05000499/LER-2011-002
| South Texas Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| 4992011002R00 - NRC Website | |
No structures, systems, or components were inoperable at the start of the event that contributed to the event.
D. NARRATIVE SUMMARY OF THE EVENT
On 11/26/11 at 2121 hours0.0245 days <br />0.589 hours <br />0.00351 weeks <br />8.070405e-4 months <br />, Unit 2 received the Stator Coil Water (SCW) Differential Temperature high alarm. The crew responded by implementing OPOP09-AN-7M03 for the A-5 annunciator. The crew identified a differential temperature greater than 14.4 °F on the Integrated Plant Computer System (ICS). None of the ICS thermocouple points indicated greater than the 174 °F criteria for notifying the System Engineer. The crew determined generator thermocouple T6144 on the SCW outlet of Coil 33T was reading higher than the other thermocouples. The crew contacted the Integrated Maintenance Team for l&C support to verify the Coil 33T thermocouple reading in accordance with the annunciator response procedure.
On 11/27/11 at approximately 0200 hours0.00231 days <br />0.0556 hours <br />3.306878e-4 weeks <br />7.61e-5 months <br />, two l&C Technicians and their Supervisor arrived at the Generator Terminal Board at the east side of the Main Generator. The l&C Technicians reported to the Unit 2 Control Room that the Coil 33T thermocouple was reading 160.7 °F. Shortly after the l&C Technicians disconnected their equipment at approximately 0222 hours0.00257 days <br />0.0617 hours <br />3.670635e-4 weeks <br />8.4471e-5 months <br />, thermocouple 33T rose to 168 °F and it continued rise unnoticed until it read greater than 175 °F. The crew mistakenly believed the higher temperature indicated a malfunctioning thermocouple due to l&C activities based on the report that the Coil 33T thermocouple was reading 160.7 °F at the generator. At 0235 hours0.00272 days <br />0.0653 hours <br />3.885582e-4 weeks <br />8.94175e-5 months <br />, the Coil 33T thermocouple rose to 180.35 °F but the crew was not trending or monitoring the point because it was considered an invalid indicator.
At 0244 hours0.00282 days <br />0.0678 hours <br />4.034392e-4 weeks <br />9.2842e-5 months <br />, the Control Room received multiple Generator Condition Monitor (GCM) alarms from the ICS. The Control Room directed a Plant Operator (PO) to adjust the analyzer's sample flow.
Based on the behavior of the GCM alarms and other indicators, the crew determined the GCM was malfunctioning and a Condition Report (CR) was written on the GCM. At 0254 hours0.00294 days <br />0.0706 hours <br />4.199735e-4 weeks <br />9.6647e-5 months <br />, the GCM Verified Alarm cleared. At 0342 hours0.00396 days <br />0.095 hours <br />5.654762e-4 weeks <br />1.30131e-4 months <br />, the crew removed the Coil 33T thermocouple from service by substituting a known value for the thermocouple data. When the Coil 33T thermocouple was removed from service it had trended down from 181.7 °F to 178.7 °F.
FORM 366 (10-2010) and cycling between safe and alarm. The PO left the GCM to obtain a copy of procedure OPOP02- GG-0001, Generator Hydrogen and Carbon Dioxide Gas System, Addendum 4, Generator Condition Monitoring Alarm Response to address the GCM alarms.
At 0310 hours0.00359 days <br />0.0861 hours <br />5.125661e-4 weeks <br />1.17955e-4 months <br />, the Control Room received annunciator 7M03-A6, Stator Cooling System Trouble Alarm. The Control Room redirected the PO to check the Stator Cooling Water System. When the PO arrived at the Stator Cooling Water skid, he noted that the tank level was high and water conductivity was rising. The PO immediately contacted Chemistry to sample the cooling water for conductivity.
The Unit 2 Reactor tripped at 0329 hours0.00381 days <br />0.0914 hours <br />5.439815e-4 weeks <br />1.251845e-4 months <br /> on 11/29/11 due to Main Generator Lockout. A "crawl through" inspection was performed on the day after the Unit 2 Main Generator event and reactor trip and significant stator coil damage was found. Approximately three feet of stator Coil 33T (i.e., the Top coil in slot 33) was melted or missing on the exciter end.
