Tubesheet and expansion transition cracking at Doel 3 and Tihange 2

The field experience from Doel 2, Doel 3, and Tihange-2 shows that mill-annealed Alloy 600 tubes may be very susceptible to primary water stress corrosion cracking in the tubesheet area, both in the roll transition and the expanded area for the full thickness of the tubesheet. Cracks initiate at areas of high residual stress resulting from mechanical or hydraulic expansion. These areas include the roll transition (with or without a kiss roll), areas of major rolling anomalies, and also areas of minor discontinuities, even those which are considered acceptable (and are practically unavoidable) by normal manufacturing practices. A large number of cracked tubes may not be noticed by the normal inspection methods and may lead to only very small service leaks. Thus, many units with mill-annealed Alloy 600 tubing might be affected by an undetected stress corrosion problem. The long-term operational and safety significance of the degradation progress is an unclear issue. An extensive R and D program has been launched to identify appropriate preventive and corrective actions for the 4 recent Belgian units (Doel 3 and 4, and Tihange 2 and 3). A full-length rotopeening has already been performed in the tubesheet areas of the units Doel 4 and Tihange 3