyle, E. A. Geochim. Cosmochim. Acta 2008, 72, 345. (19) Brennecka, G. A.; Borg, L. A.; Hutcheon, I. D.; Sharp, M. A.; Anbar, A. D. Earth Planet. Sci. Lett. 2010, 291, 228.

Zsolt Varga received his Ph.D. in Chemistry in 2007 from the Institute of Isotopes of the Hungarian Academy of Sciences. After working there as a research fellow until 2008, he joined the nuclear forensic group at the Institute for Transuranium Elements of the European Commission Joint Research Centre as a postdoctorate researcher, and from 2011 as a seconded national expert from the Hungarian Academy. His current research interests include the elemental and isotopic analysis of illicit nuclear materials and origin assessment of uranium samples of unknown origin.

NOMENCLATURE ADU ammonium diuranate AMS accelerator mass spectrometry AS alpha spectrometry BWR boiling water reactor FTIR Fourier-transform infrared spectrometry HMTA hexamethylene-tetramine HRGS high-resolution gamma spectrometry IC ion chromatography 898


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