05000389/LER-2005-001
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded |
| 3892005001R00 - NRC Website | |
Z. OCKAt I1. FACILITY NAME 6. LER NUMBERNUMBER St. Lucie Unit 2 05000389 ZEtC1==i
Description of the Event
On February 10, 2005, St. Lucie Unit 2 was in Mode 3 less than 1750 psia, Hot Standby, as work continued to complete the SL2-15 refueling outage. At 0250 hours0.00289 days <br />0.0694 hours <br />4.133598e-4 weeks <br />9.5125e-5 months <br />, reactor coolant system (RCS) leakage of -0.4 gpm was identified by the RCS walkdown team. At 0605 hours0.007 days <br />0.168 hours <br />0.001 weeks <br />2.302025e-4 months <br />, the leakage was identified to be from two leaks on the instrument line to the 2B1 safety injection header and Technical Specification (TS) Action 3.4.6.2.a was entered for RCS pressure boundary leakage. At 1246 hours0.0144 days <br />0.346 hours <br />0.00206 weeks <br />4.74103e-4 months <br />, the instrument root isolation valve, V3236, to the pressure transmitter (EIIS:BP:PT] for the 2B1 safety injection tank (SIT) (EIIS:BP:ACC] discharge piping, PT-3339, was closed, thereby isolating one leak in ASME Class 2 tubing. The 2B1 SIT was declared out of service once the outlet valve was closed. The second leak was under the insulation and was located at the interfacing socket weld between the "rifle bore" restriction orifice and the 3/4 inch half coupling on the 12-inch safety injection header. At 1626 hours0.0188 days <br />0.452 hours <br />0.00269 weeks <br />6.18693e-4 months <br />, the safety injection header section containing the leak was depressurized. Upon evaluation, FPL concluded that this leakage was ASME Class 1 RCS pressure boundary leakage based on its location between the safety injection header check valves that interface with the reactor coolant system. TS Action 3.4.6.2.a was exited at 2256 hours0.0261 days <br />0.627 hours <br />0.00373 weeks <br />8.58408e-4 months <br />. Additionally, at 2256 hours0.0261 days <br />0.627 hours <br />0.00373 weeks <br />8.58408e-4 months <br />, an 8-hour ENS notification was made for serious degradation of a principal safety barrier.
Cause of the Event
The failure mechanism of the interfacing socket weld between the rifle bore restriction orifice and 3/4 inch half coupling on the 12-inch safety injection header was high-cycle fatigue as evidenced by ratchet marks on the weld crack surface. The subject piping is designed to ASME Section III, Class 1 requirements with a design pressure of 2485 psi at 650 degrees F. The piping code is SS-3 with the following materials of construction: ASME SA-182, F304 for the half coupling and ASME SA-182 F316 for the restriction orifice. A contributing factor to the fatigue crack initiation was the presence of weld root defects evidenced by lack of fusion.
Operationally induced vibration caused the cyclic loading that resulted in the fatigue failure.
The initiating failure mechanism of the associated ASME Class 2, 1/2 inch 316 SS instrument tubing was abrasive wear from the insulation lagging. The ultimate failure of this tubing likely resulted from low-cycle fatigue or ductile fracture.
Operationally induced vibration was the source of the causative cyclic loads.
St. Lucie prepared and implemented a modification to repair the pressure boundary through-wall leaks. This modification restored the associated piping to its original or equivalent design configuration, except that the welds were modified to address high-cycle fatigue. The modification also specified a tie-back support for instrument isolation valve V3236 to address operationally induced vibration. Once the modifications were complete to the PT-3339 process line, the 2B1 SIT and its associated piping were returned to service.
Analysis of the Event
This event is reportable under 10 CFR 50.73(a)(2)(ii)(A) as serious degradation of a principal safety barrier.
--2.uOtAtti 1. FACILITY NAME 6. LER NUMBER 3. PAGENUMBER � t.1ffS.MIE=.2.
- .*.� Analysis of Safety Significance The leaks were associated with SIT pressure instrumentation. There are four SITs that provide a passive means of injecting emergency core cooling water into the reactor vessel. Each SIT is connected to one of the four reactor vessel inlet lines.
Each SIT contains borated water at refueling concentration and is pressurized with nitrogen. In the event of a LOCA that results in RCS depressurization, the borated water is forced into the RCS by the expansion of the nitrogen.
Active safety injection is provided whereby borated water from the refueling water tank is injected into the reactor vessel by two low pressure and two high pressure safety injection (LPSI and HPSI) pumps. The HPSI and LPSI pumps are actuated by a safety injection actuation system.
The design capacity from the combined operation of one high pressure pump, one low pressure pump and three safety injection tanks provides required core cooling for any size loss of coolant accident which results in RCS depressurization.
The leaks were classified as a degraded condition of a principal safety barrier. The source of the water leak was the water volume within the SIT and not the RCS; the intervening RCS check valve remained seated during the event. Regardless, had the leakage come from the RCS, the maximum leak rate in the PT-3339 instrument line was approximately 0.4 gpm, well below the 44 gpm makeup capability of a single charging pump. Additionally, there was no evidence of leakage prior to the refueling outage.
Furthermore, in Mode 3 with RCS pressure less than 1750 psia, the TSs only require that three of the four SITs be operable. Therefore, this event had no adverse impact on the health and safety of the public because the failure occurred during shutdown conditions, the leak was small, and there was no actual RCS leakage.
FPL identified no generic implications that required immediate action. Visual inspections of the St. Lucie Unit 2 socket welds and tubing at 2A1, 2A2, and 2B2 safety injection header locations were performed with satisfactory results. Although a vibration restraint was added to the pressure transmitter line for PT-3339, no other safety injection loop header instrumentation lines have vibration restraints on either St. Lucie Units 1 or 2. Therefore, Engineering is evaluating the need to install vibration restraints and/or perform vibration monitoring of the remaining St.
Lucie Units 1 and 2 safety injection loop header instrumentation lines. If any and Unit 2 refueling outages.
'Corrective Actions 1. Plant Change Modification 05029 was implemented to return the 2B1 SIT to service.
2. St. Lucie is evaluating the need to install vibration restraints and/or perform vibration monitoring on the remaining St. Lucie Unit 1 and Unit 2 safety injection loop header instrumentation lines. Required modifications will be implemented during the SL1-20 and SL2-16 refueling outages.
Other Information Failed Components Identified Field-implemented ASME Class 1 3/4 inch socket weld ASME Class 2 1/2 inch 316 SS tubing Similar Events None