The Interaction of Iodine with Insoluble Residue in the Dissolution of Simulated Spent-Fuel Pellets

To properly control radioiodine (¹²⁹I) when reprocessing nuclear fuels, it is important to understand the interaction between iodine and the insoluble residue produced during the dissolution of spent fuels. Simulated spent-fuel pellets (∼1 g each) equivalent to spent fuel with a burnup of 5% fima were dissolved in 4.1 M HNO₃ or a simulated spent-fuel solution to examine this interaction and the material balance of iodine. In dissolution in 4.1 M HNO₃, 2 to 5% of the iodine in the pellet is conveyed to the insoluble residue (8 ± 1 mg), 1 to 5% remains in solution, and the balance volatilizes into the off-gas. The process that incorporates iodine into the residue is the formation of slightly soluble iodides, such as PdI₂ and AgI, on the surface of the residue. The quantity of iodine in the residue averages 1.1 ± 0.5 µg I/mg of residue. Pellet dissolution in simulated spent-fuel solutions with a uranium concentration of ≧170 g U/ℓ and corresponding amounts of fission product elements causes a marked increase in the amount of residue and a significant increase in the amount of iodine involved. This phenomenon is due to the secondary precipitation of some metal molybdates. The PdI₂ and AgI in the residue are in equilibrium with Pd²⁺, Ag⁺, and I^- in the solution. The I^- can be oxidized into I₂ in a hot nitric acid solution bubbled with NO₂. The action of NO₂ causes part of the iodine in the residue to be eluted into the solution and then volatilized into the off-gas during the operation to expel iodine () from the solution. A process consisting of (a) heating of the residue in a -concentrated HNO₃ at 100°C and (b) introducing NO into the solution at 100°C will transfer 50 to 90% of the iodine in the residue to the gas phase. The remaining iodine is probably inside the residue as it is difficult to remove.