Influence of NO_x and HNO₂ on Iodine Quantity in Spent-Fuel Solutions

The quantity of iodine in spent-fuel solutions tends to decrease with an increase in the dissolution rate. This phenomenon is ascribed to the presence of nitrous acid (HNO₂) generated in the dissolution process because of the following three findings: (a) in a hot nitric acid solution, the steady-state HNO₂ concentration increases with an increase in the rate of its production and decreases with an increase in temperature, (b) the HNO₂ decreases the quantity of colloidal iodine (the main component of residual iodine in a simulated spent-fuel solution) in proportion to its concentration up to ∼3.0 × 10⁻³ M, and (c) a higher dissolution rate of UO₂ causes a higher HNO₂ production rate, hence, a higher HNO₂ concentration in the solution. The HNO₂ did not appear (i.e., [HNO₂] <2 × 10⁻⁴ M) in the dissolution of a UO₂ pellet (∼1 g) with a low dissolution rate, 0.4 g/h of UO₂ at 100°C. When high concentrations of I₂ and NO₂ (263 parts per million of I₂ and 38% of NO₂) in an N₂flow were passed through a simulated spent-fuel solution at 100°C, the predicted colloid of AgI was produced as a chemical equilibrium product of the reaction AgI(s) + 2HNO₃(aq) = I₂(aq) + AgNO₃(aq) + NO₂(g) + H₂O(l). This finding suggests that colloidal iodine may be produced secondarily in the dissolver of reprocessing plants; this can be one of the reasons why the residual iodine quantity in spentfuel solutions is higher in reprocessing plants than in laboratory-scale experiments.