Nuclear forensics sleuths trace the origin of trafficked material
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.
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.
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).