CSI: Karlsruhe

Nuclear forensics sleuths trace the origin of trafficked material

When the former Soviet Union broke up, the new independent states couldn’t control their supplies of nuclear material and some of it got “lost.” In 1994 alone, 45 confirmed cases of nuclear material trafficking were reported, according to the International Atomic Energy Agency (IAEA). The numbers have decreased to around 10 reported incidents a year, but poorly guarded and easily stolen nuclear materials still pose a serious problem because of the radiological hazards associated with improper transport, handling, and storage. And while the dumping of nuclear material in a landfill or salvage yard is very serious, even more dire consequences could occur if the material ended up in the hands of a terrorist.

When a cache of stolen or dumped nuclear material is intercepted, routine forensics techniques are used to answer the questions of who and how and what. Answering what the material is, where it came from, and what it could be used for is a nuclear whodunit worthy of the “CSI” television series and has resulted in the development of a new branch of science called nuclear forensics.

Since the early 1990s the Institute for Transuranium Elements (ITU) in Karlsruhe, Germany, has been involved in developing the methodology of nuclear forensics to answer the questions of chemical makeup, origin, and use. Tracing where the material came from will help governments improve physical protection of the site of origin and prevent future thefts or illegal disposal. The science is based on analytical techniques related to the nuclear fuel cycle: radiochemistry, nuclear physics, reactor physics, and materials science.

Klaus-Richard Lützenkirchen of ITU’s Nuclear Safeguards and Security Unit recently visited Los Alamos and gave a talk on nuclear forensics activities at ITU. The visit was sponsored by the Seaborg Institute for Transactinium Science. Lützenkirchen discussed typical cases that have been analyzed at ITU and described the various analytical techniques that led to the successful determination of where the materials, specifically plutonium and uranium, came from. Three of the cases are discussed below.

ITU’s nuclear forensics methodology takes data and analytical methods from nuclear safeguards, materials science, and isotope geology to determine the isotopic composition, elemental composition, impurities, macroscopic appearance, microstructure, and age. The data reveal two general classes of information: endogenic, or self-explanatory (age, intended use, production mode), and exogenic, which requires reference data (place of production, last legal owner, and smuggling route). Twenty-one seizures analyzed at ITU between 1992 and 1997 included natural uranium, low-enriched uranium fuel pellets, highly enriched uranium, plutonium, and contaminated scrap metal.

First case: uranium pellets
Uranium dioxide (UO₂) pellets are used as fuel in nuclear power reactors. In June 2003, ITU received four uranium pellets from Lithuania. The pellets were analyzed for uranium content and isotopes; chemical impurities, which would point to the source of the raw product; age, which would point to the production time; and microstructure, which would point to the production process.

All of the pellets showed identical geometry; they had a central hole and they were dished. The pellets were weighed and their dimensions measured. The four pellets were measured individually with a high-resolution gamma spectrometer for the first indication of the isotopic composition. The spectra showed gamma lines belonging only to uranium, and analysis showed an average uranium-235 enrichment of 2%. Because the pellets were identical in dimensions as well as in isotopic composition of the uranium, only one of them was dissolved for further analysis.

The isotopic composition of the uranium was determined by mass spectrometry. Mass spectrometry techniques are able to provide accurate results for the minor abundant isotopes (uranium-234 and -236), which is not the case with gamma spectrometry. The measurement technique routinely used for uranium and plutonium isotope analysis is thermal ionization mass spectrometry (TIMS). An inductively coupled plasma mass spectrometer with multi-collector detection system (MC-ICP-MS) was used to compare the accuracy and precision between these two methods.

The uranium content in solution was determined by three different methods: potentiometric titration, hybrid K-edge densitometry (HKED), and isotope dilution mass spectrometry (IDMS). All three methods determined that the uranium content corresponded to the stoichiometry of uranium dioxide (UO₂) whose theoretical value is 88%. Impurities in the sample were determined after complete dissolution by sector-field ICP-MS using rhodium-103 as an internal standard.

Determining the age of the material, and thus the date when the material was produced, helps identify the production campaign or batch. The radioactive decay of the uranium isotopes provides a unique chronometer that is inherent to the material. This clock is reset to zero each time the decay products (daughter nuclides) are chemically separated from the uranium. The half-life of the uranium isotopes in question is very long, therefore the short periods between the preparation of the uranium fuel and the seizure of the material generated extremely minute amounts of daughter nuclides. Nevertheless, the age could be determined from these parent/daughter ratios. The age of the uranium was calculated using the equation of radioactive decay and its derivatives.

