Speciation analysis of ¹²⁹I in spent nuclear fuel solutions

The lack of a suitable method for the speciation analysis of iodine-129 (¹²⁹I) in spent nuclear fuel solutions has hindered both comprehensive understand and precise control of ¹²⁹I prior to plutonium and uranium separation in reprocessing nuclear fuel. In this work, a sequential separation procedure is established to quantify multiple ¹²⁹I species in spent nuclear fuel solutions, namely I₂, IO₃^-, and colloidal PdI₂ and AgI, based on their distinct physicochemical properties. I₂ was initially transferred into an upper mesitylene phase via N₂ sparging from an acidic spent nuclear fuel solution. Colloidal PdI₂ and AgI were then isolated through ultrafiltration with centrifugation and subsequently dissolved for precise measurement. The remaining IO₃^- in the filtrate was reduced to I₂ using NH₂OH·HCl and extracted into the mesitylene phase. Cross-contamination among the iodine species was verified to be less than 2%, and the sum of all ¹²⁹I species was consistent with the total spiked ¹²⁹I amount. ¹²⁹I in each fraction was quantified by liquid scintillation counting, enabling accurate analysis of ¹²⁹I speciation in spent nuclear fuel solutions. The developed method has been successfully applied for elucidating the distribution and transformation mechanisms of ¹²⁹I in reprocessing process, providing critical insights for optimizing radioactive iodine management strategies in spent nuclear fuel reprocessing.