6
The Emerging Science of Nuclear Forensics
Key Issues
The key issues noted here are some of those raised by individual workshop participants, and do not in any way indicate consensus of workshop participants overall.
• There are strong scientific capabilities in nuclear forensic science but our ability to interpret these data is still in a state of development.
• Expanded databases with information on nuclear material around the world are needed.
• Greater understanding of how materials change as they undergo reprocessing, processing and other processes is needed.
• No single technique provides the needed information for all or even any material.
• Nonproliferation nuclear forensics requires a focused international cooperative effort.
Promising Topics for Cooperation Arising from the Presentations and Discussions
These promising topics for collaboration arising from the presentations and discussions are not those representing the consensus of the participants, but are rather a selection of those topics offered by individual participants throughout the presentations and discussions.
• Development of national nuclear forensics libraries.
• Sharing of best practices.
• Detection and age-assessment of uranium and plutonium in environmental matrices.
• Cooperation through the International Atomic Energy Agency (IAEA) and the International Technical Working Group Round Robins.
An Emerging and Still Inexact Science
Ian Hutcheon gave an overview of nuclear forensic analysis as it is practiced in the United States and some examples of different types of applications, particularly with regard to the international arena and what is sometimes called nonproliferation nuclear forensics.
Nuclear forensics is the technical means by which nuclear materials are characterized. It is an emerging science because even though nuclear forensic analysis was first applied in the United States in 1949 to diagnose the first Russian nuclear explosion, nuclear forensic analysis as we apply it today really began only in the mid 1990’s, and in an international context it has really been applied only within the past 10 years. It is an imperfect science because even though analysts can use sophisticated analytical equipment to characterize material such as interdicted highly enriched uranium (HEU), they often lack the knowledge to identify fingerprints that will allow for the identification of perpetrators and put them in jail.
Weapons-useable material today is found in at least 40 different countries. Because the threats of nuclear proliferation and nuclear terrorism rise above all others, methods are needed to prevent illicit trafficking of this material throughout these 40 different countries. International safeguards and nuclear forensics must work together to make sure that these materials remain under lawful control and in the event that illicit trafficking does occur, the perpetrators are rapidly identified. Meeting the threat of nuclear nonproliferation is a critical 21st century issue that no single country can hope to solve independently; it requires global cooperation.
Figure 6-1 is a timeline of seizures of weapon-useable material going back to 1992. It differs from the IAEA illicit trafficking database in that it contains only HEU and plutonium. There were a large number of events in the early 1990’s associated with the fall of the Soviet Union. What is particularly troubling is that nuclear material continues to be smuggled: interdictions occurred in 2001, 2003, 2006, 2010, and even in 2012. Altogether, this interdicted material amounts to about 15 kilograms of HEU and about 400 grams of plutonium. In itself, this is not enough to make a weapon, but these are only the cases of successful interdiction. If the drug trade is indicative of our success rate for interdiction, this could be as little as 10 percent of the total amount of material being trafficked.
FIGURE 6-1 Interdictions of weapons grade material from 1992 to 2012. SOURCE: Hutcheon, 2012.
After using analytical techniques to characterize nuclear material by its isotopic composition, major and trace elements, its microstructure, morphology, and age, an evaluation process begins. In rare cases, a subject-matter expert may recognize what the material is. In most cases, evaluators have to compare the characterization with material in a database or with some information about the process history to reach technical conclusions; most importantly, who is responsible. Work is needed to improve in this area of reaching technical conclusions based on fairly sophisticated signature analyses.
Signatures are created and destroyed throughout the nuclear fuel cycle. The key for nuclear forensics is to understand how these signatures are created and how they are modified so that if material from the back end of the nuclear fuel cycle is interdicted, the history can be reconstructed and can help to identifY where the material was taken from lawful use. But there is no silver bullet: No single signature identifies nuclear material. Figure 6-2 is a Venn diagram that shows that nuclear forensics succeeds by finding the point of intersection between many of these different signatures, including both nuclear and traditional signatures like packaging, fingerprints, hairs, and fibers, and in lucky cases there is a single point of intersection that leads back to the perpetrators.
