05000389/LER-2015-003
| St. Lucie Unit 2 | |
| Event date: | 10-19-2015 |
|---|---|
| Report date: | 12-18-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3892015003R00 - NRC Website | |
Description of the Event
On October 19, 2015, St. Lucie Unit 2 was in Mode 5 at 0% reactor power. During corrective maintenance activities on valve FCV-26-3 [EIIS:BD:ISV], the containment penetration 52B inside isolation valve for the 'B' channel containment air radiation monitor [EIIS:IL], maintenance personnel discovered that the valve actuator position limit switches [EIIS:33] were not wired correctly. FPL reviewed the work order history for this valve and determined that the valve actuator limit switches were miss-wired during the SL2-13 December 2001 refueling outage when the environmentally qualified limit switches were replaced.
The limit switch wiring was corrected, the post-maintenance testing was completed satisfactorily, and the valve was returned to service.
Cause of the Event
The limit switch miss-wiring is considered a legacy maintenance human performance event. FPL is investigating potential weaknesses in the post-maintenance test procedure as a potential contributor to this latent failure.
Analysis of the Event
Valve FCV-26-3 is the inside containment penetration isolation valve for containment penetration 52B which serves the 'B' channel containment atmosphere radiation monitor (FCV-25-4 is its companion outside isolation valve). Each containment isolation valve actuator contains limit switches for the open and closed position. As a containment isolation feature, the valve has closure time requirements.
Per procedure, valve closure time is verified by observation of the control room remote valve position indication. Containment penetrations 52A, 52B, and 52C share isolation valve position indication by ganging the limit switches together for the inside isolation valves (i.e., FCV-26-1, FCV-26-3, and FCV-25-5) and outside isolation valves(i.e., FCV-26-2, FCV-26-4, and FCV-25-6). The limit switch arrangement indicates dual position for the ganged inside and outside isolation valves when each of the three respective flow control valve limit switches are not in the open or closed condition. The discrepant wiring prevented valve FCV-26-3 from providing its limit switch condition to the common inside containment isolation valve indication scheme for penetrations 52A, 52B, and 52C.
No matter what position FCV-26-3 was actually in, as the ganged valves were stroked it would appear as normal so long as both FCV-26-1 and 5 operated normally. Because Technical Specification Surveillance Requirement 3/4.6.3.3 verifies valve stroke timing via the remote control room position indication, the containment isolation timing for FCV-26-3 has not been verified since December 2001. This resulted in operation of the facility in a manner prohibited by the Technical Specifications, as the wiring discrepancy invalidated previous FCV-26-3 closure time surveillance results.
However, every refueling outage FCV-26-3 and its companion valves have their local and remote position indications verified. Though not a requirement of the procedure, local personnel have observed the valves' open and close strokes as they are remotely operated from the control room. Although this does not validate the timing of FCV-26-3, it does provide reasonable assurance that FCV-26-3 stroked in concert and consistent with its companion valves.
In addition to timing requirements, containment isolation valves also have pressure integrity requirements. Pressure integrity for the containment penetration is tested in accordance with the integrated and local leak rate testing (I/LLRT) requirements, and FPL determined that there have been no containment penetration 52B I/LLRT failures between October 19, 2015 (e.g., the time of discovery) and December 2001 (e.g., when the legacy maintenance occurred) Based on these test results, FPL concludes that although the actual position indication for FVC-26-3 was inoperative, the penetration integrity was tested satisfactory during the I/LLRT tests, proving the valve was positioning and functioning as a containment isolation feature during the time period between December 2001 and October 2015.
The remaining St. Lucie Unit 2 system 26 isolation valve ganged limit switches will be inspected to ensure that the limit switches are properly wired.
Safety Significance
Although valve closure timing was not tested, the valve did respond to closure signals as demonstrated by successful I/LLRT test results. Opportunities to observe the valve stroke during local/remote valve position verifications provide reasonable assurance that FCV-26-3 operated with speeds comparable to the other penetration 52A, 52B, and 52C containment isolation valves. Additionally, FCV-26- 4 passed all surveillances during the subject time period, assuring redundant penetration isolation capability for penetration 52B. Based on the above, containment integrity was not challenged by this legacy condition.
In addition to post-accident containment isolation matters, had the valve failed closed during normal operation without indication, the 'B' channel containment atmosphere radiation monitor sample pump would trip on low flow, alerting the operators of a potential problem.
Based on the above, the inoperative FCV-26-3 valve position indication and the resultant lack of available closure time test data was not an adverse impact on the health and safety of the public.
Corrective Actions
1. The limit switch wiring for FCV-26-3 was restored to design under work order 40418516.
2. The remaining Unit 2 containment penetration 52A, 52B, and 52C isolation valves extent of condition will be verified by work order 40433093.
3. Engineering is reviewing stroke time test methodology for improvement opportunities.
Failed Component(s) Component: Limit Switch Manufacturer: NAMCO Model No.: EA180-11302 Similar Events None.