The Influence of Steel Alloying on the Flow-Accelerated Corrosion of Steels in the Loops of NPPs with VVERs

Ensuring the design service life of equipment and pipelines by reducing the flow-accelerated corrosion (FAC) in them and preventing deposition of corrosion products in the steam generator are among the key objectives pursued in designing the NPP’s secondary coolant system. The optimization of design solutions consists in controlling the flow-accelerated corrosion processes by selecting a certain combination of inhibiting agents and using steels with the necessary alloying degree in different loop sections. To minimize the FAC during operation at elevated temperatures, it is necessary to increase the concentration of amines. To do so, a need arises to introduce a second point for metering inhibiting agents downstream of the deaerator, replace the amines used in VVER reactors by more efficient ones, or use alloyed steels for manufacturing the equipment. In view of constantly tightening environmental requirements for power units, which are designed for a service life of 60 years and longer, there is a need to decrease possible toxic water effluents in the case of using organic inhibiting agents while retaining the water chemistry (WC) quality in terms of minimal concentration of corrosion products in the steam generator’s feed water. This can only be achieved by using alloyed steels for the loop sections in which inhibiting agents are unable to protect the structural material against FAC. In view of an adverse environmental impact caused by discharges of regeneration solutions from the secondary coolant working medium treatment plants, a need arises to optimize the combination of inhibiting agents and the scope of using alloyed steels in the NPP loops. The requirements for decreasing toxic water effluents prompt engineers to search for optimal solutions on selecting the type and concentration of inhibiting agents and using alloyed steels in sections where the FAC criterion is less than unity. The temperatures of coolants with different pH values in the secondary coolant circuit of NPPs with VVERs above which the FAC optimal minimization criterion is less than unity are determined. The influence of water chemistry and test conditions on the FAC ratio for different steels is considered. The correlation between the FAC ratio of steels and their chemical composition and magnetite equilibrium solubility in the coolant is shown.