05000277/LER-2006-003
| Peach Bottom Atomic Power Station Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| 2772006003R00 - NRC Website | |
Unit Conditions at the Time of Discovery was restarted from the P2R16 Refueling Outage on 10/6/06. There were no other structures, sYstems or components out of service that contributed to this event.
Description of the Event
On 10m06 at 1802 hours0.0209 days <br />0.501 hours <br />0.00298 weeks <br />6.85661e-4 months <br />, an Unusual Event was declared for Unit 2 as a result of the discovery of a leak at an elbow (EIIS: PSF) for piping that penetrates the Primary Containment Suppression Pool (i.e. Torus) (EIIS:
NH). The 4" piping is the High Pressure Coolant Injection (HPCI) / Reactor Core Isolation Cooling (RCIC) Torus Flush line. This line is normally isolated from the HPCI (EIIS: BJ)/ RCIC systems by a closed motor operated valve (MIS: ISV) and is only used during testing activities. The leak was discovered by an equipment operator at approximately 1741 hours0.0202 days <br />0.484 hours <br />0.00288 weeks <br />6.624505e-4 months <br /> during a planned inspection associated with a RCIC (EIIS: BN) system check valve (EIIS:V). The inspection was scheduled to occur during startup activities associated with the P2R16 refueling outage. At the time of the discovery, the RCIC system was being operated in the test mode with the return flow being returned to the Torus. The leaking elbow is common piping for the HPCI / RCIC 'Flush' lines that return to the Torus. The leak occurred on the intrados of a 45 degree elbow of the 4" piping.
The elbow was located approximately 1 foot above the Torus penetration (i.e. the leak was outside of Primary Containment).
Licensed Main Control Room operators declared Primary Containment inoperable at approximately 1750 was manually scrammed (and therefore, entered Mode 3) at approximately 2016 hours0.0233 days <br />0.56 hours <br />0.00333 weeks <br />7.67088e-4 months <br />. This resulted in expected Primary Containment Isolation System (PCIS) Group II and III isolations (including a start of the Standby Gas Treatment system) as a result of reactor water level reaching the PCIS Level 3 reactor water level set point.
The PCIS Group II and III isolations were reset by 2035 hours0.0236 days <br />0.565 hours <br />0.00336 weeks <br />7.743175e-4 months <br />. The Standby Gas Treatment system was secured by 2043 hours0.0236 days <br />0.568 hours <br />0.00338 weeks <br />7.773615e-4 months <br /> and the normal Reactor Building ventilation system was restarted. At 2055 hours0.0238 days <br />0.571 hours <br />0.0034 weeks <br />7.819275e-4 months <br />, the Main Steam Isolation Valves (MSIVs) were manually closed to maintain appropriate reactor pressure and temperature control. The scram was reset at approximately 2104 hours0.0244 days <br />0.584 hours <br />0.00348 weeks <br />8.00572e-4 months <br />. The unit achieved Mode 4 and the Unusual Event was terminated by 0513 hours0.00594 days <br />0.143 hours <br />8.482143e-4 weeks <br />1.951965e-4 months <br />.
This report is being submitted pursuant to the following reporting requirements:
10CFR 50.73(a)(2Xi)(B) — Condition Prohibited by Technical Specifications — The plant entered Mode 2 on 10/6/06 at approximately 1659 hours0.0192 days <br />0.461 hours <br />0.00274 weeks <br />6.312495e-4 months <br />. Primary Containment was declared inoperable on 10/7/06 at approximately 1750 hours0.0203 days <br />0.486 hours <br />0.00289 weeks <br />6.65875e-4 months <br />. Since Primary Containment was unknowingly inoperable for a period of time greater than allowed by Technical Specification 3.6.1.1, a condition prohibited by Technical Specifications existed.
Description of the Event, continued 10CFR 50.73(aX2XiXA) — Completion of a Plant Shutdown Required by Technical Specifications — Because the plant entered into Mode 3 at approximately 2016 hours0.0233 days <br />0.56 hours <br />0.00333 weeks <br />7.67088e-4 months <br />, a condition existed where a plant shutdown was completed as a result of a Technical Specification Required Action.
