05000400/LER-2013-001
| Shearon Harris Nuclear Power Plant, Unit 1 | |
| Event date: | 05-15-2013 |
|---|---|
| Report date: | 07-12-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(a) |
| 4002013001R00 - NRC Website | |
Energy Industry Identification System (El IS) codes are identified in the text as poq.
There were no systems, structures, or components that were inoperable at the start of the event that contributed to the event.
Event Description
On May 13, 2013, an independent third party review of ultrasonic data of the reactor pressure vessel head penetration nozzles [RPV-NZL] [Manufacturer: Chicago Bridge and Iron, Serial Number: T40] obtained during Harris Nuclear Plant (HNP) spring 2012 refueling outage 17 (R017), identified a condition that required further evaluation. The ultrasonic test data was being reviewed in preparation for R018. After further review, on May 15, 2013, the indication was determined to be characteristic of primary water stress corrosion cracking (PWSCC) and similar to other flaws that were identified and repaired during the R017 in 2012. Because the indication was identified while at power, Technical Specification 3.4.6.2 required a reactor shut down due to potential reactor coolant system pressure boundary leakage.
On May 15, 2013, while at 98% power in Mode 1, HNP commenced a Technical Specification required shutdown to Mode 6 to repair the flaw in nozzle 49. A bare metal visual inspection of the exterior surfaces of the reactor head and penetration was performed in accordance with ASME Section XI Code Case N-729-1, with no indication of through-wall leakage. Ultrasonic test data indicated that the flaw was approximately 0.314 inches long, situated mostly in the penetration nozzle material, and axial in orientation with penetration of about 25% of the nozzle material. An analysis of the data prior to the shutdown could not determine the exact extent of the indication in nozzle 49, noting that it straddled the J-groove weld area and may have extended into the weld.
The nozzle was subsequently repaired utilizing the inside diameter temper bead welding process. From time of discovery on May 15, 2013, at 18:49, until the nozzle was repaired on May 31, 2013, at 20:55 was a total of approximately 16 days.
Similar reactor vessel head indications were found in 2012. Four nozzles (5, 17, 38, and 63) were identified with indications that exhibited characteristics of PWSCC and were repaired using the inner diameter temper bead welding process. None of the PWSCC indications exceeded 75% through-wall and no evidence of leakage was observed based on bare metal visual examinations. This condition was not initially recognized as being reportable in 2012 per 10 CFR 50.73(a)(2)(ii)(a), for any event or condition that resulted in the condition of the nuclear power plant, including its principal barriers, being degraded, and this event report serves as the formal report to the NRC.
Cause of Event
The cause of the flaws in nozzle 49 and the other four nozzles was attributed to PWSCC. The root cause evaluation determined that the missed identification of the indication in nozzle 49 was due to the lack of mitigating programmatic governances to specify process independence and fatigue/distraction controls.
Contributing causes include lack of independent vendor analyses of data, potential fatigue, potential distractions by the noisy, active environment, and formal pre job briefs were not performed and documented as required.
Safety Consequences There was a flaw in a fission product barrier which caused HNP to not meet code requirements. A bare metal visual inspection of the exterior surfaces of the reactor head and penetration did not indicate any through-wall leakage, thus there was not a breach in a fission product barrier, and structural integrity of the reactor vessel was not significantly compromised. The flaw was not through- wall and no leakage of reactor coolant occurred. The flaw was such that it would not have propagated under any of the design basis events (earthquake, loss of coolant accident, etc.).
Containment was intact as an additional fission product barrier. Therefore, there was no significant impact to the health and safety of the public.
Per the 2004 Edition of ASME Section XI Acceptance Criteria in Table IWB -3663-1 General Note (a), "Linear surface flaws of any size in the partial penetration nozzle to vessel (J-groove weld) are not acceptable." Section 3.2.4 of NUREG-1022, Event Reporting Guidelines, identifies that defects in the reactor coolant system pressure boundary that cannot be dispositioned as acceptable per ASME Section XI are reportable per 10 CFR 50.73. Therefore this event is reportable in accordance with 10 CFR 50.73(a)(2)(ii)(a), for a condition that a principal barrier was degraded, and 10 CFR 50.73 (a)(2)(i)(A), completion of any nuclear plant shut down required by the plant's technical specification.
Corrective Actions
Completed Corrective Actions
- Nozzles 5,17,38,49 & 63 were repaired utilizing the inner diameter temper bead welding process.
- Conducted a 100% review of all R017 ultrasonic data and confirmed that no additional indications exist which were not properly analyzed and no additional indications required repair
- Updated Shearon Harris ISI Program to require volumetric exams to be performed on the reactor vessel head every refueling outage.
Planned Corrective Actions
- Create mitigating programmatic governance for providing oversight for complex automated Non-Destructive Examination (NDE) inspections through the generation of new procedure(s).
- Revise the vendor contract for R018 to require analysts to perform independent analyses of ultrasonic data, develop work-hour restrictions, and incorporate requirements for an environment conducive to independence and analysis.
- Reinforce the need to review and follow the requirements of applicable oversight procedures pertaining to the current assignment with the responsible individuals.
Previous Similar Events
No previous, similar Licensee Event Reports were identified.
Commitments This report contains no regulatory commitments.