Investigation of hideout return of steam generator in new nuclear power plants with pressurized water reactors

During normal operation of pressurized water reactor (PWR) nuclear power plant, some ionic impurities harmful to steam generator (SG) and secondary side can enter SG through feedwater. In the conditions of high temperature and pressure, and concentration factor value higher than 70, impurities concentrate in local regions and form harmful environment. Through monitoring the shutdown chemistry, one can predict and speculate the water chemistry situation in the flow-restricted regions. In this work, evaluation and analysis were made based on 4 practical hideout return testing data from three new PWR nuclear power plants. The results indicated that the monovalent ions such as Na⁺, K⁺, Cl^-, F^- return to the bulk water promptly during the power reduction process. However, the multivalent ions such as Ca²⁺, Mg²⁺, SO₄^2-, PO₄³ and organic ions require longer time. In comparison to the plants after long-term operation, the return amounts of PO₄^3-, formate,and acetate were relatively higher, which might be introduced during equipment manufacturing. Therefore, it is suggested that the hideout return time should be extended especially for new plants, so that the impurities could be completely returned to the bulk water, and removed by the blowdown system. The ALC, MRI and pH (T) modeling results used to determine the local SG chemical environment were also calculated based on 4 hideout return experiments, suggesting the alkaline environment for the SC during operating. (authors)