High-precision isotope ratio measurements of uranium and plutonium in the environment
A high-precision mass spectrometry method has been developed to measure 240Pu/239Pu at low concentrations in environmental samples. A reproducibility of <1% for sample sizes of > 0.5pg has been achieved using a highly sensitive and precise multi-collector ICP-mass spectrometer (Micromass IsoProbe). The research has involved applying the developed methods to a number of environmental studies. Application I: A chronology for 240Pu/239Pu is presented. This chronology over the period 1945-1980 is derived from grass samples from the IACR Rothamsted Archive at Harpenden (Hertfordshire, UK) and an Alpine ice core from Dome du Gouter, Mont Blanc, French Alps. Activity concentrations for 137Cs and 239,240Pu and the 238U/235U atomic ratio were also determined in the same samples. The activity concentrations clearly reflect the yearly yields of atmospheric testing. A higher abundance of 235U can be linked to nuclear testing between 1950-1963. A higher abundance of 238U is found in the period 1970-80 in the grass, but not in the ice. The 240Pu/239Pu ratio is compared with Polar ice core and stratospheric data that have appeared in the literature. It is particularly notable that the UK herbage samples clearly identify fallout from the early U.S. tests in the Nevada desert (1952 and 1953) and that 1954-68 samples show isotope ratios reflecting stratospheric fallout. Ratios for the 1970s cannot be uniquely attributed to a single source. Complementary measurements of the 240Pu/239Pu have been carried out on stratospheric air filters from Sweden and ground-level air filters from Germany. Application IIA: Plutonium isotopes have been measured in a sediment core from the Wyre saltmarsh. Radionuclide contamination in this area is much higher than in many other UK sites because of its incorporation of discharges from the BNFL Sellafield. A model for the transport of radiolabelled sediment from the Sellafield area is also presented. Application IIB: Plutonium isotopic ratios in a mudflat core from Poole Harbour reflect the pattern that is expected from weapons fallout in the pre-1970 period. The post-1970 sediment shows an additional source that is considered to be the discharges from AEA Winfrith. Application III: Radioactive contamination of the environment by Pu and U as a result of operations at the AWE sites at Aldermaston and Burghfield has been investigated. Isotope ratio results show that contamination of the environment around the sites is patchy and its concentration very small. It is interesting to note that measurable contamination is not expected from the annual discharge records when a dispersion model is applied.