Release of radioiodine from UO2 during reactor operation

Abstract

The formation mechanisms of radioactive organic iodides have been studied during reactor operation and during scram using a specially designed experimental set-up. The fractional release rates of iodine nuclides 132 I, 133 I and 135 I from UO 2 pellets at 250°C were determined as 8.3 × 10 −6 ( R/B ): the direct recoil process is the most predominant release process. Subsequently, these released nuclides were plated-out on the wall. The apparent desorption rates of the nuclides 131 I, 133 I and 132 I from the decay of 132 Te were averaged at (3.2 ± 0.7) × 10 −4 h −1 for organic iodides and at (2.2 ± 1.7) × 10 −4 h −1 for inorganic iodides during reactor operation. Under the reactor scram, the desorption rates of organic iodides decreased to (6.5 ± 1.2) × 10 −5 h −1 for 131 I and 133 I, while the value increased by one order for 132 I. These experimental observations have been explained by the radiation-induced radical reactions, leading to the formation of organic iodides.