10CFR 50.73(a)(2Xii)(A) — Principal Safety Barrier Degraded — Because a pipe crack existed which resulted in a leak from Primary Containment, a principal safety barrier was considered as degraded.
10CFR 50.73(aX2)(v) — Condition that Prevented the Fulfillment of a Safety Function - Because Primary Containment was considered to be inoperable as a result of the pipe crack in piping attached to the Torus, a condition existed that prevented the fulfillment of the Primary Containment safety function.
Cause of the Event
Detailed failure analyses of the cracks in the HPCI / RCIC Flush return line elbow were performed. The failure analysis determined that axial and circumferential flaws developed as a result of cavitation and abrasive erosion and /or water jet cutting. The underlying cause of the issue was due to excessively high flow velocities through this piping during test conditions in conjunction with an apparent lack of fusion between the weld backing ring and the weld root at the elbow weld. These analyses identified that a portion of the original installation weld-backing ring was missing. This observation suggests lack of fusion between the weld root and the backing ring when performed during original plant construction. Evidence suggests that flow underneath the backing ring created localized erosion immediately adjacent (downstream) to the weld root.
Analysis of the Event
There were no actual safety consequences associated with this event.
The leaking elbow is in common piping for the HPCI / RCIC 'Flush' line that returns to the Torus. This line is normally isolated from the HPCI / RCIC systems by a closed motor-operated valve and is only used during testing activities. Therefore, there was no impact on HPCI or RCIC system functional capability.
The 4" carbon steel piping is attached to the Torus and is not isolable from the Torus (i.e. Primary Containment). The piping terminates under the normal Torus water level and therefore, the water in the Torus serves as another barrier to prevent radioactive gaseous releases from the Torus air space during design basis events. Therefore, there were no actual gaseous releases involved with this event.
The HPCI / RCIC 'Flush' line is pressurized during Integrated Leak Rate Tests (ILRTs). The leakage would have been detected during this test. The last ILRT was successfully completed on 10/4/00 and there was no leakage identified at that time.
NRC FORM 366AU.S. NUCLEAR REGULATORY COMMISSION (1-2001) FACILITY NAME (1) DOCKET (2) LER NUMBER (6) PAGE (3) Analysis of the Event, continued Further examination of the leaking elbow noted that axial and circumferential cracking existed at the elbow intrados. Failure analyses of the elbow determined that only minimal leakage existed at the elbow with the as found indications. This minimal leakage only occurred when the HPCI or RCIC system was being operated in the test mode involving return flow being routed to the Torus.
In the unlikely event that a worst-case design basis event had occurred and the elbow did not maintain its integrity, additional leakage would have occurred. If both subsystems of containment cooling (including containment spray) were used during the design event, the Torus water level would only be minimally impacted. If only one subsystem of containment cooling were used with no containment spray, then water leakage would have occurred until the HPCI / RCIC 'Flush' line became uncovered (approximately 5 feet below normal Torus water level) and a gaseous release could have occurred. The water leakage would be contained within the Torus Room. The gaseous leakage would be processed through the secondary containment and Standby Gas Treatment System. The Torus Room is within the Secondary Containment boundary.
Safety systems operated as designed during the unit shutdown.
Preliminary assessments indicate that this event would not be risk significant.
Corrective Actions
The leaking elbow was replaced and non-destructive testing was performed. The similar pipe on Unit 3 was examined and no significant concerns were noted. Extensive walk downs of similar piping that is attached to the Torus were conducted for both Units 2 and 3. There were no similar deficiencies discovered. Selected ultrasonic testing was performed on Unit 2 and 3 Torus attached piping that involved higher flow rates. These examinations also did not identify any similar concerns.
Plant test procedures were revised to prevent using the HPCI / RCIC Torus Flush line at high flow conditions.
Additional corrective actions including extent of condition evaluations are being evaluated in accordance with the Corrective Action Program.
Previous Similar Occurrences There were no previous similar events identified.