Anthropogenic 236U at Rocky Flats, Ashtabula river harbor, and Mersey estuary: three case studies by sector inductively coupled plasma mass spectrometry

Abstract

236U (t 1/2=2.3×10 7 y) is formed as a result of thermal neutron capture by 235U. In naturally occurring U ores, where a high neutron flux is present from spontaneous fission of 238U, 236U/ 238U atom ratios are ~10 −4 ppm. In the natural Earth’s crust, unaffected by nuclear fallout, these ratios are expected to be on the order of 10 −8 ppm. Reactor-irradiated U, however, exhibits high 236U/ 238U atom ratios approaching 10 4 ppm. As a result, the presence of very small quantities of reactor-irradiated U will significantly enhance the ‘background’ 236U/ 238U atom ratio. When sufficiently elevated 236U/ 238U ratios are present, the determination of 236U/ 238U by rapid inductively coupled plasma mass spectrometric (ICPMS) methods is attractive. We have used sector ICPMS at medium resolving power (R=3440) to measure 236U/ 238U atom ratios with a determination limit of 0.2 ppm. The limiting factors in the measurement are the 235U 1H + isobar and background signal at m/z 236 arising from the 238U + peak tail. Based upon the analysis of replicates and considerations of possible systematic errors, uncertainties of ±5% are found for 236U/ 238U atom ratios of 1−100 ppm. This procedure has been demonstrated in studies of anthropogenic 236U in the environment at three locations: (a) offsite soils from the vicinity of the Rocky Flats Environmental Technology site (Golden, Colorado, USA); (b) sediments from the Ashtabula River (Ohio, USA); and (c) sediments from the Mersey estuary (Liverpool, UK). In each of these three locations, definite plumes of elevated 236U/ 238U are identified and characterized. Maximum 236U/ 238U atom ratios observed in RFETS-vicinity soils, the Ashtabula River, and the Mersey Estuary are 2.8, 140, and 4.4 ppm, respectively.