Augmented Inspection Team
On March 19, 2012, the NRC initiated an Augmented Inspection Team (AIT) reactive inspection to assess the circumstances surrounding the tube leak and unexpected wear of tubes in the Unit 3 SGs. The AIT charter was subsequently revised on May 16, 2012, in part to also evaluate unexpected wear in the Unit 2 SGs. The AIT team presented their findings in a public exit meeting near SONGS on June 18, 2012. The AIT report, issued July 18, 2012, identified probable contributing causes of the steam generator tube degradation that ultimately led to the tube leak in Unit 3. An identified common potential cause is related to design control issues, associated with the thermal/hydraulic modeling of the steam generators. The AIT also assessed the licensee’s response to the event and whether the steam generator major design changes were appropriately reviewed in accordance with the 10 CFR 50.59 requirements. The AIT identified ten unresolved items that warranted additional follow-up inspection and review.
A follow-up inspection of the unresolved issues identified by the AIT was conducted from August 20 to September 28, 2012. The follow-up inspection report, dated November 9, 2012, closed 8 of the 10 unresolved items. The two remaining unresolved items are related to the mechanistic cause of the tube-to-tube vibration resulting from fluid-elastic instability. The NRC will conduct subsequent inspections and reviews to disposition the remaining unresolved items.