The sample solution was spiked with thorium-228 and uranium-233 before the uranium/thorium separation. The amount of uranium-234 and thorium-230 was determined using the isotope dilution technique, i.e., relative measurements against the (known amount of) spike isotope. The age of the material was determined to be 12.6 years ± 0.8 years. Thus the pellets had been produced at the end of 1990 (remember that the test occurred in 2003). Only the uranium-234/thorium-230 parent/daughter ratio could be used this time for the age determination because of the long half-life of uranium-235 and consequently the very small amounts of built-up daughter nuclides.

At this point, the investigators knew the dimensions of the pellets as well as the isotopic composition and age of the material. For the next step—determining where the pellets came from—the team used a relational database at ITU that contains data from several nuclear fuel manufacturers (including most of Western Europe and Russia). The database contains dimensions of pellets, uranium-235 enrichment, and typical impurities. Besides commercial reactor fuels, the database also contains information on research reactor fuels and information acquired from open literature. Additionally, results of old findings are introduced into the database for a comparison with future cases.

In the case of the four pellets from Lithuania, the database gave a very unambiguous answer. The pellet dimensions and enrichment already were enough to identify them as being made for an RBMK-1500 reactor, which is a Russian-type, water-cooled, graphite-moderated reactor. There are two models of the RBMK reactor: the 1000 and 1500. The 1000 model is older and more widely distributed, while there is only one 1500 model reactor in the world: Ignalina Unit 2 in Lithuania, which started up in August 1987 and is still operational.

Furthermore, there is only one manufacturer for this type of fuel: MZ Electrostal near Moscow. The measured impurities of the pellet material were below the maximum values given in the manufacturer’s specifications and they also agreed with the experimental data from earlier findings of the same fuel. The last confirmation parameter was the age, which fit with the production data of the manufacturer (start of fuel production: December 1989). The information contained in the nuclear materials database proved to be essential for the attribution of the material.

ITU’s nuclear forensics team was able to further deduce from the absence of uranium-236 that the fuel had been enriched from natural uranium (meaning there was no reprocessed material) and, because the pellets contained no traces of plutonium, that the fuel had never been in a reactor.

The IAEA database on trafficking of nuclear and other radioactive materials and some other open source information reported a case of a fresh fuel assembly being stolen from the Ignalina power plant in 1992. The four pellets under investigation definitively originated from Electrostal, and probably came out of that stolen assembly. This kind of fuel assembly contains about 110 kilograms of uranium. Between 1994 and 1997 more than 100 kilograms of pellets have been confiscated in several seizures; the greater part of the material has been recovered. The material itself is not useable for nuclear weapons because the uranium-235 enrichment of 2% is far too low. However, what makes this case spectacular is the amount of the material that was stolen. Efforts have been undertaken to improve the physical protection at nuclear power plants and other storage facilities for nuclear material in the former Soviet Union.

Second case: mixed-oxide (MOX) powder
In August 1994, three men were stopped at the Munich airport carrying a suitcase containing 560 grams of plutonium- and uranium-oxide powder and 210 grams of lithium metal. The powder consisted of 64.9 wt.% of plutonium and 21.7 wt.% of uranium. The plutonium-239 enrichment was about weapons-grade quality, whereas the uranium had a low uranium-235 enrichment. The piece of lithium metal was enriched to 89.4% lithium-6. The MOX powder consisted of three different particle types: plutonium-oxide (PuO₂) platelets, rod-shaped PuO₂, and hexagonal uranium-oxide (U₃O₈).

X-ray of a suitcase that contained nuclear material. In August 1994, three men who were expected to be carrying plutonium arrived in Munich from Moscow. Gamma-ray detectors did not respond when their luggage was checked, but X-rays showed a stainless steel container and tin cans.

Light-water reactors (PWR, BWR, and VVER) were excluded as the origin of the plutonium because the isotopic composition of plutonium after a typical irradiation period of three years in these reactors would have been significantly different. Materials-testing reactors using 36%–90% enriched uranium-235 were also excluded because a higher plutonium-238 abundance would have been expected in this case. Most likely, a reactor type with a softer neutron spectrum (e.g., heavy-water or graphite-moderated) was used for production.

Nuclear material seized at the Munich airport. The suitcase contained 560 grams of plutonium- and uranium-oxide powder (below left) and 210 grams of lithium metal (below center). The structure of the mixed-oxide powder was determined by scanning electron microscopy (SEM). The micrograph (below right) shows the three distinct shapes of the powder: plutonium-oxide platelets, plutonium-oxide rods, and uranium-oxide hexagons.

