Development of an optimised method for measurement of iodine-129 in decommissioning wastes using ICP-MS/MS

Abstract

Accurate, low-level measurement of the long-lived fission product ¹²⁹I is important for waste characterisation and long-term monitoring of waste facilities and the surrounding environment. Inductively coupled plasma mass spectrometry (ICP-MS) is well-suited to high throughput, sensitive measurement of ¹²⁹I as a cost-effective alternative to other mass spectrometric and decay counting techniques. Accurate ¹²⁹I measurement by ICP-MS is affected by the multiple interferences on m/z = 129, necessitating multi-stage sample preparation and/or mathematical correction. This study assesses the capabilities of tandem ICP-MS/MS for rapid and routine measurement of ¹²⁹I in nuclear wastes. The advantages of the tandem setup for removal of isobaric, polyatomic and tailing interferences are demonstrated, as are the improvements in sensitivity through matrix modification and the importance of selecting an appropriate internal standard. The optimised setup was applied to measurement of various decommissioning waste simulants following direct extraction of ¹²⁹I. The procedure achieved an instrument detection limit of 1.05 × 10⁻⁴ Bq g⁻¹ (0.017 ng g⁻¹) for ¹²⁹I, which is two orders of magnitude below the target out-of-scope limit of 0.01 Bq g⁻¹ (1.57 ng g⁻¹), with good agreement between ICP-MS/MS and liquid scintillation counting (LSC). The results show that rapid, routine, low-level measurement of ¹²⁹I is achievable by ICP-MS/MS for end users in decommissioning and environmental monitoring.