Role of aluminum and silicon species in the formation of hard sludge on the secondary side of PWR steam generators: Potential geopolymers contribution

Despite the exhaustive chemical control, modern pressurized water reactors (PWRs) steam generators (SGs) have experienced damage associated with the formation of hard sludge on top of tubesheet. This phenomenon is detrimental because it is linked to tube degradation problems on the secondary side of SGs by corrosion processes, such as outer diameter stress corrosion cracking (ODSCC) enhanced by tube deformation or denting at the top of tubesheet. There is evidence of the contribution of aluminum and silicon species to the binding and adhesion of iron oxides particles in the formation and consolidation of hard sludge during normal operation. In fact, characterization of SG deposit samples mainly constituted by hard sludge determines high concentrations of these species (up to 6.0 +/- 0.1 wt% of Al and 2.7 +/- 0.1 wt% of Si), together with high microhardness values (above 800 HV) measured at particular locations within collars or hard sludge particles. Taking into account the physicochemical conditions (low temperature and high pH) in the gap between tube and tubesheet of SGs due to the thermal contraction of both materials during transient conditions (cooldown), this work proposes and discusses the possible formation of geopolymers (inorganic aluminosilicate polymers with cement-like mechanical properties formed at low temperatures and high alkaline conditions) during this period. In order to support this proposal, the role played by Si and Al compounds in the formation and consolidation of hard sludge is reviewed, considering information obtained from hard sludge characterization and from experimental studies reported in the literature.