Kinetic analyses on startup and shutdown chemistry of BWR plant

During startup and shutdown of Boiling Water Reactor (BWR) plants, temperature and dissolved oxygen (DO) concentration of reactor water change in a wide range. The changes result in variation of conductivity and pH of the reactor water. It has been speculated that the water chemistry change is due to dissolution of the oxides on fuel claddings and structural materials. However, detailed mechanism is not known. In the present paper, trend of recent water chemistry in several BWR plants during startup and shutdown is presented. Conductivity and pH are convenient indication of coolant purity. We tried to clarify the mechanism of the change in the conductivity and the pH value during startup and shutdown, based on the water chemistry data measured. In the water chemistry data, change in chromate concentration and Ni²⁺ concentration is rather large. It is assumed that change in the chromate concentration and the Ni²⁺ concentration results in the time variation of the conductivity and the pH value. It is reasonable to consider that the increase in the chromate concentration and the Ni²⁺ concentration is ascribed to dissolution of Cr oxides and Ni oxides, respectively. A model of dissolution of the Cr oxides and the Ni oxides is proposed. A concept of finite inventory of the Cr oxides and the Ni oxides in the coolant system is introduced. The model is as follows. Chromate is generated by oxidation of the Cr oxides and the Cr dissolution rate depends on the DO concentration. The dissolution rate of chromate is in proportion to DO concentration, the inventory of Cr and difference between solubility limit and the chromate concentration. On the other hand, Ni²⁺ is formed by dissolution of the Ni oxides, and DO is not necessary in this process. The dissolution rate of Ni²⁺ is in proportion to the inventory of Ni and difference between solubility limit and the Ni²⁺ concentration. Coolant is continuously purified, and the chromate concentration and the Ni²⁺ concentration are reduced with constant fraction of the concentrations. The input parameters in the model are temperature and the DO concentration, which vary with time. During the shutdown, the Ni²⁺ concentration is dominant. After fitting parameters related to the Ni dissolution, time variation of pH and conductivity was successfully reproduced. It can be concluded that time variation of pH and conductivity during the shutdown is determined solely by Ni²⁺. After parameter fitting of the Cr dissolution during the startup, the time variation of the chromate concentration was reproduced. Parameters on the Ni dissolution were fitted so that the Ni inventory was not vanished in a short time and the maximum Ni²⁺ concentration was similar to that of the chromate concentration. Based on the calculation results of the chromate concentration and the Ni²⁺ concentration, conductivity and pH were calculated. It is concluded that water chemistry during the startup is determined by chromate and Ni²⁺. The other ionic species unlikely affect the conductivity and pH. Analyses of startup chemistry and shutdown chemistry may provide much information about corrosion products in the preceding operation cycle. For example, the present model analysis suggests possibility that quantity of corrosion products during the preceding cycle can be estimated. It is expected that the present results contribute to fundamental understanding of water chemistry not only in startup and shutdown, but also in full power operation. (authors)