05000482/LER-2006-003
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. 05000 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded |
| 4822006003R00 - NRC Website | |
Plant Conditions Prior to the Event:
MODE — 5 Power — 0 percent
Event Description:
Wolf Creek Nuclear Operating Corporation (WCNOC) performed pre-planned inservice examinations of the nozzle to safe-end dissimilar metal (DM) butt welds and safe-end to pipe stainless steel butt welds on the safety, relief, spray, and surge line connections to the pressurizer during Refueling Outage 15 in October 2006. The examinations for the DM and stainless steel butt welds used Performance Demonstration Initiative (PDI) qualified ultrasonic examination (UT) procedures. On October 11, 2006, at approximately 12:40 Central Standard Time, Engineering personnel notified the Control Room that five circumferential flaw indications had been discovered. Three indications were located in the surge nozzle DM weld, one indication was in the "C" safety nozzle DM weld, and one indication was in the relief nozzle DM weld. The locations are part of the Reactor Coolant System (RCS) pressure boundary. The indications were at or very near the pipe inside diameter (ID) and there was no evidence of RCS pressure boundary leakage. A complete description of the flaw indications was provided to the NRC in WCNOC letter ET 06-0049, dated November 29, 2006. Weld overlay of the safe-end DM welds on the pressurizer was an activity scheduled for Refueling Outage 15, subsequent to the pre-planned inservice examinations. The weld overlays were installed prior to the unit's return to power operations.
Basis for Reportability:
10CFR50.72(b)(3)(ii)(A) requires reporting of any event or condition that results in the condition of the nuclear power plant, including its principal safety barriers, being seriously degraded. NUREG —1022 provides an example of welding or material defects in the primary coolant system which cannot be found acceptable under ASME Section Xl.
Since the indications were not acceptable under ASME Section XI inservice flaw acceptance standards, WCNOC made an eight hour Emergency Notification System call in accordance with 10 CFR 50.72(b)(3)(ii)(A).
The industry, through the Material Reliability Program (MRP), has performed safety assessments that have bounded the identified flaw indications, i.e., these flaw indications were smaller than critical flaw sizes reported in MRP-113, "Alloy 82/182 Pipe Butt Weld Safety Assessment for US PWR Plant Designs". However, WCNOC did not perform flaw evaluations to specifically determine the acceptability of the flaw indications and conservatively assumed the flaws to be unacceptable and installed pre-planned full structural weld overlays for repair of the flaws. Therefore, the flaw indications are assumed to be a condition that resulted in the nuclear power plant, including its principal safety barriers, being seriously degraded, resulting in a reportable condition.
This condition is also reportable pursuant to 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 safety barriers, being seriously degraded.
Root Cause:
The most probable mechanism responsible for the indications found on the pressurizer relief, safety, and surge nozzle DM butt welds is PWSCC. The characteristics of the indications determined by UT examination and their location in PWSCC susceptible materials are consistent with PWSCC in welds that contain Alloy 600/82/182.
Corrective Actions:
Corrective actions taken to correct the indications were:
1. Installed full structural repair weld overlays on the three nozzle to safe-end DM butt welds with reported flaw indications.
2. The weld overlay of each nozzle to safe-end DM weld would preclude future examination of the adjacent stainless steel safe-end to pipe weld due to the close proximity of the two welds. Therefore, weld overlays were also installed as follows: (a) Full structural repair weld overlays were installed on the adjacent stainless steel safe-end to piping butt welds where pre-weld overlay examination included less than 90% of the code required examination.volume; and (b) preemptive full structural weld overlays were installed on the remaining adjacent stainless steel safe-end to piping butt welds.
3. Installed preemptive full structural weld overlays to the remaining pressurizer nozzle to safe-end DM butt welds to mitigate further susceptibility of the DM welds to PWSCC.
Safety Significance:
The identified flaw indications are a degradation of the RCS pressure boundary but were not through wall and therefore, did not result in any through wall leakage. The industry, through the Materials Reliability Program, performed safety assessments that have bounded the identified degradation. Therefore, these welds were capable of performing their required passive pressure boundary function.
Though less likely than axial cracks, PWSCC can cause circumferential cracks. The residual stresses and material microstructure are very unlikely to be uniform around the circumference of the weld because of inherent variations in the welding process. Because the rates of crack initiation and propagation depend on stress and microstructure, the cracks form at different times and grow at different rates around the pipe circumference. Thus, only a small circumferential length of the crack grows through wall causing a leak before the average depth of the crack around the circumference affects the structural integrity of the weld joint. Therefore, sudden rupture of a pressurizer attachment weld is considered to be very unlikely.
Industry experience of PWSCC crack growth shows there would be time available for detection of a leak within the range of installed leakage detection allowing for orderly plant shutdown to repair the weld. The likelihood of failure to detect and appropriately respond to a small leak, before it might propagate to allow significant RCS leakage, is low.
Operating Experience/Previous Events:
the steam generator (SG) C and D lower head bowl drain lines. These conditions were attributed to PWSCC and the lower head bowl drain lines were repaired. In addition, the same preventative measure was taken for the lower head drain lines on SGs A and B.