Calculation of pHt values at operating parameters of water coolant of the secondary circuit of NPPs with PWR

The main issue of the water-chemical mode (WCM) of the secondary circuit of a nuclear power plant (NPP) is to ensure trouble-free operation of the main equipment by maintaining such physical and chemical properties of the coolant that would prevent corrosion and deposition of internal surfaces equipment. The rate of corrosion depends on the pH value, which is measured in cooled samples (at 25 °C) and taken at control points throughout the steam-water circuit. Temperature increasing leads to the change in dissociation of electrolytes, their activity coefficients, and the dissociation constants of water and dissolved electrolytes change. These changes lead to the fact that the pH values measured at 25 °C do not correspond to the real values of the pH value at the operation parameters of the coolant. Therefore, it is necessary to monitor pH value at the actual temperature (pHt) and, in case of indicator deviations, to correct a water chemistry mode. The problem may be solved semi-empirically, that is to calculate pHt values using developed mathematical models with some data obtained from chemical monitoring devices. Mathematical models to calculate pHt are based on equations of the theory of electrolyte solutions. Previously obtained temperature-dependent values of the ionic product of water, specific electrical conductivity and dissociation constants of electrolytes have been used. The authors have developed mathematical models of coolant ionic equilibria under conditions of hydrazine-ammonia water chemistry mode. pHt values in aqueous solutions of carbonic acid and ammonia have been calculated. The determination of the high-temperature pH value is of great practical importance, especially for nuclear power plant units with PWR. The results obtained show that it is possible to monitor this indicator using the devices already available at the station for automatic chemical control. The values of thermal pH calculated according to the algorithms developed by the authors are comparable with the results of other studies. The results obtained may be used to develop complex algorithms to calculate pH for industrial conditions.