Refined Analytical Method for 129I in Cement-Solidified Spent Ion Exchange Resins

Spent ion exchange resins (SIERs) are commonly immobilized in cement, necessitating the reliable determination of radionuclides in cement-based waste forms. This study developed a simple and practical method for quantifying 129I, a key radionuclide in low- and intermediate-level radioactive waste (LILW) disposal. Investigation into iodine behavior during leaching, extraction, and precipitation revealed that iodate (IO3⁻) readily adsorbs onto cement minerals, reducing chemical recovery in extraction-based methods. To mitigate this, we implemented an in-situ conversion of IO3⁻ to I2, improving chemical yields from 58% to 93% in fine cement samples (φ < 0.5 mm). The optimized method was further applied to cement containing anion exchange resins, where increasing resin content led to a decrease in chemical yield, reaching 51% at 10wt% resin loading, close to the practical limit in ordinary Portland cement (OPC) waste forms. This decline was likely attributed to the re-adsorption of iodide (I⁻) onto the resins during the conversion of IO3⁻ to I2. Despite this, measured 129I activities closely matched expected values, demonstrating its reliability for routine analysis of cement-based radioactive waste.