Characterization of Impurity Beta-Emitting Radionuclides in Spent 99Mo/99mTc Generators and Evaluation for Disposal Decision-Making

This study quantified the gross-alpha, gross-beta, and major beta-emitting radionuclides—3H, 14C, 55Fe, 90Sr, 99Tc, 129I, and 147Pm—excluding gamma-emitting radionuclides, in spent 99Mo/99mTc generator columns collected after distribution and use in domestic facilities. Based on these measurements, the feasibility of clearance disposal was evaluated. For radionuclides below the detection limit, the minimum detectable activity (MDA) was substituted according to regulatory guidelines and reflected in statistical analysis and fractional summation assessment. The gross-alpha activity of all samples was found to be below 10 Bq g⁻¹, indicating negligible contribution from alpha-emitting radionuclides. Conversely, gross-beta activity exceeded 10 Bq g⁻¹ in all samples, suggesting the necessity for individual beta emitter analysis. Among volatile radionuclides, both 3H and 14C were detected in all samples; however, their activity concentrations were sufficiently lower than the clearance levels, implying that they are not limiting factors for disposal. Although 129I was below the MDA in all samples, its regulatory clearance level (0.01 Bq g⁻¹) is much lower than the measured MDA, making regulatory confirmation challenging. For non-volatile radionuclides, 55Fe and 147Pm exhibited activity concentrations well below their respective clearance levels (1,000 Bq g⁻¹), implying minimal impact. In contrast, 90Sr approached the clearance threshold in some samples when considering analytical uncertainty. Particularly, 99Tc was detected at high levels in all samples, far exceeding the clearance level of 1 Bq g⁻¹, and was thus identified as the dominant radionuclide determining the non-clearance of the waste. Fractional summation analysis for clearance assessment revealed that, even when considering only beta-emitting radionuclides, the summed fraction exceeded unity under average conditions, indicating that clearance was not achievable. When the literature-reported gamma emitter 125Sb was also considered, the fraction further exceeded unity, reaffirming the infeasibility of clearance. Consequently, the disposal suitability of spent 99Mo/99mTc generator columns is primarily governed by 99Tc, with 90Sr and 125Sb contributing as secondary controlling radionuclides in certain cases. In conclusion, the radionuclide-specific quantitative data and fraction-based clearance assessment presented in this study provide essential foundational evidence to enhance the validity and reliability of decision-making for the disposal of spent generator columns.