05000499/LER-2011-001
| South Texas Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4992011001R00 - 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 May 14, 2011, during the weekly performance of the Engineered Safety Features (ESF) power availability surveillance, B Train 480 Volt ESF Load Centers (LC) E2B1 and E2B2 were found with local indication for voltage at 520 Volts which exceeds the surveillance acceptance criterion of a maximum allowed bus voltage of 506 volts.
Based on the failed surveillance, the Class 1E 4160V Bus 2B was declared INOPERABLE at 21:32.
The following Technical Specification action statements were entered:
- TS 3.8.3.1 action a was entered requiring re-energizing the associated bus within 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> or apply the requirements of the Configuration Risk Management Program (CRMP) or be in at least HOT STANDBY in the next 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and in COLD SHUTDOWN within the following 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br />.
- TS 3.8.1.1 ACTION a was entered which requires demonstration of the OPERABILITY of the remaining A.C. sources by performing Surveillance Requirement 4.8.1.9.1.a within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> and at least once per 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> thereafter, within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> restore the offsite circuit to OPERABLE status or apply the requirements of the CRMP, or be in at least HOT SHUTDOWN within the next 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> and in COLD SHUTDOWN within the following 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
On 05/15/11 13:26, the ESF Transformer 2B Load Tap Changer was placed in manual per plant procedures and lowered bus voltage.
On 05/15/11 15:21, the 4160 V BUS E2B was declared OPERABLE after adjusting both 4160 V E2B voltage and 480 V LC E2B voltage to within specified bands. TS 3.8.3.1 action a and TS 3.8.1.1 action a were exited including the requirements of the CRMP.
FORM 366 (10-2010) FORM 366 (10-2010) Since there is firm evidence that the associated ESF buses were inoperable longer than the allowed Technical Specification action statement duration of eight hours including the 6 hour6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> requirement to be in HOT STANDBY, this is considered a condition prohibited by the plant's Technical Specifications.
E. METHOD OF DISCOVERY
The over-voltage condition was discovered by reviewing the local voltage indicators during the weekly performance of ESF Power Availability surveillance.
II. EVENT-DRIVEN INFORMATION
A. SAFETY SYSTEMS THAT RESPONDED
N/A
B. DURATION OF SAFETY SYSTEM INOPERABILITY
Per the ICS, Class 1 E 4.16 kV ESF Bus E2B and downstream 480 Volt Load Center Buses E2B1 and E2B2 were inoperable from approximately:
- 06:15 on May 5 to approximately 06:15 on May 7
- 09:00 on May 8 to approximately 13:30 on May 15
C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT
Per engineering evaluation, there is reasonable assurance that the condition would not have prevented the affected safety-related switchgear and associated loads from performing their safety functions. Therefore, any increase in Core Damage Frequency or Large Early Release Frequency was insignificant. Consequently, there is no adverse effect on the health and safety of the public.
III. CAUSE OF THE EVENT
1. The range of voltage values allowed by the bandwidth of the Load Tap Changers (LTC) on the Unit Auxiliary Transformer and the ESF B Transformers was not conservatively modeled. An electrical set- point methodology for modeling LTCs did not exist at STP resulting in knowledge-based task performance by the user and reviewer. Program inputs/assumptions used in the computerized Electrical Transient Analyzer Program (ETAP) Calculation are not clear and were not adequately documented which impeded the capability to review the calculation.
2. Failure Modes and Effects Analysis (FMEA) for the ESF transformer LTC did not accurately identify the severity of all failure effects, and did not conclude that the method of failure detection was inadequate. An adequate FMEA would have identified the need for instrumentation and alarms. Full understanding of the potential failure modes would have identified the potential voltage range in which the LTC could fail and would have determined the voltage limits on the 480V bus could be exceeded.
2. DOCKET
00
- 001 3. Management oversight did not have a method to identify, classify, control and monitor complex modifications.
IV. CORRECTIVE ACTIONS
A. Root Cause 1:
a. Develop and issue an electrical design calculation methodology guideline or procedure for EC calculations EC-5000, EC-5001, EC-5002, EC-5008, and EC-5036.
b. Provide Training on the requirements/availability of the electrical design calculation methodology guidance and the requirements of the calculation pre-job brief.
B. Root Cause 2:
a. Develop and issue a guideline or a procedure for completing a Failure Modes and Effects Analysis using INPO and industry best practices.
b. Provide Training on the completion of FMEA documentation using the guideline or procedure developed in correction action (IV.B.a).
C. Root Cause 3:
a. To identify modifications that require the management oversight, develop and implement a definition for complex modifications in the design change package procedure including defining the requirements for management oversight which may include adequate resources, development of project schedules and site involvement in the rigorous reviews of project documents.
b. Provide training for design engineering and staff augmented contract engineers on the Plant Investment Plan Guidelines for modifications that are complex or meet the definition of a complex modification.
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)