FIGURE 6-2 Conceptual illustration of nuclear forensic characteristics and the domain of possible material that matches those material characteristics. SOURCE: Hutcheon, 2012.
Hutcheon described three different ways of applying nuclear forensics: point-to-population comparisons, which are used to connect a forensic sample to a known population of materials, such as uranium ore concentrate from a particular mine; point-to-point comparisons, which are useful if trying to match material to a specific source or two samples to the same source; and point-to-model comparisons, which are used to explore the possibility of origins for which comparison samples are not available or databases are inadequate. An example of each of these is listed below.
Point-to-population comparison
Uranium ore concentrate or yellowcake is the final product of uranium mining and milling and it is a fungible commodity with worldwide regulated trade. It is a good example for nuclear forensic analysis in part because it is relatively signature rich. That is, it comes in a variety of chemical forms and many chemical and isotopic characteristics vary based on the origin of the product. Yellowcake ranges anywhere from about 50 percent uranium to 80 percent uranium. The remainder is made up of elements which can be measured.
The U.S. Department of Energy (DOE) and three national laboratories (Los Alamos, Livermore, and Oak Ridge) have constructed the uranium sourcing database and library, which consists of about 300 physical samples of uranium ore concentrate or yellowcake from around the world and a larger database containing data on trace elements, isotopes, and physical characteristics for more than 4,000 samples. When a sample of yellowcake from an unknown origin is analyzed, the results can be fed into a statistical algorithm that compares the results to the entries in the database and finds the most likely matches to the unknown sample. This approach works with about 90 percent accuracy today.
Point-to-point comparison
Two samples of HEU were interdicted, one in Bulgaria in 1999 and the other in Paris in 2001. Both are black; both are uranium oxide; and both were found in glass ampoules. Are these two samples from the same source? This is a direct point-to-point comparison. It turned out that the answer is yes, and this was determined by comparison of results from similar analyses by the DOE and the French Commissariat à l'énergie Atomique: uranium isotopic composition, trace elements, determined material production age, and estimated irradiation history of the sample. There is a match for each characteristic, so both organizations independently concluded that the two samples represent material from the same production batch in the former Soviet Union, circa in the early 1990’s.
It turns out that these two samples were also contained in lead containers that looked remarkably similar. Both containers have distinctive yellow wax liners, and it turns out that both the yellow wax and the lead are remarkably similar between these two containers. So evidence from both the nuclear material
and the associated non-nuclear material indicate that these two samples were from the same source and were probably trafficked across Europe to find buyers by the same or closely related organizations.
Point-to-Model Comparison
In this case, the analyst may be evaluating what kind of reactor a sample may have gone through. The ratios of uranium isotopes in a sample vary based on the original enrichment of the fuel, the neutron spectrum of the reactor, and the length or irradiation; together these properties can reveal the reactor type.
Hutcheon concluded by saying that nuclear forensics has highly developed technology and can analyze samples ranging in size from kilograms down to picograms with high accuracy. However, our ability to interpret these data is still in a state of development and there is really no substitute for building expanded databases with information on nuclear materials around the world. Nonproliferation nuclear forensics requires a focused international effort. No single country can take this on alone and international engagement on nuclear forensics supports agreed international efforts to counter nuclear terrorism as discussed, such as the Global Initiative to Counter Nuclear Terrorism (GICNT) and United Nations Security Council Resolution 1540. This is an important problem requiring the best science. Connecting cutting edge science to nuclear forensics attracts the best and the brightest scientists, which is how nuclear forensic science will continue to progress.