In this case, the nuclear reactor would have operated with an initial fuel enrichment of 1.8% uranium-235 to yield the uranium composition, assuming of course that the uranium and plutonium were from the same reactor. This scenario was also proposed by plutonium isotopic correlation. However, the plutonium-238 and -242 abundances were too high to originate from the low-burn-up spent fuel of an RBMK-1000 reactor. Thus, most likely, the plutonium was a mixture of different spent fuels (e.g., a low-burn-up or weapons-grade plutonium and a high-burn-up fuel) and had no direct connection with the uranium present.

Platelet analysis. The plutonium-oxide (PuO2) platelets in the seized Munich mixed-oxide powder were examined in detail by scanning electron microscopy (SEM), above, to determine platelet-size distribution, and by transmission electron microscopy (TEM), below, to determine grain-size distribution. The SEM analysis shows a reference sample of PuO2 from a known fabrication plant (above left) and the PuO2 platelets from the sample seized at the Munich airport in 1994 (above center). The SEM analysis (above right) does not show a significant difference between the two samples. The TEM analysis shows a reference sample of PuO2 from a known fabrication plant (below left) and the PuO2 platelets from the sample seized at the Munich airport in 1994 (below center). It is interesting to note that both of the TEM pictures were taken at the same magnification. The TEM analysis (below right) reveals a remarkable difference in grain-size distribution, indicating that a different production process was used for manufacturing the PuO2.

Because the powder consisted of two different plutonium particle types, individual microparticles were analyzed by SIMS to determine if their isotopic compositions were identical or if the earlier determined isotopic composition for the bulk material was a result of mixing two different compositions. The plutonium-240/-239 ratios in the platelets and the rod-shaped particles were slightly different (0.1159 ± 0.0012 and 0.1245 ± 0.0026, respectively). However, the difference was much too small to conclude that one plutonium particle type originated from weapons-grade plutonium (plutonium-240/-239 < 0.05) and the other type from high-burn-up fuel (plutonium-240/-239 ~ 0.4–0.7). Therefore, the mixing must have taken place before the particles were produced.

The age of the plutonium material was determined by gamma spectrometry (bulk sample) and by SIMS (both particles types). The adjacent uranium particles interfered in the SIMS measurements, leading to biased results for the plutonium-238/uranium-234 and the plutonium-239/uranium-235 ratios (isobaric interferences for uranium-238 and plutonium-238, and for uranium-235 from uranium particles and uranium-235 from plutonium-239 decay). Because uranium-236 is a minor isotope in the uranium material, its interference with the in-grown uranium-236 from plutonium-240 decay was negligible. The ages determined for different particle types from the plutonium-240/uranium-236 ratio were similar (within the uncertainties) and they were consistent with the age obtained from the bulk measurement of the plutonium-241/americium-241 ratio by gamma spectrometry. Both methods gave a production time around the end of 1979 ± 0.5 years.

Even though the plutonium-239 enrichment is somewhat too low for military purposes, it is not impossible to produce a nuclear device with plutonium of this quality. With regard to the lithium metal, its high enrichment in lithium-6 of 89.4% is noteworthy. One of the possible uses of lithium-6 is to generate energetic tritons via the ⁶Li(n,α)³T reaction. Such energetic tritons would then be able to initiate deuterium-tritium nuclear fusion in a thermonuclear weapon. Therefore, it may not be a coincidence that plutonium and lithium-6 were found together.

Third case: radioactive waste
In July 2001, plutonium was found in a routine urine analysis of an employee who had been working in a shut-down reprocessing plant under decommissioning in Karlsruhe, Germany. His car and apartment were also found to be contaminated. In addition, his girlfriend and her daughter had incorporated americium and cesium. The employee was arrested and confessed that he had stolen a plastic vial containing a liquid and a swipe cloth. He had managed to get both items out of the reprocessing plant about half a year earlier.

The analytical task was two-fold: first, to confirm that the reprocessing plant in question was really the source of the material; second, to verify whether the two stolen items were the only sources of the contamination and the incorporation. Besides the two stolen items, analyzed samples included vacuum cleaner bags from the contaminated apartments, household gloves used to handle the stolen items, and clothing.

