Direct low-energy gamma-ray measurement of 129I in radioactive waste using LEGe spectrometry and evaluation of the minimum detectable activity.

Iodine-129, with its long half-life and high mobility, is a key radionuclide in the safety assessment of radioactive waste disposal. Meeting the domestic self-disposal criteria requires a measurement method capable of achieving a minimum detectable activity (MDA) below 0.001 Bq g-1. In this study, an analytical procedure combining volatilization-solvent extraction-precipitation with a low-energy germanium (LEGe) detector was investigated as a methodological feasibility study for the direct radioactivity measurement of iodine-129 in radioactive waste. As the AgI precipitate mass increased from 0.011 to 0.094 g, the detection efficiency decreased from 19.6% to 10.2% owing to self-absorption, with deviations from a quadratic calibration model between -4.73% and +4.94%. The maximum recovery of 76.0% was obtained at 2.0 g of KI carrier. Matrix effects were compared using Portland cement and SUS304: recovery decreased from 92.4% to 35.8% for cement and from 84.0% to 40.0% for SUS304 with increasing sample mass. The lowest MDA of 0.0047 Bq g-1 was obtained for cement; although this value is below the clearance level (0.01 Bq g-1) and useable for screening, it did not reach the regulatory MDA target (0.001 Bq g-1). Rather than presenting a fully validated analytical method, this work identifies the principal factors controlling the achievable MDA - precipitate mass (self-absorption), chemical recovery, matrix type, and counting time - and proposes an optimization strategy for approaching the regulatory target. Independent repeatability testing and comparison with reference methods remain as future work.