The coils have hollow strands throughout the coil to allow Stator Cooling Water (SCW) to flow through the coils themselves. Each strand is covered with resin used as internal fillers and insulation inside the coil. The insulation surrounding the conducting portion of the coil is called groundwall insulation and it consists of mica covered glass backed tape. The resin bonds the mica tape in the groundwall and fill voids between the groundwall and the conductor stack.
The failure analysis determined the most likely cause was that a very small leak existed in a hollow strand in Coil 33T. Analysis supports that this leak existed for a long time and allowed moisture to travel inside the coil. The moisture degraded the resin in the coil over time allowing the conductor bundle to come loose from the ground wall and allow some individual conductor strands to move and vibrate. The strand-to-strand vibrations and movement eventually wore away the insulation between the strands and created strand-to-strand shorts. The shorts caused excessive heating.
The affected area grew as the coil insulation was thermally damaged and failed. As the affected area grew, more heat was created until the coil melted and eventually arced violently across the missing melted area eventually causing the 33T coil to catastrophically fail.
The leak in a hollow strand wall could have been caused by several different mechanisms.
Inspections did not identify the leak location. The exact cause of the leak cannot be ascertained without being able to examine the leak location. Although the final analysis report is not yet available, it is likely that the small leak was located in the portion of the coil that is melted/missing. If so it will not be possible to identify the root cause of the leak. After the final failure analysis reports are received, the root cause report will be evaluated and revised if warranted based upon the findings. The LER will be supplemented if the evaluation results affect the substance of this report.
The failure of 33T coil itself did not cause the ground fault relay due to its position to neutral (first coil from neutral) and its low voltage, but when 33T failed, melted copper was expelled from the ground wall of 33T and flowed down on 33B. The melted copper degraded the groundwall insulation on 33B and a ground fault occurred.
FORM 366 (10-2010) The generator failure, reactor trip, and automatic actuation of the systems listed below were self-revealing.
II. EVENT-DRIVEN INFORMATION
A. SAFETY SYSTEMS THAT RESPONDED
All required safety systems responded as expected including the following actuations:
1. Reactor Coolant Pump Undervoltage Reactor Trip 2. Reactor Protection System P-16, Turbine Trip 3. Feedwater Isolation Actuation 4. CRE HVAC Emergency Recirculation (C Train LOOP) 5. Reactor Containment Fan Coolers (C Train LOOP) 6. Auxiliary Feedwater Actuation (All AFW pumps actuated) 7. Primary Pressure Control (Pressurizer Spray and Heaters actuated as required) 8. Secondary Pressure Control Actuation (Steam Dumps Actuated)
B. DURATION OF SAFETY SYSTEM INOPERABILITY
N/A
C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT
There was no impact to radiological safety, safety of the public, or safety of station personnel during this event.
The Incremental Conditional Core Damage Probability (ICCDP) for the Reactor Trip in Unit 2 on November 29, 2011 is 2.65E-07. The resulting Incremental Conditional Large Early Release Probability (LERP) is 5.99E-09.
III. CAUSE OF THE EVENT
Although the final analysis report is not yet available, it is likely that the small leak was located in the portion of the coil that is melted/missing. If so the root cause of the leak will not be determined. After the final failure analysis reports from ElectroMechanical Engineering (EME) and Kinectrics are received, the root cause report will be revised and the corrective action plan addressing the technical aspects of this event will be changed if warranted based upon the findings. The LER will be supplemented if the evaluation results affect the substance of this report.
FORM 366 (10-2010) Enhancement actions are planned to improve control room annunciation and indication of generator conditions.
Repairs to the Unit 2 Main Generator stator, rotor, exciter, hydrogen cooler, and associated auxiliary equipment are in progress.
As discussed above, following receipt of the final failure analysis reports, the root cause report will be revised and the corrective action plan addressing the technical aspects of this event will be changed if warranted based upon the findings.
V. PREVIOUS SIMILAR EVENTS
There have been no similar reportable events at STP within the last three years.
VI. ADDITIONAL INFORMATION
N/A FORM 366 (10-2010)