05000368/LER-2009-003
| Arkansas Nuclear One, Unit 2 | |
| Event date: | 09-08-2009 |
|---|---|
| Report date: | 11-05-2009 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3682009003R00 - NRC Website | |
10. POWER LEVEL
000
12. LICENSEE CONTACT FOR THIS LER
NAME
David B. Bice, Acting Manager, Licensing TELEPHONE NUMBER (Include Area Code) 479-858-4710
13. COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT
CAUSE SYSTEM COMPONENT MANU- FACTURER
REPORTABLE
TO EPIX CAUSE SYSTEM COMPONENT
MANU-
FACTURER
REPORTABLE
TO EPIX
14. SUPPLEMENTAL REPORT EXPECTED
- YES (If yes, complete 15. EXPECTED SUBMISSION DATE) 0 @ 0 NO
15. EXPECTED
SUBMISSION DATE MONTH DAY YEAR
ABSTRACT (Limit to 1400 spaces, i.e., approximately 15 single-spaced typewritten lines) On September 8, 2009, at approximately 0452 CDT, with the plant shutdown in Mode 6, it was discovered during the 2R20 Arkansas Nuclear One Unit 2, "B" Steam Generator (SG) inspection, that a wrong SG tube was plugged during a previous outage. During the Spring 2005 refueling outage (2R17), a SG tube with an identified flaw was plugged correctly on the cold leg end, but the wrong SG tube was plugged on the hot leg end. This condition resulted in Unit 2 operating at power since April 2005 with a SG tube characterized with an approximate 43% through wall defect in violation of the Unit 2 Technical Specification SG tube plugging limit of > 40% through wall.
Investigation revealed that the error in plugging was caused by a failure to use proper independent verification. Both the SG tube with the identified flaw and the inadvertently plugged SG tube were removed from service.
A. Plant Status At the time this condition was identified, Arkansas Nuclear One, Unit 2 (ANO-2), was in a shutdown for a refueling outage.
B. Event Description
On September 8, 2009, at approximately 0452 CDT during the 2R20 ANO-2 "B" Steam Generator (SG) inspection, it was discovered that a wrong tube was plugged during a previous outage.
During Eddy Current Testing (ECT), the examination of the "B" SG determined that a non-conforming condition existed. Tube SG-B R 139, C120 should have been plugged and removed from service during a previous outage; however, the adjacent tube, SG-B R140, C119 was found to be plugged on the hot leg side instead of the defective tube. Refueling outage 2R20 was the first SG inspection performed since 2R17 (Spring 2005). A review of the 2R17 post plugging verification video confirmed that the hot leg of tube SG-B R140, C119 was mistakenly plugged in 2005. During 2R20 plugging operations tube SG-B R139, C120 was plugged on the hot leg side, removing the tube from service. The cold leg side of SG-B R140, C119 was also plugged to remove it from service.
C. Root Cause
Investigation revealed that the error in plugging was caused by a failure to use proper independent verification. It was determined that concurrent verification with the same two individuals at the same time was performed in lieu of true independent verification.
Location verification problems compounded by the use of newly adapted robotic equipment were determined to be a contributing factor. Tube plugging during 2R17 was performed using the Westinghouse ROSA robot with adapted Genesis robot roll plugging end-effectors. This adaptation was performed during an interim period as Westinghouse transitioned the traditional Combustion Engineering roll plug capability to the traditional Westinghouse robots. The plugging process was controlled from a secondary workstation and was not fully integrated with the primary workstation robot control software. This equipment was determined to not be well designed for the task of location verification.
D. Corrective Actions
During 2R20 plugging operations, tube SG-B R139, C120 was plugged on the hot leg side, removing the tube from service. The cold leg side of SG-B R140, C119 was also plugged to remove it from service.
A review of the remaining plugged tubes in the ANO-2 steam generators was visually performed and a review of the Arkansas Nuclear One Unit 1 (ANO-1) plugging history was performed. No other discrepancies were identified.
In 2007, Westinghouse process changes, including development of a position verification procedure, independent position tracking, and uniquely identifiable location software upgrades, were instituted as a result of identified potential gaps in the miss-encode prevention barriers.
These changes adequately address the 2005 issues with independent verification.
Additionally, an expectations letter was distributed to Westinghouse SG field services personnel communicating the requirements to verify proper plug locations.
E. Safety Significance
The original flaw was identified in 2R17 (Spring 2005) as a volumetric flaw that measured 43% in depth. The calculated burst pressure for this flaw was > 5300 psi which is well above the NEI 97 06 and ANO-2 SG tube integrity limit of 4050 psi. Accounting for uncertainties, a flaw at 51% depth is still below that which would leak under any accident or normal operating limits. This is based on a burst pressure model that uses 95% probability and 50% confidence. In 2R20, the tube was retested with both bobbin and plus point. The tube was clean with the exception of the pre-existing flaw, which was measured at 45% in depth and a conservative burst estimate of -4900 psi. Statistically, the changes in depth, length, and width, were considered unchanged due to uncertainties in sizing methodologies and remained above the tube integrity limit of 4050 psi.
This SG tube would have met all of its intended safety functions. There were no challenges to nuclear safety, industrial safety, or radiological safety.
F. Basis for Reportability Technical Specification (TS) 3.4.5 b. states that all SG tubes satisfying the tube repair criteria shall be plugged in accordance with the Steam Generator Program. The failure to plug the correct tube during the previous refueling outage is a violation of the ANO-2 TSs and the identified condition is reportable under 10 CFR 50.73(a)(2)(i)(B) as an operation or condition prohibited by TSs.
G. Additional Information
There were no previous similar events reported as Licensee Event Reports by ANO.