05000335/LER-2001-006
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FACILITY NAME (1) DOLKEI NUMBER (2) LER NUMBER (6) PAGE (3) St. Lucie Unit 1 05000335
Description of the Event
On May 22, 2001, St. Lucie Unit 1 was in Mode 1 at 100 percent reactor power. During the performance of the monthly Technical Specification (TS) surveillance run, FPL discovered that the 12 cylinder 1B2 emergency diesel generator (EDG) had a radiator leak [EIIS:LB:HXJ. The subsequent inspection identified a split in the vertical seam of one of the radiator flat tubes located in the outermost row on the inlet face of the radiator. This leak was repaired and the 1B2 EDG radiator was pressure tested (to -9 psig which is higher than its normal operating pressure). No additional leaks were identified during the pressure tests and the 1B EDG was returned to service after successfully completing its Technical Specification surveillance run. The radiator was scheduled for replacement during the next refueling outage. As a conservative interim corrective action, FPL increased the surveillance frequency for the 1B EDG to twice a month.
On June 11, 2001 during the first of the bi-monthly surveillance runs, the 1B2 EDG radiator developed another leak. The 1B2 EDG was declared out of service (00S) pending replacement of the EDG radiator cores. Replacement cores were not available on site and were procured in an expedited manner. On June 15, replacement cores for the 1B2 EDG radiator were received on site, the cores were replaced, and the 1B EDG was returned to service on June 17, 2001.
Cause of the Event
Corrosion of the EDG radiator cooling fins from the humid salt-laden atmosphere resulted in a loss of structural support of the radiator flat tubes. This corrosion was originally identified by FPL during the St. Lucie Unit 1 spring 2001 refueling outage, SL1-17. The loss of structural support, combined with the thermal and pressure cycles experienced by the radiator, allowed the flat tubes to swell and vibrate and ultimately caused the failure of the soldered mechanical tube joints.
There were several contributing factors that lead to this event.
The failure mechanism was not well understood. FPL operating experience with similar EDG radiator corrosion was applied to the material condition of the St. Lucie 1B2 radiator. FPL believed that considerably more fin degradation was required in order for radiator tube failures to occur and that corrective actions implemented following the first failure, along with the rigorous post maintenance testing, demonstrated adequate operating margin.
Additionally, the preventive maintenance program did not address the expected life or recommend a replacement interval for the EDG radiators. Past EDG radiator replacement was based and predicated on the loss of thermal performance. Although the 1B2 radiator showed evidence of corrosion, it was not experiencing adverse thermal performance.
Analysis of the Event
The period of time that the 1B2 EDG radiator was susceptible to failure was greater than the 14-day TS allowed outage time (AOT) of TS 3.8.1.1.b and is therefore reportable under 10CFRS0.73(a)(2)(i)(B) as a condition prohibited by Technical Specifications.
FACILITY NAME (1) NUMBER (2) LER NUMBER (6) PAGE (3) St. Lucie Unit 1 05000335 Analysis of Safety Significance Each EDG consists of two diesel engines T (12 tandem with a 3500 kW generator coupled directly each diesel generator set has a self-contained forced circulation cooling water system which cooled radiator system which removes the heat water pump and radiator fan are driven directly Lucie Unit 2 EDG radiators are of a different verses flat cooling tubes), and as such, T are mechanism being reported in this LER. T Although evidence of corrosion, T the corrosion is not radiator. T FPL plans on replacing the 1A2 suitable work week window.
In retrospect, based on the chronology identified corrective actions implemented following the insufficient to assure the integrity of the operation of the 1B EDG. T Additionally, based failures in the 1B2 EDG radiator, FPL concludes the SL1-17 refueling outage and the surveillance insufficient operating cycles remaining in of the 1B2 EDG radiator.
Based on review of the chronological logs, risk significant structures, T systems, or components the same time the 1B EDG was assumed not operable component cooling water (CCW) heat exchanger safety assessment T (PSA) model was used to condition reported in this LER.
T During the the weather related offsite power events T (e.g., PSA model were not credible because the event season. T The offsite power recovery model offsite power events.
lA EDG OOS:
cylinder and 16 cylinder) mounted in between the engines. T Each engine in cooling system which consists of a cools the engine directly and an air from the cooling water. T The cooling from the engine crankshaft. T The St.
design T (i.e., T round cooling tubing not susceptible to the same failure the 12 cylinder 1A2 EDG radiator has to the extent experienced on the 1B2 EDG EDG radiator on-line during the next above, T it can be concluded that the first tube failure on May 22, T 2001, were 1B2 EDG radiator and thus, T the reliable on the failure mode and repeated tube that at some time between the end of failure on May 22, T 2001, T there were the 1B2 EDG radiator to support operation it is judged that the most potentially (SSCs) T removed from service at were the lA EDG and the 1A (Hx). T The St. Lucie probabilistic estimate the risk significance of the period when EDG 1B was out of service, hurricane) included in the current occurred prior to the active hurricane was refined to exclude the weather related a 0.5 non-recovery probability for the 1A to the grid. T The exposure time service T (00S)) was approximately 6.5 over a 6.5 hour5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> exposure is calculated from service of only the 1A EDG for removed from service for testing with the change in core damage probability is approximately 32 hours3.703704e-4 days <br />0.00889 hours <br />5.291005e-5 weeks <br />1.2176e-5 months <br />. T The risk assessed and found to be acceptable as program (CRMP). T The change in risk be the difference in risk with only the 1A and the 1B EDG OOS over the 32-hour lA CCW pump is not credited. T The is T 1.9E-07.
The risk assessment conservatively assumes EDG if a LOOP occurs while the EDG is connected (i.e., T the total time the 1A EDG was out of hours. T The change in core damage probability by comparing the risk associated with removal testing and the risk of having the lA EDG 1B EDG assumed not operable.
T The estimated 3.2E-08.
lA CCW Hx OOS:
The 1A CCW Hx was removed from service for associated with the planned CCW Hx work was part of the configuration risk management assuming the 1B EDG is also OOS would thus CCW Hx OOS and the risk with the 1A CCW Hx exposure time. T Recovery of CCW flow via the estimated change in core damage probability DuLICILL � 1B EDG 00S for the Assumed Fault Exposure Time:
FPL conservatively assumed that the time of interest for the risk significance determination of the degraded 1B EDG radiators was the full fault exposure time of approximately 46 days. The estimated increase in core damage probability over this time interval is approximately 8.0E-07. If the common practice (e.g., Maintenance Rule) to use half the fault exposure time is assumed, the estimated increase in core damage probability is approximately 4.0E-7.
Based on the above assessments, the change in core damage probability for this event remained less than 1E-06. Therefore, this event had no significant impact on the health and safety of the public.
Corrective Actions
1. The 1B2 EDG radiator was replaced.
2. FPL will incorporate the lessons learned from this event into the continuing engineering personnel training program.
3. The preventive maintenance program and procedures will be revised to provide cleaning and time based replacement criteria for the St. Lucie Unit 1 EDG radiators.
4. The 1A2 EDG radiator is planned for on-line replacement during the next suitable work week.
Additional Information
Failed Components Identified Manufacturer: Young Radiator Company Component: � 1B2 EDG Radiator Model Number: Young Radiator Company Drawing D260846 Similar Events None