Applications of Transuranics as Tracers and Chronometers in the Environment

Abstract

The transuranic elements (TRU) Np, Pu, Am, and Cm have prominently emerged as powerful tracers of earth and environmental processes, applicable to the recent, post nuclear-era timescale. Various long-lived isotopes of these elements are found in the earth’s surface environment, almost exclusively as a result of nuclear weapons production, testing, or nuclear fuel cycle activities. A globally recognizable signal, of consistent composition, from stratospheric fallout derived from 1950–1960 above-ground weapons tests is itself useful in tracing applications; in specific local/regional settings, stratospheric fallout is mixed with or dominated by other TRU sources with contrasting isotopic signatures. Both decay-counting and MS approaches have been utilized to measure the concentrations and isotopic ratios of TRU and are useful as discriminators for source characterization, provenance, and apportionment. Examples include the activity ratios 238Pu/239+240Pu, 241Am/239+240Pu, and 241Pu/239+240Pu; atom ratios such as 240Pu/239Pu, 237Np/239Pu, 241Pu/239Pu, and 242Pu/239Pu are also used in this context. Of the TRU elements, Pu is by far the most widely studied; accordingly, this chapter mainly emphasizes the use of Pu activities and/or atom ratios as tracers and/or chronometers. Nevertheless, Pu is sometimes measured in combination with one or more isotopes of other elements. The TRU elements offer several prominent applications in environmental/geochemical tracing: (1) chronostratigraphy of sediments and related recent Holocene deposits; (2) using fallout TRU as quantitative probes of soil erosion, transport and deposition; (3) investigating water mass circulation, the transport and scavenging of particulate matter, and tracking the marine geochemical behavior of the TRU elements themselves in the marine environment; and (4) studies of the local/regional transport, deposition and inventories of non-fallout TRU in the surficial environment.