Chemical and Isotopic Signatures for Environmental Remediation and Nuclear Forensic Analysis

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1 Chemical and Isotopic Signatures for Environmental Remediation and Nuclear Forensic Analysis Sue B. Clark Washington State University, Department of Chemistry, Pullman, WA Abstract Synergies exist between the fields of nuclear forensics and environmental remediation. In both cases, samples are analyzed with the goal of correlating characteristics such as isotopic ratios and chemical composition and forms (e.g. signatures) with knowledge of the process(es) from which the sample is derived. While methods for obtaining and using isotopic signatures in nuclear forensics are well known, the application of chemical signatures and the development of methods to reveal chemical form represent a relatively new approach. Sequential extractions have been used for many years to determine chemical forms of contaminants for environmental regulatory purposes and clean up decisions. They show promise for application in nuclear forensics, particularly when coupled with an organized collection of process knowledge, such as a national nuclear forensics library. Key Words: sequential extractions, chemical signatures, isotopic signatures, national nuclear forensics libraries Background Nuclear forensics involves a comprehensive analysis of nuclear and other radiological materials to elucidate information on the production, use, and history of an unknown nuclear material [1, 2]. The production and use of nuclear materials, whether for power production, medical purposes, military uses, or other applications, involves a production cycle that includes multiple large scale processes, such as chemical processing, irradiations, recycling, and waste disposal. Within each process, isotopic, chemical, and physical characteristics or signatures that may be unique to the process are created. Using process knowledge, these characteristics can be organized into a library of information that technical experts can use to compare with the characteristics of an unknown material. Such a catalog of information is called a national nuclear forensics library, and the International Atomic Energy Agency (IAEA) is developing guidelines to assist member states in the development of their individual libraries [3]. For an unknown material, exact identification of a specific process or event based on isotopic, chemical, and physical information may be impractical, but correlations of these characteristics are often quite useful in determining whether or not an unknown material is or is not consistent with a given process.

2 While nuclear forensic analysis is typically used to assess the provenance of seized materials that are encountered outside of regulatory control, similar methodologies are applied when attempting to develop remediation strategies for ecosystems that have been contaminated by large scale nuclear materials processing. As with any industrial scale activities, both peaceful and military nuclear activities have impacted our natural resources, leaving a significant environmental legacy that requires clean up. Examples include the recent Fukushima Daiichi nuclear disaster in Japan [e.g. 4], the Chernobyl accident in Ukraine [e.g. 5], and the environmental legacy of nuclear weapons production by nuclear weapons states [e.g. 6], including the Unites States, Russia, the United Kingdom, France, China, and others. Decisions about remediation approaches depend, in part, on identification of the source(s) of the contamination, and the history of the process(es) that caused the contamination. For both nuclear forensics and environmental restoration, the goal of comprehensive analysis of samples is to correlate characteristics of the samples such as isotopic ratios and chemical composition and forms with knowledge of the process(es) from which the sample is derived. Chemical Form from Sequential Extractions In environmental assessments, the total concentration of a contaminant is often incomplete information upon which a clean up decision or remediation strategy can be made, as chemical form is often one of the most important parameters that determines a contaminant s environmental mobility and toxicity. To address this issue, environmental scientists and regulators use chemical analysis methods intended to determine the solubility or leachability of a contaminant. These methods typically assess how refractory or insoluble a contaminant is in a given sample or environmental system. One example of this is chromium, which can exist in two different chemical forms. Its oxidized form is the hexavalent chromate anion, or Cr(VI), which is highly soluble (i.e. not refractory at all) and also highly toxic. Its reduced form is the trivalent chromium cation, Cr(III), which is much less soluble, (i.e., significantly more refractory) and an essential trace element that supports human health. In the US, the Occupational Safety and Health Administration (OSHA) limits occupational airborne exposures of Cr(VI) to 5 micrograms per cubic meter in an 8 hour period, whereas Cr(III) exposure is limited to 500 1,000 micrograms per cubic meter in an 8 hour period [7]. Thus, protocols for the analysis of environmental samples have been developed to distinguish between the chromate anion and the trivalent chromium cation. Sequential extractions are one type of such protocols. Sequential extractions involve sequential leaching of a contaminated soil or air sample using chemical extractants that are increasingly aggressive. Contaminants that are leached from a sample using mild chemical treatments such as salt water or weak extractants (e.g. dilute solutions of bicarbonate or acetic acid, for example) are considered more labile and likely more mobile in the environment compared to contaminants that are only removed via chemically aggressive treatments, such as strong acid or strong base leaching, or even complete sample digestion. Typically, environmental remediation strategies are designed to target those contaminants that represent

3 the greatest risk to human populations, such as those contaminants exhibiting the greatest mobility and/or toxicity.

