Measurement methods for low-level 129I determinations

The best methods for concentration measurements of low levels of 129I and other very long-lived radionuclides present in nuclear wastes are based on techniques which do not require the measurement of radiation from the radionuclide. Radiochemical 129I analyses based on chemical separations and counting require high radionuclide purity, have limited capability for checking the radionuclide purity, and have low sensitivity as a result of low 129I specific activity. Other more selective and sensitive measurement methods developed and applied to 129I analyses are neutron activation analysis (NAA), thermal emission negative ion mass spectrometry (AMS). These methods are at least a factor of 1,000,000 more sensitive than beta, x-ray, or gamma-ray counting methods. They are also very specific. All the methods require iodine separation from the sample prior to measurement but the more sensitive methods can use smaller sample sizes. A chemical separation method based on sample combustion, iodine trapping, and iodine purification is described. Iodine separations have been performed on a variety of solid, liquid, and off-gas waste samples. The measurement methods for the separated iodine are compared for application to low-level waste analyses. The NAA, TEMS, and AMS techniques have similar 129I detection limits. Measurements by more than one method can be used to enhance quality assurance. The mass measuring instrument are also usable for C-14 and Tc-99 determinations on waste samples. 16 refs., 6 figs., 2 tabs