[en] Since ammonia, for pH control in order to minimize corrosion product in domestic PWR secondary system, is very volatile, tends to stay with steam phase, resulting in not providing proper pHs, advanced amine application is required. On the other hand, dissolved organics in the system water should be removed during purification, since they have a negative effect on cycle pH. Therefore, the characteristics of pH control agent with ammonia, morpholine, AMP, ETA and MPA were evaluated, and the effect of dissolved organics on pH and the characteristics of photocatalysis for removal of them were studied. Based on the base strength, distribution coefficient and thermal decomposition rate of them, a single amine system of MPA seems to be available. Under consideration of wet stream area protection and low temperature feedwater pH requirement, a mixed amine system of ETA with MPA or morpholines thought to be more available. The calculated values of thermodynamic equilibrium between carbonic acid, carbonate, bicarbonate and carbon dioxide agreed with the values measured from photocatalytic decomposition experiments of salicylic acid. It showed that the conductivities of the aqueous solution decreased, while pH increased even with extremely small quantities of organic acids. While, results experimentally obtained from photocatalytic decomposition of the amines above showed photocatalysis would be an available process for removal of dissolved organics. (author). 35 refs., 11 tabs., 34 figs