Theft of radioactive waste. A plastic vial containing a liquid and a swipe cloth, which were stolen from a decommissioned reprocessing plant in Karlsruhe, Germany, were first analyzed by gamma spectrometry. The vial was found to contain plutonium-238, -239, and -241; americium-241; cesium-134 and -137; and antimony-125. In addition to these elements, europium-154 was also found in the swipe cloth (cellulose fibers from the swipe cloth are shown at bottom left and nylon fibers at right). To quantify the uranium and plutonium isotopes, parts of the samples were dissolved in nitric acid and measured by thermal ionization mass spectrometry (TIMS), inductively coupled plasma mass spectrometry (ICP-MS).

All samples were first measured by gamma spectrometry. The plastic vial contained plutonium-238, -239, and -241; americium-241; cesium-134 and -137; and antimony-125. In addition to these elements, europium-154 was also found in the swipe cloth. The other items contained the same nuclides in slightly lower activities. To quantify the uranium and plutonium isotopes, part of the samples was dissolved in nitric acid and measured by TIMS and ICP-MS.

The isotopic compositions of plutonium and uranium were similar in all samples and resembled the spent fuel last reprocessed in the plant before shutdown. The large amount of cesium ingested by the thief’s girlfriend was difficult to explain from the activity found in the two stolen items. However, the items were most probably washed before being transferred for the investigations. Because cesium is fairly soluble in water, most of the cesium might have been lost at this stage. The thief was sentenced to prison for breaking the security regulations of the reprocessing plant and for unauthorized possession of radioactive material. Decontaminating the two apartments cost about $2.5 million.

Current developments
The examples presented here are typical cases analyzed at ITU. A pellet case is often easier to solve than a powder case because information on commercial nuclear fuels is available in ITU’s database. Powder is usually not a final product but is an intermediate product or not from a commercial production cycle. To make the origin determination more accurate, researchers are continuously studying samples of known origins.

Existing analytical techniques, as used in material science, nuclear materials safeguards, and environmental analysis, have been adapted to the specific needs of nuclear forensic investigations. Characteristic parameters (e.g., isotopic composition, chemical impurities, and macro- and microstructure) can be combined into a “nuclear fingerprint” pointing at the origin of the material. Further research is being carried out aimed at identifying other useful material characteristics to reduce the ambiguities often remaining in the interpretation of the data and in the source attribution.

The new science of nuclear forensics has also required a change in how police conduct investigations. Using classical forensics techniques on contaminated items must be done in a controlled environment and with proper radiological protection. ITU has helped the police develop procedures for crime-scene management and has set up a dedicated glove box for taking DNA samples and fingerprints from contaminated items.

Classical forensics on contaminated items. The nuclear forensics team at the Institute for Transuranium Elements (ITU) in Karlsruhe is helping police departments learn how to perform investigations on radiologically contaminated evidence. The classical technique of taking fingerprints or DNA samples from evidence becomes even more challenging when it has to be performed in a glovebox.

Response to trafficking of nuclear material
Nuclear forensics is a crucial component of a comprehensive response to nuclear material trafficking. The response measures require a collaborative effort on an international level. ITU, together with several eastern countries of the European Union and countries in the Commonwealth of Independent States (CIS), has set up projects to increase the efficiency in combating trafficking. A comprehensive approach has been developed that involves all competent authorities in the individual countries.

Assistance is offered to develop a national response plan that is consistent with the Model Action Plan recommended by the Nuclear Smuggling International Technical Working Group (ITWG) on combating nuclear terrorism. The concept of a national response plan has been taken over by the IAEA, put on a broader basis, and promoted for implementation. Training sessions have been offered to law enforcement officers and scientists and demonstration exercises have been carried out in different countries to test the implementation of the Model Action Plan. In a final step, joint analyses of seized samples have been conducted by ITU and requesting countries to demonstrate the preparedness and usefulness of nuclear-forensic analysis.

These efforts are coordinated with other international activities, in particular by the United States and the IAEA, to make efficient use of available resources. On the scientific level, the ITWG serves as a forum for the exchange of experience, advancing nuclear forensics, and interacting with regulatory bodies, law enforcement, and measurement scientists. Nuclear forensics provides an element of sustainability in the fight against trafficking of nuclear material.

Further reading

K. Mayer, M. Wallenius, and I. Ray, “Nuclear forensics—A methodology providing clues on the origin of illicitly trafficked nuclear materials,” The Analyst 130 (2005).

M. Wallenius, K. Mayer, and I. Ray, “Nuclear forensic investigations: Two case studies,” Forensic Science International 156, (2006).

M. Wallenius, et al., “Nuclear forensic investigations with a focus on plutonium,” Journal of Alloys and Compounds 444-445 (2007).

Next: NNSA selects Los Alamos as preferred alternative site