Greater than one percent of
steam generator (
SG) 2-4 tubes were determined to be defective and require plugging. Diablo Canyon performs 100% bobbin inspection of
SG tubes, thus, no expanded bobbin inspection scope was required.
On May 3, 2006, during the Unit 2 thirteenth refueling outage (2R13), final analysis of eddy current data of SG 2-4 indicated that greater than one percent of the total tubes inspected were defective placing them in Category C-3. Thirty four (34) detective tubes in SG 2-4 were detected and are being plugged. Most of the pluggable indications are due to axial outside diameter stress corrosion cracking at the hot leg tube support plates. Diablo Canyon TS 5.5.9, Table 5.5.9-2, requires 'Notification to NRC pursuant to 10 CFR 50.72(b)(2)' (replaced by 10 CFR 50.72(b)(3)(ii), an 8-hour non-emergency ENS report requirement).
An NRC teleconference in accordance with TS 5.6.10.d to report the preliminary SG tube inspection results is scheduled for May 3, 2006, at 1000 PDT.
A Special Report in accordance with TS 5.6.10.a to report the number of tubes plugged in each SG, and TS 5.6.10.c describing the cause of the tube degradation and corrective measures taken to prevent recurrence will be submitted prior to returning Diablo Canyon Unit 2 to power operation.
The licensee notified the NRC Resident Inspector.
