The iodine species and their behavior in the dissolution of spent-fuel specimens

In this paper, spent-fuel specimens (∼3 g each) with a burnup of 21 to 39 GWd/t were dissolved in 30 ml of 4 M HNO3 at 100 degrees C, and the distribution of iodine and its chemical forms in the solution were studied. A small quantity of the iodine was conveyed to the insoluble residue (up to 2.3%), some remained in the fuel solution (up to 9.7%), and the balance was in the off-gas. Iodine was not deposited on the fuel cladding. Organic iodides were only ∼6.5% or less of the total amount of iodine in the off-gas. The fuel solution included iodine species that were difficult to expel by NO2 sparging alone (27 to 46% of the iodine in the solution). These species were ascribed to be the colloids of AgI and PdI2. Iodate (IO-3) was a rather minor iodine species in dissolution in ∼4 M HNO3. A thermochemical calculation also supports these results, indicating that the quantity of IO-3 is ≤1.7 x 10-4% of the iodine fed to 4 M HNO3 and that the colloid of AgI can be formed when the concentration of I- is ≥5.3 x 10-10 M