Since the severe accident of nuclear power plants happened in 2013, in many countries additional systems have been introduced or significantly considered to reinforce the safety.In Korea, for mitigating the hydrogen gas explosion problem possibly caused in the containment building, two safety systems have been considered.PAR (passive autocatalytic recombiner) which is a hydrogen removing system has been already introduced in the plants, and FCVS (filtered containment venting system) is under investigation as an additional safety system.FCVS is an emergency gas venting system, which is designed to operate at severe accidents, in order to lower the pressure of the containment building, by releasing contaminated steam and gas outside of the containment building.This FCVS can be divided into two different filter parts; the first part is wet scrubber filled with alkali aqueous solution and the second is composed of metal fiber filter.The wet scrubber part captures the most of the radioactive iodine species and particulates like Cs.Some fission products which pass through the first part can be captured in the next metal fiber filter.In present study, we investigated the chemical behavior of iodine in water under a high dose of gamma irradiation in the range of 0-10 kGy h-1.Unlike metallic radionuclides,iodine has various oxidation states and its volatility depends on the oxidation states in aqueous media.We found that the gamma oxidation of iodide (I-) and gamma reduction of molecular iodine (I2) take place with a distinct boundary at pH 6.0.And we observed that both oxidation and reduction reactions can change the solution pH considerably.Our results will be helpful to predict the behavior of iodine in water scrubber and to improve the scrubber structure of FCVS.