[en] Main roles of cooling water at LWR generation plant are, 1) keeping of nuclear fission chain reaction based on moderation of fast neutron to thermal one (neutron moderation medium), and 2) transfer of thermal energy to turbines (heat transfer medium). On the other hand, the cooling water with high temperature and high pressure causes various phenomena relating to corrosion when its contacting to structural and other materials. As the corrosion problem of materials in high temperature water is not specific to nuclear reactors, as a result of irradiation in the reactors, corrosive decomposition species formed by radiolysis of water causes corrosion acceleration of their structural materials, to make the problem complicated. Correlation between cooling water and materials, because of its deep relation to operations of whole of generation plants, it is important not to acquire local solution such as solution or response to only individual focused accidents, but to largely grasp it as a problem on whole of plant system. Here were introduced recent trends on effect of aquatic chemistry in nuclear reactors and aquatic chemistry control aiming at relaxation of formation and progress of stress corrosion cracking for the cracking of materials of important phenomena relating to safety and reliability of nuclear plants. (K.G.)