Limitations on the precision of 238U/235U measurements and implications for environmental monitoring

摘要

The ability to determine the isotopic composition of uranium in environmental samples is an important component of the International Atomic Energy Agency's (IAEA) safeguards program, and variations in the isotopic ratio (sup 238)U/(sup 235)U provide the most direct evidence of isotopic enrichment activities. The interpretation of observed variations in (sup 238)U/(sup 235)U depends on the ability to distinguish enrichment from instrumental biases and any variations occurring in the environment but not related to enrichment activities. Instrumental biases that have historically limited the accuracy of (sup 238)U/(sup 235)U determinations can be eliminated by the use of the (sup 233)U/(sup 236)U double-spike technique. With this technique, it is possible to determine the (sup 238)U/(sup 235)U in samples to an accuracy equal to the precision of the measurement, ca. 0.1% for a few 10's of nanograms of uranium. Given an accurate determination of (sup 238)U/(sup 235)U, positive identification of enrichment activities depends on the observed value being outside the range of (sup 238)U/(sup 235)U's expected as a result of natural or environmental variations. Analyses of a suite of soil samples showed no variation beyond 0.2% in (sup 238)U/(sup 235)U.