The degradation of Unit 4 SG tubing by IGA/SCC has recently limited both the operating period and end of life predictions for Unit 4 since restart in late 2003. SG4 has experienced the most significant damage with substantial increases in both initiation and growth rates from 2005 through the spring of 2007. A detailed review of the occurrence of circumferential OD IGA/SCC at the RTZ in the HL TTS region of Bruce 4 steam generator tubes has led a conclusion that it is probable that the IGA/SCC has been the result of attack by partially reduced sulfur species such as tetrathionates and thiosulfates during periods of low temperature exposure. It is believed that attack of this type has most likely occurred during startup evolutions following outages as the result the development of aggressive reduced sulfur species in the TTS region during periods when the boilers were fully drained for maintenance activities. The modification of outage practices to limit secondary side oxygen ingress in the spring of 2007 has arrested the degradation and has had significant affects on the allowable operating interval and end of life predictions for the entire unit.

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