4 A Case Study: Pu Isotopic and Chemical Forms at a US Department of Energy Facility Sequential extractions have been applied for environmental site assessments at former nuclear weapons production facilities in the US, such as the Savannah River Site [8]. This application also illustrates the utility of sequential extractions in nuclear forensics. Correlating the chemical information revealed by sequential extractions with isotopic signatures can help to indicate the process(es) from which the nuclear material in the environment originated, which is often important for defining clean up approaches. For example, plutonium released from a chemical reprocessing facility is usually chemically less refractory than plutonium derived from processes generating refractory metals or oxides such as fuels. Thus, a sequential extraction scheme applied to an environmental sample that has been impacted by activities at a reprocessing facility compared to a sample impacted by a fuel fabrication facility should indicate plutonium in different chemical forms. In addition, sequential extraction information can aid in resolving the existence of multiple sources of contamination in a given sample. In other words, this additional chemical information, when correlated with other isotopic and physical information, can be used in conjunction with process knowledge to reveal clues about the source(s) of environmental contamination. Using samples collected down gradient from a low level aqueous radioactive disposal unit at the Savannah River Site (SRS), we applied a sequential extraction method to distinguish between two different sources of plutonium contamination for environmental remediation purposes [8]. A six step sequential extraction approach was used to define the following fractions of Pu contamination: water soluble, specifically sorbed, organically bound, reducible, low refractory, and highly refractory. For the SRS samples, the sequential extractions indicated that Pu was in a different chemical form compared to 238 Pu. The majority of the Pu was associated with the organically bound fraction, whereas 238 Pu was predominantly associated with a highly refractory phase. From the perspective of environmental remediation, this information also demonstrated that any remedial action planned for the site must consider two different chemical forms of plutonium, likely derived from two different sources. For example, an in situ remediation based on chemical extraction could feasibly remove the Pu, but it would not be effective for the removal of 238 Pu. On the other hand, if these same results were to be applied in the context of nuclear forensics, the information gained from sequential extractions suggests that two different sources of plutonium processing operated in the vicinity of sample collection. Using process knowledge of the activities that were conducted at SRS, the Pu contamination likely originated from the discharge of low level aqueous effluents resulting from chemical separations activities at the site, whereas the majority of the 238 Pu was derived from the production of energy sources for deep space exploration. For either the environmental restoration or nuclear forensics context, the existence of process knowledge greatly aided in interpretation of the combination of the chemical information and the isotopic signatures. One contrast between environmental assessment

5 activities and analysis of samples for nuclear forensics is that results from the analyses for nuclear forensics and their interpretation are often required on a much shorter timeline than that for environmental restoration. Hence, a national nuclear forensics library documenting process knowledge, materials usage, and materials handling history is ideally developed in advance of the need to compare to unknown materials. Summary and Conclusions Correlating isotopic and chemical signatures for samples to reveal information about the origin and history of the radioactive contamination in the sample can be useful for both environmental restoration and nuclear forensics activities. Coupling these correlations with process knowledge and other information existing in a national nuclear forensics library enhances the ability to assess whether seized material (in the case of nuclear forensics) or an environmental sample (in the case of environmental remediation) is or is not consistent with a given process or activity. Acknowledgements The preparation of this paper was supported by the US Department of Defense, Defense Threat Reduction Agency via a grant at Washington State University. Literature Cited 1 English Chinese, Chinese English Nuclear Security Glossary (2008); Committee on International Security and Arms Control (CISAC), National Academies Press, 2 Nuclear Security Recommendations on Nuclear and Other Radioactive Material out of Regulatory Control; IAEA Nuclear Security Series No. 15; ISBN Guidelines for the Development of a National Nuclear Forensics Library; Draft publication open for comment by member states. Dated October, nuclear.org/info/fukushima_accident_inf129.html 5 nuclear.org/info/chernobyl/inf07.html 6 Legacy of Nuclear Weapons Production; 7 United States Center for Disease Control, Agency for Toxic Substances and Disease Registry, 8 S. Loyland, S. Lamont, and S. B. Clark, 2001, Environ. Sci. &Tech,