05000443/LER-2013-001
| Docket Number | |
| Event date: | 10-30-2013 |
|---|---|
| Report date: | 12-23-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 4432013001R00 - NRC Website | |
Description of the Event
On October 30, 2013, a review of actions taken for a leak in the service water (SW) supply piping determined that the station did not declare SW inoperable and enter TS 3.7.4.d, resulting in the plant operating in a condition prohibited by Technical Specifications for 24 days from August 8, 2013 until an approved non-code repair was completed on September 1, 2013.
On August 7, 2013, during routine monitoring, a system engineer identified a small leak that he believed to be a through wall wormhole based on operating experience on the train B SW strainer bypass line. A UT analysis of the leak area was completed on August 8, 2013 and determined that there was a rectangular area of 2.327" x 1.5" where UT readings could not be obtained (UT indication). Engineering review determined that the identified UT indication had no impact on the structural integrity of the piping. Based on this analysis, an immediate operability determination was performed and the piping was found to be operable but degraded. A prompt operability determination (POD) was performed that also determined the piping to be operable but degraded based on the identified leak rate of 10 drops per minute (dpm). Monitoring actions were put in place and future outage repair plans started.
On August 20, 2013, the leak increased to 90 dpm. Based on information obtained from Engineering, Operations concluded that the 90 dpm leak was still well within the bounding allowable leakage as determined by the existing POD.
On August 28, 2013, the leak rate increased to approximately 25 gallons per minute (gpm) following transfer of the SW train from the ocean supply to the cooling tower for surveillance testing. Based on the increase in leakage, the control room staff terminated the surveillance and restored train B SW to the ocean pumps. Leakage decreased to approximately 15 gpm following transfer to the ocean supply. After consulting with Engineering, Operations concluded that the 25 gpm leak was still within the bounding allowable leakage as determined by the existing POD.
NRC authorization for the non-code repair was received on August 31, 2013 and the repair was completed on September 1, 2013.
Cause of the Event
A root cause analysis was conducted for this event.
The direct cause of the event was determined to be:
Due to overconfidence, Engineering preparation of the Prompt Operability Determination (POD) for the August 7, 2013 SW pipe leak did not address UT examination results in the analysis of potential impact on system safety functions.
The root cause of the event was determined as:
The NextEra Fleet POD process did not have sufficient process barriers to ensure that the preparation error (Direct Cause) was recognized and corrected prior to POD approval.
The contributing causes for this event are:
1. The UT examiner did not initiate a CR to document and report the anomalous UT results (UT indication).
2. Operations Shift Managers lacked a questioning attitude as to the potential implications for UT indication on system safety functions.
Analysis of the Event
The function of the SW system is to transfer the heat loads from various sources in both the primary and secondary portions of the plant to the ultimate heat sink. The system is designed to supply sufficient cooling water to its heat loads under possible operating conditions.
The SW system consists of two completely independent and redundant trains with two ocean pumps [BI, P] and one cooling tower pump [BS, P] in each train. Two sources of cooling water are provided for the SW system: the Atlantic Ocean and the cooling tower. The normal source of water for the SW system is the ocean through two tunnels, one tunnel from the submerged intake structure offshore and a second tunnel that discharges cooling water to the ocean.
The cooling tower serves as the ultimate heat sink during a seismic event that results in blockage of the ocean tunnels.
While the ocean serves as the normal supply of cooling water, heat loads can be automatically or manually transferred to the cooling tower.
Technical Specification 3/4.7.4 requires that the SW System be Operable in Modes 1 through 4. This includes
- An operable SW pump house and two service water loops with an operable SW pump in each loop and,
- An operable mechanical draft cooling tower and two cooling tower loops with one operable cooling tower pump in each loop, and
- A portable cooling tower makeup system stored in its design operational readiness state.
