Fouling of Steam Generator Tubes in Nuclear Power Plants : Investigation on the Preventive Effect of Polyacrylic Acid on Iron Oxides Deposition

Abstract : In order to reduce and prevent the fouling and blockage of Steam Generators (SGs), one of the approaches of nuclear operators consists in injecting PolyAcrylic Acid (PAA) as a dispersing agent. The goal of this injection is to prevent corrosion products deposition on the surface of SG tubes and to facilitate their evacuation through the SG blowdown circuit. The aim of this study was to evaluate the preventive nature of PAA on the fouling deposits (mainly composed of iron oxides) in SGs and to determine the action mechanism(s) of PAA on iron oxides. The investigation of the effect of PAA on magnetite suspensions and on the formation of deposit layers onto (pre-oxidized) Inconel 690TT tubes was conducted. This was performed in a dedicated biphasic loop reproducing the physico-chemical conditions of pressurized water reactor SGs. The results showed an increased iron concentration at the loop exit with the injection of PAA. Characterizations of the samples indicated the apparent diminution of oxide layer thickness and a change in oxide phase from magnetite to hematite at the liquid-steam interface region in the reactor. The study of these effects led us to the critical discussion of the hypotheses which can explain the underlying action mechanisms of PAA. Moreover, the SG fouling, including an injection of PAA, has been modelled and showed an increase of iron quantity at SG blowdown. The proposed model was based on the physico-chemistry mechanisms occurring in SG. The chemical code PhreeqCEA-3.2 version dedicated to reactors application was successfully used.