Nuclear Forensics and Special Nuclear Materials: An Indian Perspective
V. Venugopal defined nuclear forensics, described many of the techniques of nuclear forensics, and explained its value using two case studies. Venugopal noted the multitude of definitions in use for nuclear forensics, but presented this one as his preferred definition:
Nuclear forensics is the technical means by which nuclear and other radioactive materials, whether intercepted intact or retrieved from post-explosion debris, are characterized as to the composition, physical condition, age, prevalence, history, and interpreted as to the provenance, industrial history, and implications for nuclear device design, etc.
Setting aside medical and industrial radiation sources because they were discussed by another presenter, Venugopal focused on fissile material or special nuclear materials (uranium-233, uranium-235, plutonium-239, plutonium-241), which can be used in making a nuclear weapon.
Nuclear forensics techniques are practiced in different contexts in India, such as analysis of nuclear fuel samples from reactor cores and radioanalytic support for India’s courts. There is a forensics laboratory in Bhaha Atomic Re-
search Centre (BARC) that assists police in investigations of crimes (gunshot residues, lead poisoning, cyanide poisoning, arsenic poisoning). When there were questions regarding seized items, they were sent to BARC. Venugopal noted that many of the samples contained sand or a resin or tail portions of uranium used as counterbalance; none ever contained uranium that exceeded 0.7% U-235. Doing this work for the Indian courts taught Venugopal that not only must the analyses be done correctly, but the interpretation and communication of the results are critically important because the audiences are not necessarily technically trained. For example, an accused thief might be sent to jail for the rest of his life because of the uranium if the analyst simply reports the numerical results of analysis but not that the uranium is natural uranium or is within the background level.
Nuclear forensics begins with an incident. Materials involved in the incident are sampled and sent to laboratories where measurements are made. The data and observations are compared to nuclear materials data bases, such as the IAEA illicit trafficking data base, the research reactor data base, and others. Other information from the incident is also factored in, and from all this, one determines where the material came from and ultimately improves the security at the facility where it originated.
When a sample arrives at the laboratory, analysts first identify what the material is. The physical form (powder or liquid) might indicate what stage of production the sample is from. The analyses typically begin with nondestructive analysis, using high-resolution gamma spectrometry, to identify the material from signature gamma-ray emissions, and x-ray crystallography to examine the microstructure, sample homogeneity—whether the sample represents single or multicomponent systems—particle morphology and size. Today, secondary ion mass spectrometry (SIMS) is so powerful that one can analyze the isotope composition of, for example, Pu oxide or uranium, whether it is a rod shape or a platelet shape. The shape is a result of the heat treatment that the material has undergone. So one can identify whether single or multiple sources of plutonium oxide are present by using particle morphology and characteristics. Unfortunately, although SIMS is available in India for other materials, India is not able to procure the latest SIMS technology for fissile material characterization. For some samples, especially for bioassay materials (such as determining whether urine samples contain plutonium or uranium) BARC may use fission track analysis to go down to very low levels.
The next step is to look at chemical signatures, including both nuclear (uranium, plutonium, fission products) and non-nuclear elements, some of which may have been used in processing the materials (for example, reducing the actinide oxides to metal), thorium, magnesium, calcium, iodine, sulfur, and acid residues, such as chlorine, fluorine, bromine, nitride, and nitrate; contaminants that are included in the metal from processing like beryllium, fluorine, iron, and silicon; and additives designed to moderate reactivity. Knowledge of the overall material, including the bulk and the composition, helps in figuring out where the sample might have originated.
Destructive analysis, thermal ionization mass spectrometry (TIMS) and high resolution inductively coupled plasma mass spectrometry, gives very precise and accurate measurements of the isotopic composition or mass abundance of uranium, plutonium, and other materials. The isotopic composition can also give indicators of the history and provenance of the material. If uranium contains U-236, we know that it was irradiated in a reactor. If the uranium sample has a higher percentage of U-234 than the natural abundance, that means that the uranium has passed through an enrichment process. The isotopic composition of plutonium tells us the burnup of the fuel in which it was produced and the neutron spectrum (hard or soft) in the reactor where it was produced. Analyzing residual isotopes using chemical processing techniques and fission yields, one can find out krypton and xenon isotopic abundance and figure out on that basis when the object might have been cast.