The SW strainer bypass line is located upstream of the Primary Component Cooling Water (PCCW) and Diesel Generator (DG) heat exchangers and is pressurized during ocean pump alignment and cooling tower operation. The line is not isolable with the system in service. If a leak at this location was large enough, cooling tower inventory and the ability to operate for seven days without makeup could be impacted. The calculated maximum acceptable boundary leakage for 7 day cooling tower operation is approximately137.25 gpm. The POD performed on August 8, 2013 for the leak discovered on August 7, 2013 concluded that the leak was estimated to be less than 10 drops per minute. A structural evaluation performed for the August 7, 2013 specifically addressed the UT indication and concluded that the structural integrity of the SW bypass line was not compromised and there was no structural impact on the service water system. The evaluation of the other SW safety functions (Cooling Tower inventory, Cooling supply to PCCW and EDG heat exchangers and PAB flooding) did not address the UT indication. Based on the leak rate of 10 dpm, both the preparer and reviewer of the POD were of the opinion that this leak was the same as previous pin hole leaks identified in service water piping. The POD documented the propagation of the leak was assumed to be a 1/2" diameter hole based on the 10 dpm leak.
On August 20, 2013, the leak increased to 90 dpm. On August 21, 2013, a second UT examination was performed of the leak area. The UT examination confirmed the indication size had not changed, however a depression (concavity) in the UT indication area of approximately 1/2" in diameter was noted. The UT examination results were evaluated by Engineering; concluding that since the size of the indication had not changed the structural integrity of the pipe is maintained. Based on information obtained from Engineering, Operations concluded that the current leak rate was within the bounding allowable leakage as determined by the prompt operability.
On August 28, 2013 at 1900, during surveillance testing of the train-B cooling tower pump, it was determined that the previously identified leak had increased to approximately 25 gpm. During the swap to the cooling tower pumps SW header pressure typically increases. In this situation, the SW header pressure increased from 48 psi to 66 psi. Based on the increase in leakage and SW header pressure, the control room staff terminated the surveillance and restored train B SW to the ocean pumps. When returned to the ocean pumps, 13' SW header pressure returned to 48 psi and leakage decreased to approximately 15 gpm. Assessment of operability at that time determined that the SW system remained operable with the increased leakage based on the operability determination performed previously for the August 7, 2013 SW leak. A housekeeping patch was placed on the pipe and discussions regarding a relief request for a noncode repair started with the NRC. The repair was completed on September 1, 2013.
On October 30, 2013, a review of the event determined that the POD performed for the August 7, 2013 leak did not provide reasonable assurance of continued operability of the SW system. Therefore, the SW leak rendered the ocean and cooling tower loops in SW train B inoperable. TS 3.7.4, action d applies to this condition and requires restoring at least one loop to operable status within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> or placing the plant in Mode 3 in the next six hours and in Mode 5 in the next 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br />. The plant exceeded the allowed outage time and shutdown time of the TS action when it operated from August 8, 2013 until September 1, 2013, when the leak was repaired. The review of the event also found that concurrent with the unrecognized inoperability of SW train B, the train A SW train was rendered inoperable for testing and maintenance activities. On August 15, 2013, train A SW was rendered inoperable on two occasions for a total time of approximately three hours. The concurrent inoperability of both trains of the SW system resulted in a condition that could have prevented fulfillment of the safety function of the SW system. Therefore, this event meets the reporting criteria of 10 CFR 50.73(a)(2)(i)(B) for an operation or condition prohibited by the TS and 10 CFR 50.73 (a)(2)(v)(D) for an event that could have prevented fulfillment of a safety function needed to mitigate the consequences of an accident.
No adverse consequences resulted from this event, and this incident had no adverse impact on the health and safety of the public or the plant and its personnel. This event did involve a safety system functional failure because both trains of the SW system were inoperable for approximately three hours on August 15, 2013. Nonetheless, this event was of minimal risk significance, having no impact on core damage frequency or large early release frequency as during the time in question, the SW system remained functional. No inoperable structures, systems, or components contributed to this event.
Corrective Actions
The Root Cause Team identified the following corrective actions:
1. Involved individuals involved in the POD preparation were coached and mentored on documentation requirements for POD preparation.
2. Revise the Fleet procedure to add appropriate process barriers to ensure the engineering evaluation adequately documents consideration of field input information and assumptions for safety functions. Include an independent review or supervisory review and require assumptions be stated (including disposition of input data), i.e., UT, NDE inspection results and observations.
3. Reinforce procedure requirement to initiate a condition report for unexpected UT results even if, the UT is being performed as part of an already identified condition.
4. Prepare and present a case study regarding this event and root cause evaluation to Engineering, Operations Shift Management and Station Leadership.
Similar Events A review of the corrective action program and LERs in the past five years identified no similar events.
Failed Components
None.
Additional Information
The Energy Industry Identification System (EllS) codes are included in this LER in the following format: [EllS system identifier].