Sequential Separation of Iodine Species in Nitric Acid Media for Speciation Analysis of 129 I in a PUREX Process of Spent Nuclear Fuel Reprocessing.

For efficient and accurate speciation analysis of 129 I in the nitric acid solution of spent nuclear fuel in its reprocessing process, a sequential procedure for stepwise separation of different iodine species in 3 mol/L HNO 3 was proposed based on the solvent extraction using CCl 4 and mesitylene. Molecular iodine (I 2 ) was first separated by solvent extraction using CCl 4 , and iodide (I - ) remaining in the aqueous phase was oxidized to I 2 by adding NaNO 2 and then extracted to mesitylene. Finally, iodate (IO 3 - ) was reduced to I 2 using NH 2 OH·HCl and extracted to mesitylene. The separation efficiency of 98-99% for tracer amounts of 129 I 2 , 129 I - , and 129 IO 3 - in 3 mol/L HNO 3 and less than 2% crossover among different iodine species were achieved. The extraction process and mechanism of different iodine species in CCl 4 and mesitylene were investigated, and the problem of crossover of different iodine species due to the low extraction efficiency of low concentration of iodine in nitric acid solution was solved. A direct transfer of IO 3 - from HNO 3 to the mesitylene phase without conversion to I 2 was observed, which was attributed to the iodination of mesitylene in the HNO 3 medium. Addition of a stable iodine species carrier and repeated extraction significantly improved the separation efficiency of iodine species, making their quantitative separation achievable. This method provides an approach for speciation analysis of 129 I in the acidic spent nuclear fuel solution, enabling us to investigate and control the behavior of volatile 129 I in the spent nuclear fuel reprocessing process.