By looking at the abundance of each material in a radioactive decay chain, one can deduce when the material was irradiated in a reactor or purified. The specific radionuclides used are called isotopic chronometers. For example, cobalt-60 decays to nickel-60. The ratio of cobalt-60 to nickel-60 tells us when this Co-60 was produced. Uranium has several isotopic chronometers. By looking at the decay daughters, one can find out when the uranium was purely separated. Radioactive decay is a built-in chronometer. One can use isotope dilution mass spectrometry, and isotope dilution alpha spectrometry to detect daughter isotopes at ultra-trace levels. For example, in the case of U-233 irradiation, one can identify the U-232 content very precisely and accurately using alpha spectrometry. This is being practiced very routinely in the laboratory.
Thermal ionization mass spectrometry is the mother of all measurements done for isotopic composition, but it is very difficult to acquire these devices. As a result, to analyze nuclear fuel to support nuclear fuel fabricators, BARC had to construct these instruments (i.e., its own TIMS). Venugopal estimates that in terms of precision and accuracy, it may compare to European equipment from about 1995 to 2000. More sophisticated TIMS equipment may be available now from Germany or the United Kingdom.
Detailed analyses of non-nuclear constituents might reveal the geographic location of production or how the sample might have been produced based on the composition. There is a database focused on such non-nuclear constituents. Alloying or cladding materials may also reveal useful information: the presence of gallium suggests that it was used for stabilization of a particular phase of plutonium. The non-nuclear composition can indicate who the producer might be.
Nuclear forensics is a piece of the international effort to combat nuclear terrorism. There are plenty of examples of interdictions of nuclear smuggling, especially in the 1990’s. The materials include HEU, Pu, and even enriched lithium. Their signatures are well known, whether irradiated or unirradiated.
There are many techniques available to characterize and report on the material (including glow-discharge mass spectrometry, x-ray fluorescence, gas chromatography-mass spectrometry, and others in addition to those already mentioned). The goal is to do this complete analysis and to report on key parts
of the analyses within specific timelines. For that report, it is important for the scientist to understand not only the science but the way the courts will use the report. Venugopal gave two examples of cases.
Case Study 1: Lauenforde
A confiscated nuclear fuel pellet was analyzed in June 2003. The data are available from the Institute for Transuranium Elements in Germany. The sample was analyzed and, based on the composition and contaminants published in open literature, one can determine where the sample came from. The laboratory subjected the sample to high-resolution gamma spectrometry. It was found to be unirradiated uranium fuel. Destructive analysis showed the uranium content was about 80 percent and using isotope dilution mass spectrometry, wherein the sample solution was spiked with uranium-236 and thorium-233, analysts found the sample to be enriched to about two percent. The constituents were separated and by comparing the abundances of parents and daughters in the uranium decay chain, the age of the uranium was found to be 12.6 years. The analysis was carried out in 2003, so the material was produced by the end of 1990. The impurity composition was also examined. Based on the discovered age, the production time, pellet dimensions, isotopic composition, impurity content, percentage enrichment, analysts compared these to entries in a database at the ITU and found the pellet to be for an RBMK 1300 Russian graphite moderated reactor.1
Case Study 2: Munich Airport
In the Munich airport in 1994 on a Lufthansa flight to Moscow, a sample of material was confiscated. It contained mixed oxide powder (uranium and plutonium oxide, or MOX) 560 grams and 210 grams of lithium metal. The sample was 64.9 weight percent plutonium and 21.7 weight percent uranium, and the lithium was highly enriched, 89.4%, in lithium-6. The isotopic composition of the MOX powder was found to include weapon grade uranium and low (1.66 percent) enriched uranium, which would probably be for a naval reactor or something like that. The most important discovery was that there were two distinct forms of plutonium oxide: hexagonal platelets and rods. This implies two different sources of plutonium for this sample. The plutonium was probably from two different lightly irradiated fuels or weapons grade high-burnup fuel with no direct connection with the uranium in the sample. Where the lithium came from is unknown. This case has not been solved.
