EDF approach on OD corrosion of SG tubes

The secondary side corrosion of steam generator tubes is the main degradation of components in operating power plants, strongly impacted by chemistry. This is why EDF has largely studied the chemical parameters in its 56 PWRs which might influence corrosion development. The results of 168 hideout returns of chemical species performed on the French plants allowed to draw conclusions on where chemical species are likely to concentrate in steam generators and on the influence of several contaminants on corrosion processes: sodium, chloride, phosphate, organic compounds, etc... Based on laboratory studies and plants feedback, new chemistry specifications were established and are now applied to EDF units to minimize corrosion and operating costs and to provide a good availability while maintaining an excellent safety. Boric acid is added in the secondary water of the 10 oldest units with Inconel 600 MA tubing, highly sensitive to intergranular stress corrosion cracking. Implementation of such a chemistry at low operating costs, without expensive remedies or too stringent guidelines has been satisfactory with an average number of 21 plugged tubes per unit for stress corrosion cracking on secondary side of SG tubes including the non affected units with Inconel 600 thermally treated or Inconel 690. For the 18 affected units with Inconel 600 MA, this corresponds to an average of 67 plugged tubes per unit. No leak or burst occurred and only one unit replaced its steam generator due to this type of degradation. (author)