Outcomes and Analyses of the Secondary Circuit Water Chemistry Strategy for the French PWR Fleet

摘要

Secondary circuit water chemistry management aims to: (i)minimize general corrosion and flow accelerated corrosion of the secondary water system components; (ii) minimize corrosion product transport to the steam generator in order to guarantee steam generator performance, whilst reducing SG fouling and TSP blockage; (iii) minimize the need for steam generator chemical cleaning; (iv) minimize chemistry-related forced outages and repair costs, and finally (v) extend plant operating life time. The specific goals of secondary chemistry management are as follows: 1. Maintain a high overall pH throughout the circuit; 2. Ensure reducing conditions in the steam generators by limiting air ingresses and adding hydrazine; 3. Control and reduce impurities entering the steam generators; 4. Optimize the steam generator maintenance program (sludge lancing, chemical cleanings...); 5. Remove copper alloys from the circuit when possible (no microbiological constraints); 6. Maintain waste releases into the environment as low as reasonably achievable. The pH control strategy of the EDF fleet consists of the use of ethanolamine and ammonia as pH control agents. Ammonia and ethanolamine have high and low volatilities respectively and their combination ensures that both steam and condensate sections are protected, by guaranteeing a high overall pH throughout the circuit. Based on a two-year measurements campaign in existing EDF plants, and by simulating the distribution of chemical species throughout the circuit and associated FAC rate, the concentrations of conditioning agents have been optimized to protect against FAC and reduce waste releases. The target concentration in final feedwater is 3.5-4 ppm of ethanolamine and 1.5 ppm of ammonia to attain a pH_(25°C) of 9.6-9.7. In some cases, when dosing only ethanolamine and hydrazine, the condenser exhaust flow rate results in a low ammonia concentration making it necessary to simultaneously inject ammonia or recycle ammonia via the condenser vacuum system. The chemical conditioning implemented in EDF fleet is currently morpholine 9.2 pH_(25°C) (14 units) for units with copper alloys, morpholine 9.6 pH_(25°C) (30 units), ammonia 9.7 pH_(25°C) (4 units) and ethanolamine 9.7 pH_(25°C) (10 units). The EDF strategy is to switch all units to ethanolamine conditioning by 2020. This paper presents EDF's chemistry strategy to minimize general corrosion and flow accelerated corrosion in the secondary circuit, and to ensure the cleanliness of the secondary part of the SGs. It also summarizes the status and results of chemical treatments as well as presenting the water chemistry improvement program.