Impact of geopolymers on hard sludge formation on the secondary side of PWR steam generators under transient conditions

Abstract

In modern steam generators (SG), some tube degradation phenomena are promoted by the formation of hard sludge (HS) on top of tubesheet. These phenomena reduce the efficiency of nuclear power plants (NPP). In some cases, SG deposits are primarily constituted by HS. This HS usually presents high concentrations of aluminum and silicon (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) at specific locations. In fact, there is evidence that the presence of aluminum and silicon is at the origin of the problematic of HS in SG during normal operation.Geopolymers are inorganic polymer materials constituted by a 3D-network of interlinked silica and alumina tetrahedra. Geopolymers present cementitious properties similar to HS. Geopolymers are formed under highly alkaline conditions and low temperature, by the polymerization of aluminosilicate and/or silicate oligomers. During NPP transient periods, geopolymer formation could be favor by the physicochemical conditions in the gaps arisen due to the different thermal contraction of SG tubes, deposits and tubesheet. Thus, it could be though that part of the HS formed in the crevice between tube and tubesheet of SG could correspond to geopolymers. In order to demonstrate this hypothesis, the mayor constituents of HS (magnetite, aluminum and/or silicon sources) have been autoclaved in representative secondary circuit transient conditions (cooldown). The effect the Si to Al ratio, the aluminosilicate source, the presence of magnetite and the chemical conditions in the crevice is also studied. The chemical, structural and morphological characterization of the resulting solids shows the presence of aluminosilicate compounds in some of the tested experimental conditions. However, these aluminosilicate compounds do not act as cementing agents agglomerating magnetite particles and therefore, they do not contribute to HS formation.