The effect of polyacrylic acid on iron oxides formed on steam generator tubes and on metallic copper present in pressurized water reactors

International audienceIn order to reduce and prevent fouling of the Steam Generators (SGs) tubes and the blockage of tubes support plates in the secondary circuit, EDF has several approaches. One of them consists in injecting a chemical agent, polyacrylic acid (PAA), to prevent the deposition of corrosion products on the surface of SG tubes and to facilitate their evacuation by the SG blowdown circuit. First, the dispersing and/or complexing PAA effect on iron oxides, in the secondary circuit physico-chemical conditions (T = 275°C, pH$_{25°C}$ = 9.2-9.6, reducing conditions fixed by H$_2$) was demonstrated and quantified. For a PAA concentration equal to 500 x 10$^{-3}$ g.kg$^{-1}$, the iron content values were 2500 times higher than the iron solubility limit at the blowdown. Then, the influence of chemical conditioning and pH on the PAA effectiveness was studied. It has been showed that the pH and the conditioning amine nature (amine volatile treatment: ethanolamine, morpholine and/or ammonia) seems to have no significative impact on the PAA effect. A similar study was performed on the dispersing and/or complexing PAA effect on metallic copper present in several EDF’s power plants. The main conclusion is that PAA seems to disperse metallic copper at room temperature but this property disappears at high temperature (275°C).Finally, the PAA action mechanism on iron oxide at high temperature is discussed. Complexation seems to be the main interaction mechanism between PAA and iron oxides observed in the studied experimental conditions and materials. The most likely mechanism would seem to be a dissolution of magnetite forming the deposit and then a complexation reaction between PAA and iron(II) ions (reducing conditions). Moreover this complexation would be responsible for the formation of hematite observed on the magnetite powder