Fe–Cr–Ni alloys have experienced localized degradation, such as stress-corrosion cracking (SCC), when used for steam generator tubes in nuclear power plants. The tube surface can be covered by a porous deposit layer resulting primarily from fouling. This porous layer acts as a barrier to the mass transfer for the chemical species in the main fluid to the tube surface. Thus, it influences the interfacial chemistry at the metal surface and the susceptibility of Fe–Cr–Ni alloys to SCC. While the chemistry of the main fluid can be controlled and monitored, this interfacial chemistry must be determined indirectly. Numerical models can be used to predict the interfacial chemistry and provide insight to SCC initiation and propagation. In the present work, a numerical model has been developed to calculate the mass-transfer rate of a chemical species, such as dissolved oxygen (DO), from main fluid to tube surface through an unreactive porous layer under single-phase liquid flow conditions. Major features of the model were validated against available literature data at room temperature (25 °C). The numerical results for high pressure (5 MPa) and high temperature (250 °C) conditions show that the effect of advection on the mass-transfer rate of DO through an unreactive porous layer dominates over that of diffusion.

No abstract

A new tube degradation mechanism was observed in a recirculating steam generator (SG) with an integral preheater tube at the clearance gap between the tube and the preheater baffle. The general pattern of the damage and material composition in the degraded region suggested that the degradation was cavitation erosion. Cavitation erosion occurs when vapour bubbles exist or form in the flowing liquid and then these bubbles collapse violently in the vicinity of a solid wall. The bubbles collapse when they contact water that is sufficiently subcooled, i.e., below the saturation temperature. In the clearance gap between the tube and the preheater baffle, secondary fluid flow exists due to the pressure difference across the baffle plate. Meanwhile, heat transfer occurs from the primary-side fluid to the secondary-side fluid within this clearance gap, driven by the primary-to-secondary temperature difference. Factors such as the tube position in the baffle hole and fouling may influence the local flow and heat transfer conditions and can cause subcooled boiling that results in cavitation. This paper presents a numerical analysis of flow and heat transfer phenomena to determine the factors contributing to cavitation erosion of tubes in the preheater of a recirculating SG. The analysis used the THIRST code for a 3-dimensional thermalhydraulic simulation of steam generator and the ANSYS Fluent® code for detailed calculations of flow and heat transfer in the clearance gaps. A detailed temperature distribution in the gap was obtained using this analysis to determine the regions where subcooled boiling could occur by comparing the local fluid temperature with its saturation temperature. The susceptibility to cavitation was found to increase with increased inclination (i.e., tilt) and eccentricity (i.e., off-centre) of the tube in the baffle plate gap, and increased fouling on baffle plate surfaces. This methodology could be applied to analyze the cavitation susceptibility for other preheater types with this “tube to baffle plate” gap, as these preheaters might have conditions where boiling followed by the rapid condensation of the steam bubbles are present.