Identification and characterization of radioactive ‘hot’ particles in Chernobyl fallout-contaminated soils: the application of two novel approaches

Abstract

The Chernobyl accident in 1986 resulted in the widespread identification of the post-accident presence of radioactive (or ‘hot’) particles across large areas of Eastern and Central Europe. Such particles arise from direct deposition and also from condensation and interactions on particle surfaces during and following the deposition of soluble fallout. Identification of the presence and nature of hot particles is necessary in order to determine the long-term ecological impact of radioactive fallout. This paper describes several techniques for the identification and characterization of hot particles in soil samples from Belarus. In addition to new results from the use of gamma spectrometry, we include two novel instrumentation approaches that have been developed and applied to Chernobyl fallout-contaminated soils. The first, ‘differential’ autoradiography, utilizes a photographic film sandwich to characterize the nature of the ionizing radiation emitted from samples. In this paper we show that differential autoradiography can not only identify hot particle presence in soil, but can also determine the dominant radionuclide in that particle. The second approach, sector field ICP-MS (ICP-SFMS), can provide rapid, high-precision determination of the actinides, including the transuranic actinides, that characteristically occur in hot particles originating from weapons fallout or fuel matrices. Here, ICP-SFMS is shown to yield sufficiently low detection limits for plutonium isotopes (with the exception of 238Pu) to enable us to confirm negligible presence of plutonium in areas outside the Chernobyl exclusion zone, but with high levels of fission-product contamination.

Preliminary evaluation of 135Cs/137Cs as a forensic tool for identifying source of radioactive contamination

“…The ratio of 135 Cs/ 137 Cs may provide a useful forensic tool, as nuclear activity from different sources may be easily distinguished. A similar tool as has been found with Pu isotope ratios, where differentiation of radioactive fallout from different sources and events has been demonstrated in ice cores (Olivier et al, 2002), sea water and sediment (Buesseler, 1997;Rodushkin et al, 1999), soil (Entwistle et al, 2003) and atmospheric test sites (Warneke et al, 2002). The value of an isotopic fingerprint for modelling dispersion of fission products in the environment after radioactive spills has also been suggested: a study of the removal of Chernobyl-derived radiocaesium from European lakes found an influx of 137 Cs from bomb-testing fallout confounded the model (Smith et al, 1999), and inventories of the Kyshtym and Karachay accidents in the Ural mountains suggested that the site was also contaminated by Chernobyl 137 Cs (Aarkrog et al, 1997).…”

“…Some discrepancy between the 137 Cs determined by gamma spectrometry and in the dissolved sample determined by ICP-MS may be due to gamma counting of samples having an uneven distribution of activity. Because this technique is sensitive to geometry of the sample, activities determined in samples containing hot particles will vary according to the location of the hot particle within the sample, and do not follow a normal distribution where standard deviation is defined by the inherent measurement error (Entwistle et al, 2003). The 135 Cs/ 137 Cs ratio in the samples is likely dominated by hot particle activity, as no trend with depth of sediment was evident in the core sample.…”

Inductively coupled plasma mass spectrometry (ICP-MS) and laser ablation ICP-MS for isotope analysis of long-lived radionuclides

“…Therefore, 236 U/ 238 U isotope ratio measurements are of interest for the characterization of hot particles which were found in soils contaminated by Chernobyl fallout. Hot particles were characterized by ICP-SFMS with respect to uranium isotope ratios by Entwistle et al [68] and Boulyga and Becker [69]. 236 U/ 238 U isotope ratios in the range from 10 −5 to 10 −3 were found [69].…”

New horizons in microparticle forensics: Actinide imaging and detection of 238 Pu and 242m Am in hot particles

“…A great number of radioactive particles were released during the accident of 26 April 1986 in reactor block number 4 of the Chernobyl nuclear power plant (16)(17)(18). Even today, many of these so-called hot particles can be found, predominantly within the Chernobyl exclusion zone (CEZ) 30 km around the reactor (19,20). They are distinguishable from the surrounding bulk soil due to their high specific gamma activity ( 137 Cs content of the fuel particles) and due to their high density and high atomic number (fuel fragments).…”