05000364/LER-2013-001
| Joseph M. Farley Nuclear Plant, Unit 2 | |
| Event date: | 05-29-2013 |
|---|---|
| Report date: | 09-13-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3642013001R01 - NRC Website | |
Westinghouse - Pressurized Water Reactor Energy Industry Identification Codes are identified in the text as [XX].
Description of Event
Farley Nuclear Plant steam flow transmitters are calibrated each refueling outage utilizing instrument data from the previous operating cycle. Following the initial post-refueling ascension in power the steam flow transmitters are normalized utilizing beginning-of-cycle data per procedure FNP-2-ETP-4462 (Power Ascension Following a Unit Refueling Outage).
On May 29, 2013 at 1339, with Unit 2 operating in Mode 1 at 100% power, Engineering personnel performing beginning-of-cycle normalization calculations determined that 2C Steam Generator Steam Flow Transmitter FT-494 [FT] was outside of Technical Specification accuracy requirements. Technical Specifications require that the high steam flow bistable actuates at less than 110.3% of full steam flow. However, the power ascension data indicated that the actual high steam flow setpoint for FT-494 was 111.38% of full steam flow.
As a result of the determination by Engineering, FT-494 was declared inoperable and the Technical Specification 3.3.2 (Engineered Safety Feature Actuation System Instrumentation) required action for the inoperable channel was entered. The affected bistable was placed in the tripped condition within the required 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> from the declaration of inoperability. FT-494 was subsequently re-calibrated and the high steam flow bistable was verified to trip at a setpoint less than the Technical Specification limit of 110.3% of full steam flow. FT-494 was returned to operable status at 1642 on June 1, 2013. FT-494 is a model number 764 flow transmitter manufactured by ITT Barton.
The data utilized in the Engineering determination was obtained on May 14, 2013. For the purpose of determining reportability, this date is considered the point of discovery.
Consequently, the completion times associated with the applicable Technical Specification required actions were not met. This represents an operation or condition prohibited by Technical Specifications and is reportable under 10CFR50.73(a)(2)(i)(B).
The Engineering review also identified that a similar condition existed at the beginning of the previous fuel cycle. At 1311 on January 4, 2012, with Unit 2 operating in Mode 1 at 100% power, Engineering review of beginning-of-cycle instrument data identified that the FT-494 high steam flow bistable setpoint was 111.36% of full steam flow which is above the maximum Technical Specification allowable setpoint of 110.3% of full steam flow. This resulted in a declaration of inoperability for FT-494. FT-494 was recalibrated and returned to service at 0935 on January 7, 2012 which is within the time requirement of the applicable Technical Specification required action based on the time of declaration of inoperability. The data utilized for this review was obtained on November 29, 2011, which is the discovery time for the purpose of determining reportability. Based on this discovery time, the completion time associated with the applicable Technical Specification required action was not met. This also represents an Operation or Condition Prohibited by Technical Specifications and is reportable under 10CFR50.73(a)(2)(i)(B). Due to an oversight by the plant staff this reportability requirement was not recognized and the required Licensee Event Report was not submitted.
Cause of Event
Causal analysis of the out-of-tolerance condition identified on January 4, 2012 was performed utilizing the Apparent Cause Determination process. This causal analysis determined that the out of tolerance condition could be due to transmitter issues such as the equalizing valve leak- by or partial clogging of the flow transmitter sensing lines. This causal analysis included a review of associated procedures and work order packages for calculation or implementation errors with no errors being identified. As a result of this analysis, actions were put in place to monitor the performance of FT-494 over a six-month period, collect and review data to resolve possible sensing line blockage, and to investigate the condition of the transmitter equalizing valve during the Spring 2013 Unit 2 refueling outage. The causal analysis also specified performance of a re-evaluation of the event causes following the completion of the above mentioned actions. This re-analysis was incorporated into the Enhanced Apparent Cause Determination for the May 29, 2013 event.
The out-of-tolerance condition identified on May 29, 2013, was evaluated using the Enhanced Apparent Cause Determination process. This analysis determined that the direct cause of FT- 494 being out of tolerance for both events was due to transmitter shift possibly related to temperature changes. The apparent cause was found to be a previous revision to the preventive maintenance program that extended the service life of the transmitter from 20 years to 35.5 years and resulted in the deferment of a scheduled transmitter replacement in 2011, A contributing cause for the repeat event was determined to be an ineffective causal determination from the first event.
Safety Assessment Steam Flow Transmitter FT-494 performs a safety function by providing a high steam flow input to main steam line isolation logic circuitry. Each of the three steam generators is equipped with two redundant steam flow transmitters. A high steam flow signal from one of the two steam flow transmitters on two of the three steam generators coincident with a low-low reactor coolant system (RCS) average temperature signal from two of three RCS temperature channels generates a main steam line isolation signal that causes closure of all main steam line isolation valves.
During the time periods of the FT-494 out-of-tolerance condition, the redundant steam flow transmitter remained capable of performing its safety function. Therefore, sufficient inputs to the main steam line isolation circuitry were available to actuate a main steam line isolation at the proper setpoint. At no time was there a loss of safety function.
A diverse means of providing a main steam line isolation in the event of a steam line break is the low-steam-pressure main steam line isolation signal. This function remained fully capable of performing the main steam line isolation function during the periods that FT-494 was known to be out of tolerance.
Based on the above considerations and on the FT-494 setpoint being slightly above allowable tolerance but otherwise operable this condition is considered to have low safety significance.
Corrective Action FT-494 was rescaled per work order SNC 459224 and returned to service at 1642 on June 1, 2013. To address the transmitter shift experienced by this instrument, the transmitter will be replaced during the next refueling outage in October 2014. Corrective action has been implemented to re-evaluate the replacement strategy in the preventive maintenance program for transmitters used in Technical Specification scaling and to adjust transmitter replacement schedules accordingly. Training on the requirements and expectations for trending, licensing basis, and failure mechanisms will also be provided to the applicable engineers and their supervisor.
Subsequent to the causal analysis for the first event, management oversight of the causal analysis process was strengthened by the implementation of a causal analysis grading process that is utilized during management review of analyses.
The failure of the plant staff to identify the event that occurred on January 4, 2012 as requiring a Licensee Event Report has been entered in the plant corrective action program as condition report 675409.
Additional Information
A review of previously submitted Licensee Event Reports identified no similar issues being reported within the previous three years. There have been three other instances in the previous three years of steam flow transmitters requiring re-normalization following the evaluation of beginning-of-cycle data. In all of these cases, the high steam flow setpoints remained within the accuracy requirements of Technical Specifications and operability of the steam flow channel was not affected. These occurrences are documented in condition reports 55575 (for Unit 1 FT- 485), 485511 (for Unit 1 FT-484), and 646824 (for Unit 2 FT-485).