Determination of uranium concentration and isotopic composition by means of ICP-MS in sequential extracts of sediment from the vicinity of a uranium enrichment plant

A method has been developed for the determination of the U-238/U-235 isotope abundance ratio in sequential extracts of freshwater sediment by means of ICP-MS. Instrument parameters were optimised, and sample pre-treatment procedures developed, to improve the quantification of U-235. Extracts prepared according to the three-stage BCR sequential extraction protocol were passed through a 2 cm column of anion exchange resin (BioRad AG1-X8), and uranium eluted with 8 mol l(-1) HNO3 prior to analysis. The precision obtained for measurement of the U-238/U-235 ratio in solutions containing 20 ng ml(-1) U-238 and 0.2 ng ml(-1) U-235 was 0.18%. Analysis of stream sediments from the vicinity of a nuclear fuel enrichment plant revealed an enhancement in uranium levels (up to 27 mug g(-1) dry weight) which decreased with distance from the effluent discharge point. The U-238/U-235 ratios measured in sequential extracts of the stream sediments were significantly less than 100 (cf. the natural value of 137.9), confirming that Rivacre Brook is contaminated with U-235-enriched uranium. Results of the sequential extraction also suggested that uranium is discharged from the plant in, or rapidly converted to, a relatively refractory form.