Venugopal closed by noting that these and other examples illustrate that radioanalytical chemists need to have many kinds of technical skills and have to interpret the data so that they will withstand legal scrutiny.
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1 RBMK – Reaktor Bolshoi Moshchnosti, Kanalnii or High Power Channel-Type Reactor.
DISCUSSION
In the case of bullet-lead composition, it has been found that there is as much variation within a batch as there was across batches. Hutcheon noted that in nuclear forensics some batches are homogenized by the manufacturer, but some samples are heterogeneous. Indeed, some samples that appear homogeneous on a bulk scale are actually heterogeneous when examined grain by grain using SIMS. This is an area in which more research is clearly needed. We need to understand how trace element or isotope signatures are imparted into different types of nuclear material.
While the focus of the workshop is on fissile material, the question was raised whether nuclear forensics can determine the provenance of radiological sources that are found, interdicted, or used in an incident. Venugopal described analyzing a sealed source of cesium-137 in what turned out to be a moisture density gauge, which was handled without difficulty. He also reiterated that in the case of a source like cobalt-60, the age of the radioactive material since it was purified can be found.
One member of the audience noted experience with the Oklo natural reactor where billions of years ago natural uranium went critical on and off for millennia producing tons of plutonium. Geochemists and other scientists today determined when it happened, how it happened, how frequently it came on and was switched off, and then where the plutonium went. Some of the same analytical techniques used for the Oklo reactor are used for nuclear forensics.
Another audience member noted that the current state of nuclear forensics as an emerging and imperfect science is dangerous. Particularly in India where nearby there are hostile parties who function on the basis of plausible deniability or even implausible deniability, so long as it cannot be proven. This should be a ripe area for international cooperation and particularly a prime area for collaboration between Indian and U.S. scientists with reference to everything we know about India’s western neighbors. Venugopal replied that this work requires a very high level of radioanalytical capability, and while there are some courses being conducted, India is unable to obtain some of the most useful state-of-theart tools for analysis—SIMS and even the latest TIMS equipment—which are needed for analyzing the minimum sample size. If Indian scientists had such equipment, he said, they could do work on par with those of other international scientists.
Hutcheon observed that nuclear forensic science has improved substantially since the Munich seizures in 1994, and concluded that the worldwide community would do a better job today. He also said that scientists in the United States are eager to cooperate in nuclear forensics. IAEA is also encouraging all member states to cooperate on nuclear forensics and the agency offers an annual training course on nuclear forensic analysis. Another participant noted that nuclear forensics was discussed in the preparatory meetings for the 2012 Nuclear Security Summit in Seoul and the GICNT is preparing a document on nuclear forensics, so the topic is now getting the attention it is due.
The question was raised whether the IAEA could put together a comprehensive databank of material that is under IAEA safeguards. Hutcheon explained that the IAEA has adopted a policy whereby it is encouraging countries to develop their own national nuclear forensic libraries, and then to make them available for query in the event of an international incident. Such libraries are actively being developed in the United States, South Africa, Ukraine, the European Union, France, and the United Kingdom. There is not an on-going effort to develop a single database that would be worldwide in coverage.
Finally, an audience member asked how to ensure that natural uranium used for armor penetrating munitions does not cause occupational hazards for those working with the uranium. Several experts answered that the major hazard from depleted uranium is not radiological, but the chemical hazard as a heavy metal. So it should be treated in the same way as any potentially toxic heavy metal. This problem has been analyzed by the defense laboratory at Jodhpur.
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