| Root cause of failure of stainless steel heat exchange tubes withdrawn from high pressure feed water reheater of a foreign nuclear power plant has been studied intensively by visual inspection, chemical composition analysis, mechanical testing, microstructure observation. Analysis results show that Cl" accumulation in crevices between heat exchanger tube and the hole on support plate and tubesheet is the major factor that induced stress corrosion cracking of austenitic stainless steel tubes. Operation history review found a long time unexpected outage after a period of power operation. The heat exchangers were not protected during the outage, and were contaminated by ingression of chloride ion. Chloride built up in crevices after restart of heat exchangers, and cracked the heat exchanger tubesResearches on stress corrosion tests also showed high residual stress in the as received heat exchanger tubes for making those high pressure feed water reheaters. Meanwhile, ANSYS stress analysis shows bending stresses due to mis-alignment of heat exchanger tube holes on the tube support plates and residual stress due to expansion of tube-to-tube sheet joints also accelerated the propagation of stress corrosion cracks.Lessons learned from the accidents can be listed as follows. Firstly, the as received stainless steels for making heat exchangers must be carefully examined and the risk of stress corrosion cracking must be evaluated based on chemical composition, microstructure and residual stress. Second, cleanness during manufacturing process must be maintained and cleaning is needed before restart after outage. Thirdly, high precision of holes on tube support plates must be achieved before inserting heat exchange tubes, in order not to induce bending stress on heat exchange tubes after insertion into the holes. And finally, protection from ingression of hazardous ions into the heat exchanger must be provided during long time outage. |
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