05000389/LER-2008-001
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded |
| 3892008001R00 - NRC Website | |
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3)
Description of the Event
On January 29, 2008 at 0200, Unit 2 control room personnel completed the daily RCS leak rate calculation. Results showed that the RCS leak rate was 0.20 gpm.
Investigation into additional reactor containment building indicators showed that the reactor sump level had increased, containment particulate radiation levels had increased and the RCP 2B1 showed controlled bleedoff flow to the 2B1 RCP had decrease. At 0250, a second RCS leak rate calculation was completed showing a 0.22 gpm RCS leak rate. At 0325 on January 29, 2008, St. Lucie Unit 2 commenced downpower as directed by plant procedures and manually tripped the reactor at 0531.
Cause of the Event
A root cause evaluation was completed and concluded that a synergistic effect of four separate causes involving: localized abnormal chemistry conditions, line resonance vibration levels, inherent stress concentration effects of socket welds and elevated RCP vibration levels in all combinations caused the through wall crack.
Analysis of the Event
This event is reportable under 10 CFR 50.73(a)(2)(ii)(A), as any event or condition that resulted in a manual or automatic reactor trip.
Analysis of Safety Significance The upper seal cavity line provides pressure indication of the RCP seal for confirmation of seal performance. Leakage from this line is limited by the pressure breakdown tubes which would only allow 1.3 gpm maximum from the RCS. Failure analysis performed on the cracked weld concluded there was evidence of corrosion at the crack initiation point at the weld root, with subsequent propagation through the throat of the weld. The failure mechanism was low stress high cycle fatigue caused by vibration induced cyclic loading.
The RCP leak was discovered during daily RCS leak rate calculations. If the leak had not been discovered at that time, normal RCS leak rate monitoring would have discovered the increased leakage long before a significant degradation of the piping had occurred. Experience has demonstrated that with this weld failure mechanism, there is ample time from the discovery of increased RCS leakage to allow a controlled shutdown to cold shutdown conditions without challenging the emergency core cooling system (ECCS) or small break loss-of-coolant accident analysis (SBLOCA).
Although stainless steel components are not susceptible to boric acid corrosion, the spray from the cracked weld deposited wet boric acid onto the external surfaces of numerous carbon and low alloy steel components, including pressure retaining bolting.
Inspection of the affected areas revealed only minor surface corrosion with no measurable material degradation or wastage. The inspection results were consistent with the expected corrosion rates provided in the industry guidelines for the specific leak parameters such as temperature and boric acid concentration.
Based on the above, the safety consequences of the event are judged to be low and there was no adverse impact on the health and safety of the public.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3)
Corrective Actions
The corrective actions and supporting actions are entered into the site corrective action program. Any changes to the proposed actions will be managed under the commitment management change program.
Completed Corrective Actions:
Repaired cracked socket weld by replacing the controlled bleedoff as well as the upper and middle seal cavity piping spool pieces'for all four RCPs with 2:1 taper socket welds.
Long Term Corrective Actions 1.Forensic metallurgical evaluations of selected socket weld fittings on RCP seal lines to address environmental issues.
2.Establish long term RCP pump and coupling maintenance practices to reduce vibration.
3.Actions to be performed on the Unit 1 RCP sensing lines during the upcoming refueling outage include: chemical analysis of water samples, replacement of criteria, utilization of 2:1 taper socket weld design or butt weld and flushing of certain lines to achieve normal levels ( contaminants (chlorides, fluorides and sulfates) in each of the eight lines.
Similar Events There have been (3) three similar failure events'at PSL Unit 2 involving cracked RCP seal lines: 1) Inside diameter initiated low stress high cycle fatigue of the 2B1 RCP lower seal injection line socket weld; 2) outside diameter initiated low stress high cycle fatigue of the 2B2 RCP seal injection vendor J groove weld; 3) inside diameter initiated environmental assisted cracking under cyclic loading. While each of these events involved a different failure mechanism, vibration loading was a common key contributor.
A search of the INPO Operating Experience database, Industry Owner Groups, and other Utilities identified multiple plant events involving RCP seal injection line with cracked welds relating to vibration, environmental, water chemistry, and weld material, which each independently or collectively contributed to these types of events within the nuclear industry.
Failed Components
RCP 2BI Upper Seal Cavity piping 34 inch schedule 160 304 SS piping socket weld at flange.