05000348/LER-2013-003
| Joseph M. Farley Nuclear Plant, Unit 1 | |
| Event date: | 11-5-2013 |
|---|---|
| Report date: | 2-21-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(i) |
| 3482013003R01 - NRC Website | |
Westinghouse - Pressurized Water Reactor Energy Industry Identification Codes are identified in the text as [XX].
Description of Event
Following the initial post-refueling ascension in power, station procedures required the steam flow transmitter loop to be normalized utilizing beginning-of-cycle data obtained during power ascension.
On November 5, 2013 with Unit 1 operating in Mode 1 at 100% power, Engineering personnel performing beginning-of-cycle normalization calculations determined that 1C Steam Generator Steam Flow Transmitter FT-495 [FT] was outside the acceptance criteria for normalization. As a result of the determination by Engineering, FT-495 was declared inoperable and Technical Specification 3.3.2 (Engineered Safety Feature Actuation System Instrumentation) Required Action for the inoperable channel was entered. The effected 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. The steam flow transmitter loop was subsequently re-calibrated and the high steam flow bistable was verified to trip within the allowable tolerance. FT-495 was returned to operable status on November 6, 2013.
The data utilized in the Engineering calculation was obtained on October 31, 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 10 CFR 50.73(a)(2)(i)(B).
Cause of Event
The apparent cause of the out-of-tolerance condition identified on November 5, 2013 has been determined to be ineffective responses to previous events involving steam flow instruments not meeting normalization acceptance criteria. Previous causal analyses focused on individual instrumentation components and did not adequately consider the normalization process. In the normalization process, pre-normalization instrument calibrations that are based on previous cycle data may not meet normalization acceptance criteria due to actual full power steam flow changes between the previous and current fuel cycle. Additionally, previous corrective actions intended to reduce the duration of the normalization process to within the time limit of the applicable Required Action Statement were ineffective.
Safety Assessment Steam Flow Transmitter FT-495 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.
Prior to being rescaled, the high-steam flow bistable for FT-495 would have actuated at 111.62 percent steam flow as compared to a maximum Technical Specification allowable set point of 110.3 percent steam flow, During the time period of the FT-495 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.
Another 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-495 was known to be inoperable.
Based on the above considerations and on the FT-495 setpoint being outside the allowable tolerance but otherwise functional, this condition is considered to have low safety significance.
Corrective Action An immediate corrective action of rescaling the FT-495 loop was performed. The loop was declared operable on November 6, 2013.
Several actions are being taken to address the recurring failures to meet normalization acceptance criteria. The steam flow instrument pre-normalization calibration will be modified to either lower the high steam flow bistable setpoint or to conservatively reduce the instrument gain settings so that potential normalization process inaccuracies do not result in a high steam flow bistable exceeding the maximum allowable setpoint. The feasibility of widening the normalization acceptance band will also be assessed.
To improve the timeliness of the normalization process, additional training has been provided to site engineering personnel and improved scaling methods will be investigated.
Additional Information
A review of previously submitted Licensee Event Reports identified two similar previous events on Unit 2, both were reported in Farley Unit 2 LER 2013-001-00, in which the 2C Steam Generator Flow Transmitter FT-494 was inoperable longer than allowed by Technical Specifications.