Effect of Heat Flux on the Concentration Factor in Crevices of Nuclear Steam Generators

To investigate the denting phenomenon, the rates of corrosion occurring in simulated tube/support plate crevices were examined by using seven model boilers at 1.11 MPa. The model boilers were operated with all-volatile treatment (morpholine + hydrazine) with 15- and 100-ppm chloride concentrations constituting 10% FeCl₂, 30% NaCl, and 60% CaCl₂. It was found that corrosion rates increased with heat flux. A model was proposed to explain this observation, based on mechanisms that acid chloride is concentrated in the tube/support plate crevices. The model is expressed by the following equation for empty heated crevices: Good agreement was obtained by comparing the results predicted by the model with Brown’s data and the present data for empty heated crevices, and with Pathania’s data obtained at high heat flux. Based on the above equation, a model was developed to describe chloride concentration within the crevices versus heat flux for given condenser leakage rates in nuclear steam generators. Results predicted by the model show that a small increase in condenser leakage rates gives a considerable increase in chloride concentration within the crevices.