Estimation of pH in Iodine Solution on Equilibrium Condition

It is recognized that a radioiodine is potentially one of the most hazardous fission products that could be released from the fuel of nuclear reactors during severe accidents. The nuclear fuel has large inventories of various isotopes on iodine, and released iodine from the fuel makes complex chemical forms under a severe accident. According to the thermodynamic calculations and various experiments under the severe accident condition, the iodine released from the fuel would be primarily in its reduced state as cesium iodide in the containment. This would be dissolved in the water by discharging of coolant and safety spray system, and be re-vaporized into the iodine by oxidization or radiolysis process. There exist two basic approaches, that is, mechanistic and empirical models, to modeling on the generation of volatile iodine, I₂, from non-volatile iodide, I^-, in the aqueous phase by the radiolytic oxidation. The empirical models are practically used in the severe accident codes such as IODE, ASTEC, IMPAR etc. The mechanistic modeling tries to represent the whole set of elementary radiochemical reactions (typically included hundreds of the important reactions in water) and solves for the kinetics of these reactions using some ordinary differential equation solver. The typical codes adapted in the mechanistic approach are the INSPECT (IodiNe SPECiation and Transport) developed in AEAT, which has been integrated with some changes into MELCOR 1.8.5 and the LIRIC model (Library of Iodine reactions in containment) developed in AECL. We prepared a pool scrubbing code using the current iodine retention models, and performed sensitivity studies on the iodine retention such as the pool temperature, vapor velocity in a bubble, and iodine concentration. We also issued a new correlation to estimate the iodate concentration in an acidic pool under irradiation conditions by identifying the key reactions and solving the differential equations based on the rate law. The correlation was also verified with experimental data using NaI solutions under gamma irradiation. The important chemical parameters in aqueous solution at equilibrium on the iodine chemistry are pH, chemical potential, temperature, and total iodine concentration. The chemical potential and pH at a given temperature and iodine concentration are usually used to describe the behavior of iodine water system at equilibrium. Generally, Gibbs energies minimization is used to calculate species concentration in either solid, gas or mixture phase at equilibrium. In this paper, the Gibbs energies minimization method is set to calculate the iodine species and the concentration of H⁺ or pH in solution. Gibbs energies minimization method is successfully set to calculate the iodine species and the concentration of H⁺ or pH in solution. The pH increases from acidic to alkaline solution as the concentration of total iodine in the solution decreases.