Application of isotopic fingerprinting in nuclear forensic investigations: A case study

Naturally occurring chemical elements show only small variations in isotopic composition. Material irradiated in a nuclear reactor undergoes nuclear reactions, prominently induced fission (n, f) and neutron capture (n, g), which lead to materials of an isotopic composition determined by reactor conditions (initial fuel composition, neutron energy spectrum, irradiation time, cooling time, etc.). Treatment, such as chemical separation as done in the reprocessing of spent nuclear fuel, does not change the isotopic composition of a chemical element. Consequently, the isotopic composition serves as an identifying fingerprint. Accurate and highly sensitive methods are required to determine variations in isotope ratios. Gamma spectrometry, alpha spectrometry, thermal ionization mass spectrometry and inductively coupled plasma mass spectrometry are employed to measure isotopic compositions of plutonium and uranium. Extensive use of this methodology was made in the context of a case of theft of radioactive material from a nuclear installation being decommissioned. Two contaminated items were reportedly stolen: a sample vial containing a small amount of waste solution, and a cleaning wipe (cellulose cloth). Radioactive contamination was detected in the suspect's apartment, where he had stored the items for more than six months. Radioactivity was detected also in the apartment of a person associated with the suspect, and, to lesser extents, at other locations, indicating cross-contamination. Samples were collected at various locations (e.g. vacuum cleaner bags. clothes, packaging of stolen items, stolen items) in order to determine whether the contamination could be uniquely related to the two (reportedly) stolen items. The investigations showed that the ratio of radionuclides (e.g. Cs-137/Am-241) in the various samples was not constant, which can be explained only in terms of differences in sample treatment (e.g. weathering, rinsing, or washing). Solubility tests using micro-amounts of the sample were carried out to investigate relative differences under experimental conditions mimicking leaching through exposure to rainwater or washing-machine conditions. Such "treatments" may not have been performed intentionally on the stolen items; thus great care was necessary in the interpretation of these results. In contrast to that, isotope ratios (e.g. U-235/U-238, Pu-240/Pu-239) do not change through such chemical treatments. Consistent results were found for most of the samples, indicating that the observed contaminations were related to the two stolen items. However, one sample showed a significantly different Pu-238/Pu-239 isotope abundance ratio.