Batch-type tests on formations of volatile iodine species from aqueous iodide solution under 60Co γ-rays irradiation

Test programs with a dynamic flow type VITA (Volatile Iodine Test Apparatus) system are in progress in JAERI for elucidating the radiation- induced formations of volatile iodine species and their transport behavior under natural and/or forced convection conditions. In order to survey dominant factors affecting the radiation-induced reactions of volatile iodine formation in advance of the VITA programs, batch tests were carried out using closed pyrex-glass ampoules (200 ml), in which 100 ml of aqueous iodide solution was filled with organic and various chemical additives. The test solutions were irradiated by 60Co γ-rays at 10 kGy/h (106 rad/h) for one hour. The formations of volatile iodine species (I2, CH3I and so on) from aqueous iodide solution were measured. We will describe the test results and discuss mechanisms of volatile iodine, especially methyl iodide, formations. From results of batch tests, the following conclusive remarks are derived on the radiation-induced formations of Iox forms, iodate and methyl iodide species: - The mechanism and the rate coefficients will be considered to be established theoretically, at least 25 deg. C, on radiation-induced reaction of Iox formation from aqueous I - solution. - The empirical calculations of Iox yield explain very well the measurements in an I0- concentration above 10-5 mol/dm3, although radiation-induced oxidation of HOI is assumed to be in the rate determining step of Iox formation. - In an I0- concentration below 10-5 mol/dm3, IO3- species is formed in higher yields. But the formation mechanism has not been characterized enough yet. - The utilization of organic additives for CH3I formation may be independent of the chemical properties of organic additives such as types and forms. - The CH3I yields may depend on redox potentials of γ-irradiation and the iodine/iodide systems, independent of concentrations of initial iodide and organic additive. - a simple reaction sequence may be proposed for formations of volatile iodine species by using overall rate constants. (authors)